摘要:

J O U R N A L O F C H E M I S T R Y Materials Ionic conductivity, ferroelectricity and chemical bonding in TKWB type ceramics of the K6Li4Ta10O30–Pb5Ta10O30 system Virginie Hornebecq,* Jean-Maurice Re�au, Antoine Villesuzanne, Catherine Elissalde and Jean Ravez ICMCB-CNRS-Avenue du Dr. A. Schweitzer-33608 Pessac, France Received 6th May 1998, Accepted 3rd August 1998 Correlations between Curie temperature and chemical bonding were found in ferroelectric tetragonal potassium tungsten bronze type ceramics [TKWB] using the extended Hu�ckel tight-binding (EHTB) method.The influence of Pb for K,Li substitution is shown. AC complex impedance measurements were performed on ceramics with compositions corresponding to Pb5xK6(1-x)Li4(1-x)Ta10O30 (PKLT) in a wide temperature range. The conductivity relaxation parameters of Li+ conducting PKLT ceramics were determined.Transport properties in these materials appear to be due to a Li+ ion hopping mechanism. The influence of the substitution 6K++4Li+�5Pb2+ on the mobility is discussed with the assistance of the results obtained from EHTB calculations. frequency range, the permittivities were determined, under Introduction vacuum, from capacitance and dielectric losses tan d (the ratio Ferroelectric ceramics are of great interest for applications between the imaginary part er and the real part e¾r of the such as dielectrics for capacitors, infrared detectors, electro- relative permittivity) using a Wayne–Kerr Component mechanical converters, electrooptical modulators, etc.Analyser Model 6425.The temperature range was 100 to Dielectric losses often need to be reduced when the sample 300 K. The heating rate was close to 0.2 K min-1. has to be polarized or for high quality capacitors. In contrast, the conductivity may also be at the origin of other applications Results such as CTP (coeYcient temperature positive), CTN The temperature dependence of the real permittivity e¾r shows (coeYcient temperature negative), microwave absorbants, etc.a maximum corresponding to the ferroelectric–paraelectric Recent works have therefore been devoted to the study of transition temperature for the various compositions studied conductivity in ferroelectrics.1,2 (x=0.10; 0.25; 0.50; 0.65; 0.75; 0.80). Fig. 1 gives as an The aim of the present work is to prepare and to characterize example the curve (e¾r versus temperature) obtained for the new ferroelectric ceramics with ionic conductivity only.The composition corresponding to x=0.75 at 103 Hz. The trans- cation Ta5+, whose oxidation state is very stable, was chosen ition temperature increases significantly when K+ and Li+ are to prevent electronic conductivity. replaced by Pb2+ (6K++4Li+�5Pb2+).Furthermore, the The compositions PKLT selected here belong to the maximum of e¾r is wide and implies a diVuse ferroelectric– K6Li4Ta10O30 (Tc=7 K)–Pb5Ta10O30(Tc=538 K) system.3,4 paraelectric phase transition which can be explained by Furthermore, the relation between the Curie temperature Tc a domain of variable composition implying a distribution and chemical bonding has been recently determined in ferroof transition temperatures.Fig. 2 gives the variation of electric perovskites. The approach in the present paper consists the ferroelectric–paraelectric transition temperature versus firstly of research into the relation between the transition composition. temperature and chemical bonding in TKWB tantalates. Secondly, EHTB calculations were also helpful in understand- Correlation between the Curie temperature and the chemical ing the eVect of cationic substitution on the Li+ ionic bonding conductivity.Recent works have been devoted to the role of chemical bonding in the Curie temperature and microwave relaxation Preparation frequency in tantalates.7,8 It was shown that, in significantly PKLT samples with composition Pb5xK6(1-x)Li4(1-x)Ta10O30 are prepared from K2CO3, Li2 CO3, PbO and Ta2O5.The cationic substitution corresponds to 6K++4Li+�5Pb2+. A special process was used in order to keep the initial stoichiometry, in particular to prevent PbO losses by volatilization. Details of the ceramic densification are given elsewhere.5 The samples are discs of diameter about 7 mm and thickness about 1 mm.X-Ray diVraction analysis revealed the ceramics to be single phase for 0x0.80. They crystallise with a tetragonal tungsten bronze structure.6 Ferroelectric study of PKLT ceramics Experiments The dielectric measurements were performed on ceramic discs. Electrodes were formed by depositing gold on the top and Fig. 1 Temperature dependence of e¾r ( f=103 Hz). bottom of circular surfaces by sputtering.In the 102–2×105 Hz J. Mater. Chem., 1998, 8(11), 2423–2428 2423Table 1 Extended Hu� ckel parameters: atomic orbital energies and Slater-type exponents and coeYcients. Double-f expansions are used for d orbitals Element Orbital Hii/eV f1 c1 f2 c2 Ta 5d -12.10 4.760 0.6597 1.940 0.5589 6s -10.10 2.280 1 6p -6.86 2.241 1 O 2s -32.30 2.275 1 2p -14.80 2.275 1 K 4s -4.34 1.000 1 4p -2.73 1.000 1 Pb 6s -15.70 2.350 1 6p -8.000 2.060 1 Li 2s -5.400 0.650 1 2p -3.500 0.650 1 Fig. 2 Composition variation of the ferroelectric–paraelectric transition temperature. but correspond to the K++Li+=Pb2+ substitution mode in covalent systems such as tantalates and niobates, covalency K6Li4Ta10O30. They were chosen in order to define a tractable tends to inhibit the ferroelectric distortion, by strengthening unit cell for the computation and to allow the study of the Pb the metal–oxygen bond and stabilizing the paraelectric phase.for K,Li substitution eVects. For ionic radius reasons, Pb In tantalates and niobates, this eVect appears to dominate the atoms were located in the A1 site (C.N.=12) (Fig. 3) for the well known softening eVect of covalency on short-range second composition and in both A1 and half A2 (C.N.=15) interatomic repulsions.9 sites for the third one.Here, covalency means the amount of mixing of metal d The extended Hu�ckel parameters used in the present work orbitals and oxygen 2p orbitals to form valence bands. This are given in Table 1. is a more complete view than the one given by electronegativity Fig. 4 shows the density of states and COOP curves for scales, for which covalency only means the departure from the K4Pb2Li2Ta10O30, calculated with the EHTB method. Li- and purely ionic picture. The bond covalencies were evaluated by K-character bands are located well above the Fermi level EF the computation of crystal orbital overlap populations and are out of the energy range of this figure.All bonding (COOPs) in the framework of the extended Hu�ckel tight- Pb–O and Ta–O bands lie below EF; the upper occupied band binding (EHTB) method.10,11 This quantum chemistry semi- corresponds to very covalent crystal orbitals with both Pb 6s empirical method is particularly well suited for the study of and O 2p atomic orbital contributions.It is of strong antibondthe interplay between chemical bonding and electronic struc- ing character against Pb–O bonds. Those very covalent Pb–O ture of molecules or crystals.12–17 Valence Slater-type atomic interactions are expected to weaken the Ta–O bond covalency. orbitals are attached to each atom; the Fock matrix elements The COOP values for Ta–O and Li–O bonds, summed up are computed, using the Wolfsberg–Helmholz formula,18 on to EF, are given in Table 2.The COOP, i.e. the covalency of the basis of atomic orbital overlaps and tabulated energies. Ta–O bonds decreases when the Pb content increases. The COOP is the extension to the crystalline solid of the Moreover, this increase is more significant for those Ta–O Mulliken overlap population for molecules.19,20 It is pro- bonds close to Pb atoms in the structure (competing bonds portional to two quantities closely related to covalency: the eVect).16 This evolution of Ta–O COOPs is due to the decrease overlap of atomic orbitals and the product of the correspond- of electron density (Fig. 5) along those Ta–O bands aVected ing LCAOch is maximal for equal coeYcients.by the Pb for K substitution. The very low COOP values for The band structure and the Ta–O and Li–O COOPs were calculated with the EHTB method, for the compositions K6Li4Ta10O30, K4Pb2Li2Ta10O30 and K2Pb4Ta10O30. The two latter compositions are close to K6(1-x)Pb5xLi4(1-x)Ta10O30, Fig. 4 Density of states and COOP curves for K4Pb2Li2Ta10O30. The main atomic orbital contributions to the bands are indicated. 6s and 6s* bands are very covalent with both Pb 6s and O 2p character. B1 Fig. 3 Structure used to model the TKWB network. and B2 are two crystallographic sites occupied by Ta ions (Fig. 3). 2424 J. Mater. Chem., 1998, 8(11), 2423–2428Table 2 COOP values for PKLT ceramics calculated with the EHTB PbTa0.5Sc0.5O3 and PbNb0.5Sc0.5O3; in these significantly method.B1 and B2 are two crystallographic sites occupied by Ta ions covalent systems, the bulk energy stabilisation and the metal– (see Fig. 3) oxygen bond strengthening, due to covalency, tends to inhibit the ferroelectric distortion.8 The transition temperature can be COOP (e-/bond) increased by any chemical means leading to an increase of the Composition Ta(B1)–O Ta(B2)–O Li–O metal–oxygen network ionicity.K6Li4Ta10O30 0.6444 0.6299 -0.0111 Impedance study of PKLT ceramics K4Pb2Li2Ta10O30 0.6324 0.5882 -0.0176 K2Pb4Ta10O30 0.5953 0.5663 In the Pb5xK6(1-x)Li4(1-x)Ta10O30 solid solution, it is probable that, as in Ba5Li2(Ti2Nb8)O30, Li+ ions occupy only the triangle sites with 9-fold coordination (sites C).21 The partial presence of Li+ ions in those sites is favorable for the occurrence of Li+ ionic conductivity in the direction parallel to the c-axis.Experiments AC measurements were performed on the same samples as those used for dielectric measurements; they were carried out under vacuum. Each experimental temperature was maintained by a Eurotherm-902-S Controller for 0.5 hour with an accuracy of ±0.5 K before collecting data.AC measurements were recorded out using a 1260 Solartron frequency analyser in the frequency range 102–106 Hz for several temperature cycles between 300 and 700 K. Results Complex impedance diagrams of Z/V as a function of Z¾/V, i.e. Cole–Cole plots, are presented in Fig. 6 for the composition corresponding to x=0.75 at various temperatures: the bulk ohmic resistance corresponding to each experimental temperature is the intercept on the real axis of the zero phase angle extrapolation of the highest frequency curve.22,23 Fig. 5 (Top) Contours (logarithmic scale) of the computed valence electron density for K6Ta10O30.A and B sites are occupied by K and The temperature dependence of conductivity between Ta atoms, respectively. The C site is occupied by Li atoms in the 300 and 700 K is given in Fig. 7 for some PKLT series. K atoms in A1 (A2) sites lie 2.10 A° (2.07 A° ) above and Pb5xK6(1-x)Li4(1-x)Ta10O30 compositions as a plot of log(sT ) 1.83 A° (1.86 A° ) below the figure plane. Ta and O atoms lie within against inverse temperature: an Arrhenius type behavior is 0.16 A° and 0.02 A° from the figure plane, respectively. (Bottom) clearly exhibited.A linear fit to sT=s0 exp(-DEs/kT ) is Contours ( logarithmic scale) of the diVerence in computed valence shown, with correlation coeYcient R=0.98. Electrical data electron density between Pb4K2Ta10O30 and K6Ta10O30. Pb atoms are located in A1 and A2* sites. Solid lines: positive values. Dashed lines: relative to the samples are listed in Table 3: a slight increase negative values. of activation energy DEs and a decrease of conductivity can be observed when x increases.Conductivity relaxation parameters have been calculated Li–O bonds confirm their almost purely ionic character; the slight antibonding Li–O interactions (from the covalency point from the complex impedance data in the complex modulus formalism M*=1/e*=j(vC0)z*, where j2=-1, v (v=2pf ) of view) increase with the Pb content.The COOP evolution with Pb content can be related to the is the angular frequency and Co is the vacuum capacitance of the cell. This formalism discriminates against electrode polariz- changes of transition temperature. The lowest temperature (7 K) corresponds to the highest Ta–O covalency ation and other interfacial eVects in solid electrolytes.Plots of normalized modulus (M/M¾max) versus log( f ) are given at (K6Li4Ta10O30); the increase of transition temperature observed upon Pb insertion is correlated to the decrease of various temperatures for the composition corresponding to x=0.75, for instance, in Fig. 8: the curves are non-symmetric, Ta–O covalency. This eVect was already observed in the K(Ta1-xNbx)O3 system and in the comparison of in agreement with the non-exponential behavior of the conduc- Fig. 6 Complex impedance plots at various temperatures. J. Mater. Chem., 1998, 8(11), 2423–2428 2425When the temperature increases, modulus peak maxima shift to higher frequencies (Fig. 8). Fig. 9 gives the temperature dependence of the fp=1/2pts relaxation frequency relative to Mmax for the composition x=0.75: Arrhenius-type behavior is shown.The temperature dependence of conductivity is reported in Fig. 9: its behavior is also of Arrhenius-type. Both lines are quasi-parallel, the activation energies issued from the impedance (DEs) and modulus (DEf) spectra are very similar (Table 3), suggesting that the Li+ ion transport in the materials studied is probably due to a hopping mechanism.27 Analogous results were obtained for the other compositions studied (Table 3).The composition dependence of log (s600K ) is given in Fig. 10: log (s600K) slightly decreases, quasi-linearly, when x increases whereas DEs slightly increases (Table 3). In contrast, the b parameter appears to be independent of x (Table 3). Its value (b#0.75) can be attributed to the existence of a distribution of relaxation times.28,29 This carrier polarization mechanism appears as weakly dispersive, of the same order of magnitude as the lattice one.5 Discussion Conductivity. For a given ionic conductor, the low frequency Fig. 7 Inverse temperature dependence of log(sT ) for some limit sdc of the bulk AC conductivity determined by impedance Pb5xK6(1-x)Li4(1-x)Ta10O30 compositions.spectroscopy is governed mainly by the hopping rate of free charge carriers and by the charge carrier concentration N(T ): Table 3 Electrical data and conductivity relaxation parameters relative to some various Pb5xK6(1-x)Li4(1-x)Ta10O30 compositions studied sdc=eN(T )m(T ) sdc=e2N(T )ca2h(n0/kT )exp(Sm/k) exp(-Em/kT ) x=0.10 x=0.25 x=0.50 x=0.75 x=0.80 where ah is the hopping distance, c is a geometrical factor log s600K/V-1 cm-1 equal to 1/6 for isotropic media, n0 is an attempt frequency to (±0.02) -6.92 -7.05 -7.25 -7.55 -7.58 overcome potential barriers, Sm is the migration entropy, Em DEs/eV (±0.02) 0.84 0.84 0.91 0.92 0.93 is the migration energy, the other parameters having their log s0/V-1 cm-1 conventional meaning.30–32 Equating DEs to Em, the pre- (±0.02) 2.91 2.78 3.17 2.96 3.18 exponential factor s0 in the sT=s0 exp(-DEs/kT ) equation DEf/eV can be given by the following expression: (±0.02) 0.85 0.85 0.93 0.91 0.94 b 0.77 0.76 0.73 0.77 0.74 s0=(e2a2hn0/6k)N(T )exp(Sm/k) Considering all Li+ ions as charge carriers in the partial tivity relaxation, which is well described by the empirical range (0.10x0.80) of the Pb5xK6(1-x)Li4(1-x)Ta10O30 solid stretched exponential Kohlrausch function Q(t)=exp[-t/ts]b solution, a maximum of charge carriers could correspond to (0<b<1).24–26 In this expression, ts and b are the conductivity the composition relative to x=0.50 where the C-sites are halfrelaxation time and the Kohlrausch exponent, respectively.occupied by Li+ ions.Such a result cannot be deduced from The smaller the value of b, the larger the deviation of the the small composition dependence of log(s0) (Table 3), which relaxation with respect to a Debye-type relaxation (b=1). Whatever the temperature, the full width at half-height (FWHH) of the M/M¾max spectrum is wider than the breadth of a Debye peak (1.14 decades) (Fig. 8) and it results in a value of b=1.14/FWHH for the Kohlrausch parameter, which can be considered as independent of temperature in the range studied.Fig. 9 Temperature dependences of log(sT ) and log fp, where fp is Fig. 8 Plots of normalized modulus (M/M¾max) versus log f at various temperatures. the Mmax peak frequency. 2426 J. Mater. Chem., 1998, 8(11), 2423–2428Fig. 10 Variation of log s600K with x for Pb5xK6(1-x)Li4(1-x)Ta10O30.depends on both charge carrier concentration and migration bonding of Li atoms is then weakened and their mobility increases with x. entropy parameters. The variation of electrical properties inside the Pb5xK6(1-x)Li4(1-x)Ta10O30 solid solution is relatively weak. Mobility. Charge density maps were obtained with the EHTB method. Calculations were performed for the In order to pinpoint precisely the relative importance of various steric and electronic eVects, it would be interesting to Pb4K2Ta10O30 and K6Ta10O30 compositions, in order to visualise the changes in electron density induced by Pb for K study the eVect of the insertion of lead on compounds containing a same number of Li+ cations.Work is in progress to substitution. Li atoms were omitted in these calculations in order to visualise, without the perturbation due to the lithium correlate insertion of lead, mobility of Li+ cations and ionic conductivity.charge density, the evolution of the electron density in the triangular tunnel section connecting C sites, which will be important in the remainder of this paper. The Fermi level for Conclusions K6Ta10O30 was located at the top of the 2p oxygen bands in The EHTB method was used to investigate the chemical order to reproduce the electron count of K6Li4Ta10O30. bonding in TKWB type ceramics.The eVect of the Pb for Fig. 5 shows the calculated valence electron density for K,Li substitution was studied in PKLT tantalate compounds. K6Ta10O30 in the TaO2 planes (containing both the equatorial Considerations such as covalency allowed us to compare Ta–O Ta–O bonds and the triangular tunnel sections) and the bonds in these compounds.diVerence in electron density between Pb4K2Ta10O30 and A correlation between chemical bonding and Curie K6Ta10O30. No significative valence charge density occurs in temperature was evidenced: the Curie temperature increases A sites for K6Ta10O30 since the 4s and 4p potassium bands when Pb atoms are inserted in the network. The very covalent are empty.Pb 6s charge density clearly appears in A1 and lead–oxygen bonds appear to have a significant influence on A2* sites even if Pb atoms are not in the plane of the figure. the metal–oxygen network, decreasing its covalency and fav- The decrease of electron density in those Ta–O bonds close to ouring the ferroelectric distortion.Pb atoms leads to the decrease in the Ta–O COOP calculated Furthermore, the conductivity parameters DEf and b were above. However, Ta–O bonds close to K atoms appear to be determined in the complex modulus formalism for various Li+ reinforced by the Pb for K substitution. The decrease in containing PKLT ceramics.The activation energies taken from electron density in the triangular tunnel section connecting C the impedance and modulus spectra are very similar, suggesting sites, when the Pb content rises, could play a role in the Li that Li+ ion transport is probably due to a hopping mechan- ion mobility. ism. The conductivity relaxation is well described by a Several antagonist eVects can aVect the mobility of Li+ ions Kohlrausch function Q(t)=exp[-t/ts]b.The value of b (b= in such a system. Steric eVects: (i) a slight decrease of the 0.75) can be attributed to the existence of a distribution of lattice constants, perpendicular to the tunnel direction, when relaxation times; it shows that the charge carrier polarization x increases, leads to a diminution of the tunnel sections mechanism is weakly dispersive. The eVects of the substitution containing Li+ cations.5 This eVect tends to increase the 6K++4Li+�5Pb2+ on the Li+ ion mobility were also potential barrier and to reduce the mobility of Li+ cations, discussed.but is expected to be weak (Da/a#Db/b<1%); (ii) as Pb is inserted in the network, a transfer of electron density from Ctunnels (around Li atoms) to Pb–O bonds is expected because References Pb–O bonds are more covalent than K–O bonds and because 1 M.Dong, J. M. Re�au, J. Ravez and P. Hagenmuller, J. Solid State Pb2+ is more polarizing than K+ (Fig. 5). Furthermore Pb, Chem., 1995, 116, 185. K, Li and O atoms are in the same plane (structure). This 2 M. Dong, J. M. Re�au and J. Ravez, Solid State Ionics, 1996, leads to a higher mobility of Li+ cations.Electronic eVects: 91, 183. (iii) the Li–O COOP has been calculated with the EHTB 3 T. Fukuda, Jpn. J. Appl. Phys., 1970, 9, 599. method (Table 2). It clearly shows that Li–O bond is fully 4 E. C. Subarrao and G. Shirane, Acta. Crystallogr., 1960, 13, 226. 5 V. Hornebecq, C. Elissalde, J. M. Re�au and J. Ravez, Phys. Status ionic, with a very low antibonding covalent contribution which Solidi, submitted.increases with x, giving a slight increase of Li ion mobility; 6 A. Magne�li, Arkiv Kemi, 1949, 1, 213. (iv) from an electrostatic point of view, the replacement of 7 C. Elissalde, A. Villesuzanne, J. Ravez and M. Pouchard, K+ ions by Pb2+ tends to decrease the absolute value of the Ferroelectrics, 1997, 99, 131.negative Madelung potential in C-tunnels, because of the 8 A. Villesuzanne, C. Elissalde, M. Pouchard and J. Ravez, Eur. diVerence of charge of the two ions and because the O2- ions Phys. J. (B), in press. 9 R. E. Cohen, Nature, 1992, 358, 136. net charge is reduced (covalency eVect). The electrostatic J. Mater. Chem., 1998, 8(11), 2423–2428 242710 R.HoVmann, J. Chem. Phys., 1963, 39, 1397. 22 K. S. Cole and R. H. Cole, J. Chem. Phys., 1941, 9, 341. 23 J. E. Bauerle, J. Phys. Chem. Solids, 1969, 30, 2657. 11 M.-H. Whangbo and R. HoVmann, J. Am. Chem. Soc., 1978, 24 G. Williams and D. C.Watts, Trans. Faraday Soc., 1970, 23, 625. 100, 6093. 25 K. L. Ngai and S. W. Martin, Phys. Rev. B, 1989, 40, 10550. 12 R. HoVmann, Solids and Surfaces: A Chemist’s View of Bonding in 26 F. S. Howell, R. A. Bose, P. B. Macedo and C. T. Moynihan, Extended Structures, VCH, New York, 1988. J. Phys. Chem., 1974, 78, 639. 13 J. K. Burdett and S. A. Gramsh, Inorg. Chem., 1978, 33, 4309. 27 B. V. R. Chowdari and R. Gopalakrishnan, Solid State Ionics, 14 E. Canadell and M.-H. Whangbo, Chem. Rev., 1991, 91, 965. 1987, 23, 225. 15 J. K. Burdett, Chemical Bonding in Solids, Oxford University 28 B. V. R. Chowdari and K. Radhakrishnan, J. Non. Cryst. Solids, Press, New York, 1995. 1989, 108, 323. 16 A. Villesuzanne and M. Pouchard, C. R. Acad. Sci. Paris, 1996, 29 J. Kawamura and M. Shimoji, Mater. Chem. Phys., 1989, 23, 72. 310, Se� rie II, 155. 30 N. F. Uvarov and E. F. Hairetdinov, J. Solid State Chem., 1986, 17 A. Simon, Angew. Chem., Int. Ed. Engl., 1997, 36, 1788. 62, 1. 18 J. H. Ammeter, H.-B. Bu� rgi, J. C. Thibeault and R. HoVmann, 31 D. P. Almond and A. R.West, Solid State Ionics, 1987, 23, 27. J. Am. Chem. Soc., 1978, 100, 3686. 32 N. F. Uvarov, E. F. Hairetdinov, J. M. Re�au, J. M. Bobe, 19 T. Hughbanks and R. HoVmann, J. Am. Chem. Soc., 1983, 105, J. Se�ne�gas and M. Poulain, Solid State Ionics, 1994, 74, 195. 3528. 20 R. S. Mulliken, J. Chem. Phys., 1955, 23, 1833. 21 M. Dong, Thesis, 1997, Universite� Bordeaux I, France. Paper 8/03412E 2428 J. Mater. Chem., 1998,

ISSN:0959-9428    

DOI:10.1039/a803412e

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Preparation of 2,5-diphenyloxazole doped sol-gel glasses and their application to radio-analytical chemistry Ian Hamerton, John N. Hay, John R. Jones* and Shui-Yu Lu Department of Chemistry, School of Physical Sciences, University of Surrey, Guildford, Surrey, UK GU2 5XH Received 5th June 1998, Accepted 9th September 1998 Transparent, colourless, monolithic sol-gel glasses doped with 2,5-diphenyloxazole (PPO) at diVerent concentrations were prepared via the polymerisation of tetraethyl orthosilicate (TEOS) in a H2O–AcOH–EtOH–PPO solution.The gel samples were aged at room temperature, and dried in two stages: first at room temperature, then at 70 °C to reduce possible fracture. They have pore diameters in the range 1.8–2.1 nm.The leaching of PPO from the sol-gel glass was monitored using ultraviolet (UV) and X-ray photoelectron spectroscopy (XPS). The results show that the sol-gel glasses dried at the higher temperature leach less than those dried at room temperature. These sol-gel glasses, either in monolith or powder form, are eVective in detecting b- radioactivity. should be kept to a minimum.Hence, a counting method Introduction requiring minimum use of solvent and recycling of the primary/ The development of liquid scintillation counting in the early secondary solute would represent a major advance in radio- 1960s has had a dramatic eVect on the use of radioisotopes, analytical chemistry technology. particularly in the physical and life sciences.1,2 The main The sol-gel method is a convenient low temperature route reason for this is that the two most widely used radioisotopes to converting metal alkoxides into the corresponding inorganic in this area, 3H and 14C, are both weak b- emitters, so that gels and glasses under relative mild conditions.6 An approother counting techniques such as ionisation, Geiger–Mu� ller priate precursor, such as tetraethyl orthosilicate (TEOS) and proportional counting are either impractical or have undergoes acid catalysed hydrolysis [eqn.(1)], followed by serious limitations. The advantages of liquid scintillation condensation polymerisation [eqn. (2)] at low temperature in counting for weak b- emitters are many: (a) high eYciency, a suitable solvent to form the polymeric silica network.typically >50% for 3H and >90% for 14C; (b) high sample Si(OEt)4+4H2O�Si(OH)4+4 EtOH (1) throughput as instrumentation is easily automated; and (c) a wide range of samples, from simple organic to complex n Si(OH)4�n SiO2+2n H2O (2) biological, can be analysed.3,4 The low temperature provides a clean route for doping The need for further improvement stems from the general inorganic gels and glasses with organic molecules, and in the tightening in radioactive waste legislation that has been in process makes it possible to develop composite materials with evidence over the last decade.5 A liquid scintillator consists of specific properties, and hence opens the way to various potena solvent, a primary solute (e.g. 2,5-diphenyloxazole) and, tial applications.For example, organic doped sol-gel glasses, sometimes, a secondary solute {e.g. 1,4-di[2-(5-phenyl- ranging from monoliths to thin films, have been developed as sensors for metal cations,7–10 protons (H+),9–13 anions,14,15 neutral species,16–19 oxygen20 or carbon monoxide14 in water, and oxygen21 or carbon monoxide21,22 in the gas phase. We sought to incorporate PPO into a sol-gel silica monolith in order to exploit fully its eventual sensing properties in radio-analytical chemistry technology.The chemical and physical properties of the PPO are hopefully retained, whilst the pore network allows external molecules, tritiated water (HTO) in this case, to diVuse into the matrix and interact with the solute. In this paper we report a preliminary investigation into the preparation and characterisation of sol-gel glasses doped with PPO at diVerent concentrations.Pore size, surface area and leaching tests and application in detecting 3H radioactivity are of particular interest. O N N O O N POPOP PPO Experimental oxazolyl )benzene], POPOP}. The radioactive sample is either dissolved directly in the scintillator, or if insoluble, through Materials the addition of a blending agent or secondary solvent— TEOS (Aldrich, 98%), TMOS (Acros, 99%), glacial acetic acid dioxane is frequently used for this purpose.Cocktails have (Fisons, Anal.), absolute alcohol 100 (Hayman Ltd., 99.86%), been developed so as to maximise the amount of aqueous PPO (Acros, scintillation grade), toluene (Fisons, low in samples that can be dissolved in the scintillator.3,4 After sulfur, Anal.) were used as received.counting it is customary practice to dispose of the radioactivity via the drains or absorb it onto vermiculite. Large numbers of Instrumentation samples can generate a considerable volume of waste. Ideally the radioactive waste should be stored in as compact a form BET surface areas and pore sizes were measured on a Coulter SA3100 Surface Area and Pore Size Analyser.The samples as possible and the work necessary to bring it to this state J. Mater. Chem., 1998, 8(11), 2429–2432 2429Table 1 Preparation of PPO doped sol-gel glassesa were outgassed at 120 °C for 240 min. UV spectra were recorded using a Philips PU8740 UV/VIS Scanning Composition of each vial/mmol Colourless Spectrophotometer. Liquid scintillation counting was carried Fracture- monolith out using a Packard Tri-Carb 1500 Liquid Scintillation No.b TEOS EtOH H2O AcOH PPO free gel? upon drying? Analyser, toluene was the preferred solvent.XPS measure- 1 4.58 13.73 9.15 0.35 0.000 Yes Yes ments were made using a VG Scientific ESCALAB Mk II 2 4.58 13.73 9.15 0.35 0.014 Yes Yes Spectrometer interfaced with a VG500S data system based on 3 4.58 13.73 9.15 0.35 0.027 Yes Yes a DECPDP 11/73 computer.The operating conditions were 4 4.58 13.73 9.15 0.35 0.054 Yes Yes as follows: the X-ray source (Mg-Ka 1253.6 eV radiation) was aDuplicates of 10 vials were prepared. The mixture was acidic (pH operated at a power of 450W (i.e. 13 kV potential and 34 mA 3.5). Gelation and aging were carried out at room temperature.The emission current). The spectrometer was operated in the fixed final drying temperature was 70 °C. bThe same numbering system was analyser transmission mode at a pass energy of 50 eV. The used throughout Table 1–5. Each number refers to the monolithic base pressure in the sample chamber during analysis was sol-gel glasses which were prepared in the same batch with identical PPO concentration.approximately 3×10-8 mbar. The sample was kept in the preparation chamber under vacuum overnight. Preparation of PPO doped sol-gel monolith ture with around 5% (w/w) weight loss per day for one week. Fracture-free, transparent and disc-shaped sol-gel glasses were Typically a mixture of TEOS (10.2 cm3, 45.8 mmol), water obtained after further drying at 70 °C overnight.The tempera- (1.65 cm3, 91.5 mmol), ethanol (8.1 cm3, 137.3 mmol), acetic ture is just slightly below the melting point of PPO (72–74 °C). acid (0.2 cm3) and PPO (amount appropriate for concentration All operations were carried out under normal atmosphere. required) was obtained as a clear, colourless solution. A portion of the solution (2.0 cm3) was sealed in a glass vial Surface area and pore size (20 cm3).Usually 10 such vials were prepared for one particular PPO concentration at a time. These vials were then kept Surface area and pore size data are summarised in Table 2. at room temperature to gel (1 week) and age (1 week) under The average pore diameter (d) is estimated from d=4V/S, normal atmosphere. The gels were then allowed to dry at where V is the total pore volume and S is the surface area.room temperature for one week, followed by drying at 70 °C The Coulter SA3100 Analyser uses the gas sorption method; overnight. Sol-gel glasses were obtained as clear, colourless the inert gas adsorbate is nitrogen. The BET (Brunauer, and fracture-free monoliths in cylindrical disc form. Emmett, Teller) calculation is used for the determinof the sample specific surface area.The BJH (Barrett, Joyner, Leaching test Halenda) calculation yields the pore size distribution. Coulter VacJack sample tubes, which minimise the eVects of changing Leaching tests were carried out using UV and XPS methods. liquid cryogen level as cryogen boils away during long pore For UV monitoring, sol-gel glass was ground to a powder parameter analysis, were used.By using these sample tubes form. To an UV cell (quartz, 1 cm) filled with toluene (3 cm3), BET surface area reproducibility was better than ±2%.23 was added a known amount of sol-gel glass sample. The These PPO doped sol-gel samples have very similar surface mixture was shaken at regular intervals to be mixed thorareas.PPO concentration (up to 2×10-2 M) did not seem to oughly. UV spectra were recorded at various time intervals to aVect the surface area or average pore diameter. obtain the profiles of PPO concentration in the solution, while the solids separated from the solution and settled at the bottom Leaching of the cell. For XPS analysis, four sol-gel glass discs, representing PPO concentrations at 0, 1, 2 and 4 g dm-3, were PPO was reported to display a high intensity absorption band used without further treatment.Another four sol-gel glass in the 300–335 nm region due to the p–p* through conjugate discs were packed in a small column, separated by filter papers transition in the aromatic structure. Fine details of this absorpand thoroughly washed by a constant toluene stream that was tion band are recognisable in cyclohexane solution, with peaks maintained at a rate of 2 cm3 min-1.The sol-gel glasses were at 302 and 318 nm, plus shoulders at 310 and 333 nm.24 In then dried at 70 °C overnight and then mounted on the XPS toluene, PPO (9×10-3 M) also displays a very similar absorpsample stand using ‘super glue’ for analysis.tion band with peaks at 307 and 319 nm, plus shoulders at 311 and 335 nm. Hyperchromic eVects were observed when Liquid scintillation counting procedure toluene was replaced by EtOH or TEOS as solvent. However, as observed in the BET surface analysis experiments, sol-gel To a micro-vial (0.5 cm3) was added stock HTO solution in glasses dried at 70 °C only lose a further 5% weight when 1,4-dioxane, toluene (0.4 cm3) and sol-gel glass powder subjected to heat treatment at 120 °C for 4 h.This means that (0.05–0.2 g). The micro-vial was then inserted into a standard the level of any volatile residue, such as solvent (EtOH), glass scintillation vial (25 cm3), and counted for 5 min for catalyst (AcOH) or unreacted starting materials (water, good statistical disintegration per minute (DPM) values.TEOS) will not exceed 5% in weight. A 100 mg sol-gel glass sample can release ca. 5 mg impurity to the 3 cm3 of toluene Results and discussion in a UV cell. UV absorptions of PPO in toluene spiked with 0.2% (v/v) impurity such as EtOH, AcOH or TEOS were all Sol-gel preparation identical. The cut-oV wavelength for toluene is 285 nm25 and Both TMOS and TEOS were used in trials to prepare PPO doped sol-gel glasses.At room temperature, gelation for both Table 2 Surface area and pore size measurements HCl catalysed systems was slow. Acetic acid catalysed systems gave fracture-free gels. Without catalyst, precipitation of white No. Drying temp./ °C Surface area S/m2 g-1 Average pore solids was observed in the TEOS system. We therefore decided diameter d/A° to use the system containing TEOS–EtOH–H2O–AcOH (see 1 70 610 21 Table 1).To avoid fracture, the gels were aged at room 2 70 540 20 temperature for a week, whilst the vials containing the gel 3 70 506 20 were kept tightly closed to avoid the loss of solvents. After 4 70 515 21 ageing the gels were dried in two stages: first at room tempera- 2430 J.Mater. Chem., 1998, 8(11), 2429–2432Table 4 Counting eYciency for PPO doped sol-gel glasses in EtOH–toluene mixture Sol-gel HTO Radioactivity/ Relative No. glass/g sample/g dpm g-1 eYciency (%)a 1 0.076 0.3616 22904 74.6 2 0.059 0.3602 25318 82.5 3 0.057 0.2608 26863 87.5 4 0.060 0.3545 27367 89.2 aCompared to a toluene-based scintillator (3.4 g dm-3 PPO). Standard HTO radioactivity=30694 dpm g-1.Table 5 Counting eYciency for PPO doped sol-gel glasses after toluene wash in EtOH–toluene mixture Sol-gel HTO Radioactivity/ Relative No. glass/g sample/g dpm g-1 eYciency (%)a 1 0.125 0.4116 60409 66.8 2 0.104 0.4130 65965 73.0 3 0.111 0.4139 59698 66.0 4 0.059 0.4115 70140 77.6 aCompared to a toluene-based scintillator (3.4 g dm-3 PPO). Standard HTO radioactivity=90408 dpm g-1.of PPO. This observation also supports the UV results that the PPO concentration increased in the toluene solution due to leaching at the sol-gel surface or outer layers. However, PPO was also encapsulated in the inner pores, and suYcient concentration was retained after the toluene treatment. These sol-gel glasses still gave more than 60% relative eYciency when used in liquid scintillation counting (see Table 5).Liquid scintillation counting using PPO doped sol-gel glasses The eYciency of PPO doped sol-gel glasses to detect b- radioactivity (from tritiated water, HTO) was investigated using a liquid scintillation counting method. Preliminary Fig. 1 Changes in UV spectra, representing PPO leaching from results are shown in Table 4 and 5, and the counting eYciency (A) sol-gel glass (No. 4 in Table 1) dried at room temperature and was compared with the standard PPO solution in toluene as (B) sol-gel glass (No. 4 in Table 2) dried at 70 °C. These spectra were scintillator. When sol-gel glasses were used with toluene alone, recorded at (a) 0, (b) 2, (c) 5, (d) 10, (e) 20, ( f ) 30 and (g) 60min after mixing the sol-gel glass with toluene in a UV cell.the relative eYciency was very low, possibly due to some preferential absorption of water molecules onto the glass. It is also possible that hydrogen–tritium (H–T) exchange takes the main absorption band for PPO is in the 300–335 nm place on the surface Si–OH groups.26 This can gave rise to region. With these factors in mind, together with the advantage chemical quenching because at this stage it appears that the of monitoring the leaching in toluene which is the major PPO released from the freshly broken surface was the eVective scintillation solvent, the choice of toluene as solvent was fluor.The energy of radiation was not freely passed between entirely satisfactory. the solvent and fluor molecules, thus fewer photons were UV spectra representing the PPO concentrations in toluene generated.However, with careful adjustment of the scintil- are shown in Fig. 1. The increase of PPO concentration reflects lation cocktail, in this case by adding EtOH to suppress the the leaching from sol-gel glasses in a semi-quantitative manner. surface H–T exchange or preferential absorption, high The rate of leaching was greater in the earlier stage of the eYciency, typically >60%, can still be achieved.The liquid mixing, then decreased gradually for sol-gel glasses dried at scintillation counting method provides important clues about room temperature [Fig. 1(A)]. Sol-gel glasses treated at higher the eVectiveness of sol-gel encapsulation. Furthermore it may temperatures should have shrunk structures, and tend to leach yield more information about the mechanistic details as to less than their room temperature dried counterparts during how radioactivity interacts with the solvent and fluor within/ the same period [Fig. 1(B)]. XPS analysis (Table 3) of the outside the sol-gel cages. Further work designed to improve toluene-treated sol-gel glasses showed that the surface was free our understanding is in progress.Table 3 XPS analysis of PPO doped sol-gel glasses before and after The authors thank the EPSRC for a ROPA award (S.-Y.L). treatment in toluene We also thank Dr J. F. Watts and Mr S. J. Greaves for Before (atomic %) After (atomic %) assistance with XPS analysis. No. C O Si N C O Si N References 1 8.2 72.4 19.0 0.5 7.8 72.8 19.5 0.0 1 Isotopes5Essential Chemistry and Applications II, ed.J. R. Jones, 2 10.6 70.3 18.0 1.2 6.5 73.9 19.6 0.0 The Royal Society of Chemistry, London, 1988. 3 10.3 69.4 18.9 1.4 8.1 71.5 20.5 0.0 2 E. A. Evans, Tritium and Its Compounds, 2nd edn., Butterworths, 4 18.9 63.1 14.8 3.3 8.6 72.7 18.7 0.0 London, 1974. J. Mater. Chem., 1998, 8(11), 2429–2432 24313 A. Dyer, An Introduction to Liquid Scintillation Counting, Heyden, 14 B.C. Dave, B. Dunn, J. S. Valentine and J. I. Zink, Anal. Chem., 1994, 66, 1120A. London, 1974. 15 D. D. Dunuwilla, B. A. Torgerson, C. K. Chang and 4 C. T. Peng, Sample Preparation in Liquid Scintillation Counting, K. A. Berglund, Anal. Chem., 1994, 66, 2739. The Radiochemical Centre, Amersham, 1977. 16 A. S. Yamanaka, F. Nishida, L.S. Ellerby, C. R. Nishida, 5 P. E. Ballance, A. G. Richards and R. N. Thomas, B. Dunn, J. S. Valentine and J. I. Zink, Chem.Mater., 1992, 4, 495. Tritium5Radiation Protection in the Laboratory, HHSCHandbook 17 L. S. Ellerby, C. R. Nishida, F. Nishida, A. S. Yamanaka, NO 11, H and H Scientific Consultants Ltd., Leeds, 1992. B. Dunn, J. S. Valentine and J. I. Zink, Science, 1992, 25, 1113. 6 (a) C. J. Brinker and G. W. Scherer, Sol-Gel Science5The Physics 18 D. N. Simon, R. Czolk and H. J. Ache, Thin Solid Films, 1995, and Chemistry of Sol-Gel Processing, Academic Press, San Diego, 260, 107. 1989; (b) D. A. Loy and J. K. Shea, Chem. Rev., 1995, 95, 1431; 19 U. Narang, P. N. Prasad, F. V. Bright, A. Kumar, N. D. Kumar, (c) D. Avnir, Acc. Chem. Res., 1995, 28, 328.B. D. Malhotra, M. N. Kamalasanan and S. Chandra, Chem. 7 P. W. Wong and J. D. Mackenzie, in Better Ceramics Through Mater., 1994, 6, 1596. Chemistry, ed. C. Sanchez, M. L. Mecartney, C. J. Brinker and 20 K. E. Chung, E. H. Lan, M. S. Davidson, B. Dunn. J. S. Valentine A. Cheetman, Mater. Res. Soc. Symp. Proc., MRS, Pittsburgh, and J. I. Zink, Anal. Chem., 1995, 67, 1505. 1994, vol. 346, p. 329. 21 O. Lev, Analysis, 1992, 20, 543. 8 B. I. Kuyavskaya, I. Gigozin, M. Ottlenghi, D. Avnir and O. Lev, 22 J. I. Dulebohn, S. C. Haefner, K. A. Berglund and K. R. Dunbar, J. Non-Cryst. Solids, 1992, 147–148, 808. Chem. Mater., 1992, 4, 506. 9 S. Braun, S. Shtelzer, S. Rappoport, D. Avnir, M. Ottolenghi and 23 Coulter SA3100 Series Surface Area and Pore Size Analyser O. Lev, Anal. Chim. Acta., 1992, 256, 65. Product Manual, Coulter Corporation, Miami, Florida, 1996. 10 L. Yang and S. S. Saavedra, Anal. Chem., 1995, 67, 1307. 24 A. T. Balaban, L. Birladeanu, I. Bally, P. T. Frangopol, 11 N. Aharonson, M. Altstein, G. Avidan, D. Avnir, A. Bronshtein, M. Mocanu and Z. Simon, Tetrahedron, 1963, 16, 2199. A. Lewis, K. Liberman, M. Ottolenghi, Y. Polevaya, C. Pottman, 25 R. D. Braun, Introduction to Instrumental Analysis,McGraw-Hill, New York, 1987. J. Sammuel, S. Shalom, A. Strinkovski and A. Turniasky, in 26 I. Hamerton, J. N. Hay, B. J. Howlin, J. R. Jones, S. Y. Lu, ref. 7, p. 519. G. A.Webb and M. G. Bader, High Perform. Polym., 1997, 9, 281. 12 J. E. Lee and S. S. Saavedra, Anal. Chim. Acta, 1994, 285, 265. 13 B. D. MacCraith, V. Ruddy, C. Potter, B. O’Kelly and J. F. McGilp, Electron Lett., 1991, 27, 1247. Paper 8/04267E 2432 J. Mater. Chem., 1998, 8(11), 2429–2432

ISSN:0959-9428    

DOI:10.1039/a804267e

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Crystallization behaviour of the series of solid solutions ZrxTi1-xO2 and PbyZrxTi1-xO2+y prepared by the sol–gel process Rotraut Merkle* and Helmut Bertagnolli Institut fu�r Physikalische Chemie, Universita�t Stuttgart, PfaVenwaldring 55, D-70550 Stuttgart, Germany Received 16th June 1998, Accepted 11th August 1998 A series of solid solutions of zirconium titanium oxide ZrxTi1-xO2 (0x1) and PbyZrxTi1-xO2+y (0y1.15, 0x1) up to lead zirconate titanate PbZrxTi1-xO3 was prepared from zirconium and titanium n-propoxide and lead acetate, dissolved in 2-methoxyethanol, by the sol–gel process.The crystallization was investigated by diVerential thermal analysis (DTA), and the apparent activation energies, crystallization enthalpies and Avrami exponents were determined.The crystalline phases were identified by X-ray diVraction (XRD). Within the series of solid solutions ZrxTi1-xO2, the activation energy changes from ca. 200 kJ mol-1 for samples with low (x0.14) or high (x0.9) ZrO2 content, to ca. 800 kJ mol-1 for intermediate ZrO2 contents. Within the series of solid solutions PbyZrxTi1-xO2+y, PbO lowers the activation energies and hinders the crystallite growth.solution in PrnOH, Ti(OPrn)4 98 wt% solution in PrnOH, Alfa] 1 Introduction in 2-methoxyethanol (MOE, distilled before use). During the The sol–gel process allows the simple preparation of oxidic addition of 5 mol H2O per mol Zr/Ti, a slightly turbid gel was solid solutions with a continuous variation of the stoichiometry formed. It was dried at 90 °C, 20 mbar and crushed to a fine and a homogeneous distribution of all components at the powder.For gels containing more than 10% ZrO2, a steam molecular level. It is frequently used for the synthesis of treatment7 was applied to prevent the formation of carbon electrical and magnetic ceramics with optimized properties.1 residues during pyrolysis which severely disturb the following Owing to the high homogeneity of the amorphous oxide DTA measurements. For gels with a ZrO2 content of up to mixture which is the product of the pyrolysis of the dried gels, 10% the steam treatment cannot be applied because small crystallization can occur without long range diVusion at mod- anatase crystallites are formed during this treatment.erate temperatures. Thus, desired or undesired metastable Additionally, the solvent was varied and samples of ZT(X) phases can be the first crystallization products2 which trans- were prepared with n-propanol (PrOH, distilled before use) form to the thermodynamically stable structures at higher instead of MOE. During hydrolysis, a voluminous precipitate temperatures. instead of a gel was formed.It was dried under the same In the first step, the binary system of ZrxTi1-xO2 [ZT(X) conditions as the gel samples. These samples pyrolyze without with X=100x=content of ZrO2 in %] prepared by the sol–gel any formation of carbon residues even without steam treatprocess is investigated to study the eVect of substituting Zr by ment. Samples of ZT(X) prepared in PrOH with and without Ti on the crystallization behaviour.Zr and Ti can be inter- steam treatment [labeled ZT(X)-PS and ZT(X)-P] are examchanged easily in many ternary systems, e.g. in the perovskites ined to study the eVect of the steam treatment on the ABO3 on the B sites with A=Pb, Ba or Sr. Nevertheless, in crystallization. the binary system ZrO2–TiO2 the diVerences between zir- Lead containing samples were prepared analogously to conium and titanium may have larger eVects.The hydrolysis ZT(X) by dissolving appropriate amounts of lead acetate of TiMOR groups and the condensation of TiMOH groups trihydrate (99.5%, Fluka) in methoxyethanol (100 ml per are faster than the corresponding reactions of ZrMOR and 0.1 mol of Zr/Ti alkoxide), adding Zr/Ti n-propoxide ZrMOH groups.3–5 While titanium is mostly octahedrally and hydrolyzing.After the steam treatment, samples of coordinated by oxygen, zirconium prefers larger coordination P(Y )ZT(45) with 0Y115% pyrolyze without carbon resi- numbers of 7 to 8. dues. In the series P(Y )Z, samples with a PbO content larger In the second step, the PbO content in the ternary system than 10% form carbon residues and cannot be studied further.PbyZrxTi1-xO2+y [P(Y )ZT(X) with Y=100y=content of Samples of P(Y )T must be prepared in n-propanol (with PbO in %] is varied at fixed ZrO2 content X, and gives a series anhydrous lead acetate to prevent precipitation before hydroly- of solid solutions, extending from P(Y )T, P(Y )ZT(45) to sis) without steam treatment, because they crystallize during P(Y )Z.Between ZrO2 and PZ, a complete series of solid the course of the steam treatment, and P(Y )T, prepared in solution can be prepared by coprecipitation,6 where the struc- MOE without steam treatment, forms carbon residues. ture changes from the tetragonally distorted fluorite structure Samples of P(Y )T-P with Y>15% crystallize in a diVerent for the lead poor samples to the pyrochlore structure for lead structure (PbTi3O7) and were not examined further.rich samples. The P(Y )ZT(X) series provides information A Netzsch STA 409 with Al2O3 crucibles was used for DTA about the role of the lead in the crystallization process. The measurements. The mass of the samples was 5–30 mg, the knowledge obtained from all these series can improve the furnace was purged with 0.5 l min-1 of dry air.All samples understanding of the crystallization of PZT prepared by show an exothermic DTA peak at about 300 °C due to the the sol–gel process. pyrolysis of the organic components, and a total weight loss of less than 10% for ZT(X) up to about 25% for PZT(X). After this pyrolysis step, no further weight loss occurs, which 2 Experimental indicates that the samples have reached the final composition and contain no further organic residues.Activation energies The samples ZT(X) with 0X100% were prepared from a 1 molar solution of Zr/Ti n-propoxide [Zr(OPrn)4 70 wt% were determined from DTA runs with heating rates from J. Mater. Chem., 1998, 8(11), 2433–2439 24331 K min-1 to 30 K min-1. Crystallization enthalpies were cal- preexponential factors p0, k0: culated from the integrals of the DTA peaks.Some samples have extremely narrow DTA peaks. Thus the total crystalliz- b-ln R Ea¾ # 1 n ln ap0k0 m m =ln c0¾ (5) ation heat is liberated in a very short time, and the sample is overheated which also accelerates the crystallization. This The molar volume contained in a can be estimated from the leads to unrealistically high values of the Avrami exponents volume of the unit cell, which is usually of the order of determined from the DTA peak shape.Therefore the samples some A° 3, and the geometry factor included in a is of the same were mixed with Al2O3 as an inert material in the mass ratio order of magnitude as m, thus the apparent preexponential of 152 for determining Avrami exponents from the DTA factor c0¾ approximately equals the geometrical average of the peak shape.preexponential factor of the nucleation rate per molecular unit X-Ray diVractograms were measured on a horizontal Stoe p0¾=p0/NA and of the growth rate k0: Stadi P diVractometer with a focussing germanium monoc0 ¾#(p0¾k0 m)1/n (6) chromator and a curved position sensitive detector (PSD).Cu-Ka radiation (40 kV, 35 mA) was used. The samples were An approximate value nGr of the Avrami exponent n can be pressed to tablets, fixed on a rotating sample holder and determined from the integral . h(T )dT and the maximum measured in reflection mode. The measured intensities were height h(Tm) of the DTA peak20 by the phenomenological corrected for absorption8 and polarization.9 The crystallite relation, deduced from simulated DTA curves: size L was determined from the full width at half maximum (FWHM) d(k) of the reflections on the k scale [k=4p sin(H)/l] nGr=0.02+C h(Tm)Tm2 Ea¾.h(T )dT (7) with the Scherrer equation10 L=2p/d(k).The measured d(k)exp was corrected for apparatus broadening d(k)app according to11 C is 2.211×10-4 for energies in eV,20 corresponding tod(k)=d(k)exp-d(k)app with the assumption that the peak 0.02133 for energies in kJ mol-1.For the crystallization of shapes are lorentzian. The apparatus broadening d(k)app was PZT prepared by the sol–gel process,21 a good agreement was determined from sintered PT and PZ samples with suYciently found between n determined from Avrami plots of isothermal large crystallite size so that d(k)exp equals d(k)app.crystallization measurements and approximate values of nGr DiVractograms of samples heated to 350 °C, where the from non-isothermal DTA measurements. pyrolysis of organic components is completed, prove that these samples are amorphous. DiVractograms of samples heated above the temperatures of the crystallization peaks, which 4 Solid solution series ZT(X) were observed in the DTA measurements, show the complete transformation to the crystalline phases. The crystalline phases formed in the course of the heating of the ZT(X) samples depend on the ZrO2 content X.The pure TiO2 and the titanium rich samples up to ZT(18) crystallize in the anatase structure. The formation of this metastable 3 Crystallization kinetics phase by a sol–gel process is also reported for TiO2 in The isothermal formation of a crystalline phase from an refs. 22–24. amorphous or another crystalline phase by a mechanism of In the intermediate range of ZT(25) to ZT(60), the samples nucleation and surface controlled growth can be described by crystallize in the orthorhombic srilankite structure (a-PbO2 the Johnson–Mehl–Avrami equation:12–15 structure), whose unit cell parameters vary linearly with the ZrO2 content.25 At temperatures below about 1200 °C, srilank- 1-x(t)=exp(-apkmtn/m) (1) ite TiZr2O6 is the only stable compound in this stoichiometry range, and above 1200 °C only ZrTiO4 is stable.26 For the x(t) is the fraction transformed at time t, m is the dimensionalsol –gel samples crystallizing at much lower temperatures, ity of the growth and n=m for a constant number of nuclei, the metastable srilankite is formed over a wide range.The and n=m+1 for a constant nucleation rate; a includes a diVractograms of the crystalline phases of ZT(25)–ZT(60) are geometry factor and the reciprocal of the molar volume; p collected in Fig. 1. and k are the rate constants of nucleation and linear crystallite The samples with a ZrO2 content of more than 60% crys- growth.They are assumed to follow an Arrhenius law: tallize mainly in the tetragonally distorted fluorite structure. p=p0 exp(-En/RT) (2) The formation of this phase in the course of the sol–gel process is also reported for ZrO2 in ref. 27, although this phase is k=k0 exp(-Eg/RT) (3) stable only at temperatures of 1100–2300 °C.The diVractograms of the crystalline phases of ZT(75)–ZrO2 are also The Kissinger plot16,17 which was deduced in order to collected in Fig. 1. ZT(75) and ZT(85) still show a very weak determine the activation energy of a simple, not interface srilankite (011) reflection. Shoulders at 2H=28.2° and 2H= controlled reaction from the dependence of the shift of the 31.5° besides the main (111) fluorite reflection indicate the DTA peak maximum temperature Tm from the heating rate a, presence of small amounts of the monoclinic ZrO2 phase can also be applied to crystallization processes obeying Avrami formed from the tetragonal phase during pressing of the kinetics.The plot of ln(a/T 2m) versus 1/Tm yields an apparent sample tablet.activation energy Ea¾ as the weighted average of En and Eg: Application of the Scherrer equation yields crystallite sizes of about 250 A° for TiO2 and ZT(10), and over 1000 A° for Ea¾= En+mEg n (4) ZT(18). The crystallite size is in the range 140–350 A° for the srilankite phases, and rises for the fluorite phases to 640 A° for ZT(95) and over 1000 A° for ZrO2.For the samples prepared For the crystallization of ZT(X) and P(Y )ZT(X) samples, both the nucleation (i.e. the growth of a subcritical nucleus to in PrOH, the steam treatment has no eVect on the crystallite sizes. The only exceptions are TiO2-P and ZT(10)-P, where a supercritical nucleus) and the crystallite growth require the reorientation of the (Zr/Ti)O6 network by breaking and the crystallite size increases significantly with the use of PrOH, and reaches about 1000 A° .reconstructing (Zr/Ti)MOM(Zr/Ti) bonds. Therefore, it is very likely that En#Eg and thus E¾a#En#Eg. In this case, The DTA curves of ZT(X), prepared in MOE, are shown in Fig. 2. TiO2 and ZT(10) crystallize at about 400 °C and the crystallization is isokinetic,18,19 and the following relation holds for the intercept b of the Kissinger plot and the 500 °C, respectively.The samples ZT(18)–ZT(75) with moder- 2434 J. Mater. Chem., 1998, 8(11), 2433–2439Fig. 1 DiVractograms of the crystalline phases of ZrxTi1-xO2 [ZT(X) with X=100x=ZrO2 content in %], shifted along the ordinate for clarity. Fig. 2 DiVerential thermal analysis curves of ZrxTi1-xO2 [ZT(X) with X=100x=ZrO2 content in %], prepared by a sol–gel process in 2-methoxyethanol.ate ZrO2 contents crystallize at temperatures above 650 °C and exhibit very narrow DTA peaks. The crystallization temperature decreases with increasing ZrO2 content down to about 400 °C for ZrO2. The crystallization temperature is independent of which crystal structure is formed, because the samples crystallizing above 650 °C crystallize in the anatase [ZT(18)], srilankite [ZT(25)–ZT(60)] and fluorite [ZT(75)] structures.All the crystallization enthalpies are in the range of about 14–18 kJ mol-1; only the samples TiO2 and ZT(10), prepared in MOE without steam treatment, show lower values of about 10 kJ mol-1. The apparent activation energies Ea¾, determined from the Kissinger plots (examples are shown in Fig. 3), are depicted in Fig. 4. They show the same trend as the crystallization temperatures with a distinct maximum at medium ZrO2 contents. For the samples prepared in MOE, the apparent activation energies of about 800 kJ mol-1 reach the values of the Fig. 3 Kissinger plots of ZrxTi1-xO2 [ZT(X) with X=100x=ZrO2 sublimation enthalpies of ZrO2 and TiO2 (811 kJ mol-1 and content in %], prepared by a sol–gel process in 2-methoxyethanol. 695 kJ mol-1 28).The crystallization behaviour of the samples prepared in PrOH with steam treatment is very similar to that of the samples prepared in MOE with steam treatment. Thus, must be broken in order to allow the linked (Zr/Ti)O6 the diVerent early stages (gel versus voluminous precipitate) octahedra to arrange according to the crystal structures.For have no eVect on the crystallization. The samples prepared in the pure components ZrO2 and TiO2 of the binary system PrOH without steam treatment show the same general trends (Zr/Ti)O2, the activation energies are similarly low at about in the activation energies, but the maximum values of about 200 kJ mol-1. This can be interpreted by the following, simpli- 550 kJ mol-1 for ZT(18)–ZT(75) are lower than for the steam fied model.The TiMOMTi bonds (and ZrMOMZr bonds) treated samples. can be broken with the same probability in both directions: The drastic change of the apparent activation energies with the zirconium content is not correlated with the changes in OMTiMOMTiMO�OMTiMO9 |+TiMO the final crystal structures.Therefore the reason for this change OMTiMOMTiMO�OMTi+|O9 MTiMO must be sought in the amorphous phase or the crystallization process. In the crystallization process, some (Zr/Ti)MO bonds thus the breaking of one (or very few) TiMOMTi connections J. Mater. Chem., 1998, 8(11), 2433–2439 2435l4 mol-4 s-1 was found.5 As the alkoxide exchange reaction M(OPrn)+EtOH�MOEt+PrnOH is fast, even at room temperature, 4 it can be assumed that the species which was hydrolyzed was mostly Zr(OEt)4, and therefore k3 and k¾5 can be compared to prove the faster hydrolysis of titanium alkoxides compared to the corresponding zirconium alkoxides.The condensation reaction of the TiMOH groups, which follows the hydrolysis step, TiMOr=TiMOMZr+ROH TiMOH+HOMTi=TiMOMTi+H2O occurs easier with ZrMOR than with TiMOH due to the larger basicity of the leaving group, and leads to the favoured formation of TiMOMZr bonds instead of the homocondensation to TiMOMTi and ZrMOMZr.In an EXAFS study at the Zr K-edge of PZT gel which was not dried,32 the first metal backscatterer around Zr was found to be Ti, indicating Fig. 4 Apparent activation energies of ZrxTi1-xO2 [ZT(X) with X= 100x=ZrO2 in %], with standard deviation calculated from the linear the preferred heterocondensation.Therefore already at moderregression. ate ZrO2 or TiO2 contents of about 20%, a large amount of ZrMOMTi bonds may be present and causes the change in the crystallization behaviour. yields fragments which are suitable for the progress of the The Avrami exponents nGr, determined from the DTA peak crystallite growth.shape, show no significant diVerence between the samples Ea¾ and thus En and Eg are drastically increased when more prepared in MOE or PrOH and with or without steam than about 15% of the other metal cation is present. Although treatment. nGr of the samples crystallizing in the anatase Zr and Ti are chemically similar, they disturb each other structure decreases from almost 4 for TiO2 to about 2 for markedly in the crystallization process. As a consequence of ZT(18).The samples forming the srilankite phase have Avrami the larger electronegativity and the larger positive charge exponents in the range of 3–4. nGr decreases to nGr#2 for density of Ti4+ compared to Zr4+, the TiMOMZr bonds are ZT(75) with the change to the fluorite phase, and increases broken preferably in one direction: again with increasing ZrO2 content to about 4 for ZrO2.The MOMTiMOMZrMOM�MOMTiMO9 |+ZrMOM closer the stoichiometry of a sample is to that of the ‘pure’ phase (TiO2 with anatase structure, ZrTi2O6 and ZrTiO4 with Thus mainly the fragments MOMTiMO9 | and ZrMOM are srilankite structure, ZrO2 with tetragonally distorted fluorite formed from MOMTiMOMZrMOM bonds.If other fragstructure), the higher are the Avrami exponents. The maximum ments, e.g. MOMTi or |O9 MZrMOM, are needed for the values of about 4 correspond to a crystallization mechanism reconstruction of the (Zr/Ti)O6 network during the crystallizof a constant nucleation rate with subsequent three-dimen- ation, additional bonds of a (Zr/Ti)O6 octahedron must be sional crystallite growth.When the growth is disturbed, the broken, which strongly increases the activation energy. This Avrami exponent is reduced more or less. will happen more often when the number of TiMOMZr bonds From the ordinate intercept b and the apparent activation is high, i.e. at intermediate zirconium contents. energies E¾a of the Kissinger plots, the apparent preexponential In order to estimate the probability of TiMOMZr bonds in factors c0¾#( p0¾k0 m)1/n can be calculated according to eqn.amorphous samples of ZT(X), the hydrolysis of the zirconium (5). They show a strong increase with increasing values of Ea¾. and titanium alkoxides shall be considered briefly. The struc- A similar trend was observed for the thermally activated tural aspects of the formation of oligomers and polymers diVusion in amorphous alloys.33–35 The plot of the logarithm during the hydrolysis of zirconium and titanium alkoxides of the Arrhenius preexponential factors versus the diVusion were studied extensively by Bradley and coworkers.29–31 activation energies of several iron–zirconium alloys, shown in Kinetic aspects of the diVerent steps comprising the complex ref. 33, exhibits a linear correlation approximately. This hydrolysis and condensation reactions were investigated relation can be deduced from the Eyring theory, which yields later.3–5 Owing to the larger charge density of the Ti4+, which a temperature dependence of the rate constants of makes the hydrogen atoms of coordinated water molecules in TiMOH2 more acidic, the hydrolysis of TiMOR groups to TiMOH is faster than the hydrolysis of ZrMOR groups.c= kT h exp(DS/R) exp(DH/RT) (8) For the formation of TiO2 from 0.1–0.2 molar solutions of Ti(OEt)4 in ethanol with a water5Ti ratio of 2–5, a rate law instead of the Arrhenius law.36 For a small temperature range, of t=k[H2O]3[Ti(OEt)4] (t is the time from the addition of the frequency factor kT /h is approximately constant, and the the water to the first turbidity of the solution) was found.3 It Arrhenius preexponential factor c0 can be identified as indicates that the hydrolysis reaction Ti(OR)4+3H2O�Ti(OR)(OH)3+3ROH c0= kT h exp(DS/R) (9) is the rate limiting step, and a value of k=6.6 l4 mol-4 s-1 was determined for the hydrolysis. In contrast, a rate law of containing the activation entropy DS‡.The Arrhenius activation energy Ea corresponds to the activation enthalpy t=k[H2O][Ti(OEt)4]2 is observed for dilute solutions [0.01–0.03 molar solutions of Ti(OEt)4 in ethanol ] with a DH‡. In contrast to gas reactions, usually DS‡ is positive in the solid state, because the transition state requires a high large excess of water (10–100 mol H2 per mol Ti).4 Instantaneous hydrolysis to Ti(OH)3(OEt) is proved by meas- mobility to enable a reorientation of the fragments into another structure.The increase of the activation energies Ea and uring the rapid decrease of the water content. The formation of precipitates is much slower, and the condensation reaction activation enthalpies DH‡ observed in refs. 33–35 was attributed to a larger number of atoms involved in a collective is the rate-limiting step. For the hydrolysis of 0.05–0.53 molar solutions of Zr(OPrn)4 motion during a diVusion step. This is accompanied with an increase of the activation entropy DS‡. In case of an approxi- in ethanol, a rate law of t=k¾[H2O]3[Zr(OPr)4] with k¾=0.9 2436 J. Mater.Chem., 1998, 8(11), 2433–2439Fig. 6 DiVractograms of PbyZr0.45Ti0.55O2+y [P(Y )ZT(45) with Y= Fig. 5 Logarithmic plot of the Arrhenius preexponential factor c¾O 100y=PbO content in %], shifted along the ordinate for clarity. versus activation energy E¾a for the samples ZrxTi1-xO2 [ZT(X) with X=100x=ZrO2 content in %], with standard deviation calculated from the linear regression.mate proportionality DS‡~DH‡, we get from eqn. (9) the observed linear relation ln c0~DS‡�ln c0~DH‡. This interpretation of the Arrhenius preexponential factors can be transferred to the crystallization of the ZT(X) samples. The increase of Ea¾ can be attributed to a larger number of (Zr/Ti)MOM(Zr/Ti) bonds which must be broken in the transition state, and therefore Ea¾~DS‡ can be assumed.In Fig. 5 the plot of ln c0¾ versus Ea¾ is shown, which clearly exhibits a linear relation between these quantities. The maximum eVect of the activation entropy on c0¾ can be estimated from the crystallization of ZT(18) taking place at about 650 °C. From c0¾#1052 min-1 and kT /h#1015 min-1, we get DS‡#700 J mol-1 K-1 which is still below the entropy of sublimation of about 765 J mol-1 K-1 estimated from the sublimation enthalpy of TiO2.Another example of the correlation of activation energies Fig. 7 DiVerential thermal analysis curves of PbyZr0.45Ti0.55O2+y [P(Y )ZT(45) with Y=100y=PbO content in %], prepared in 2- and Arrhenius preexponential factors can be found in ref. 37 methoxyethanol with steam treatment. for the crystallization of PbTiO3.For samples prepared by coprecipitation or sol–gel processing, activation energies of 260–270 kJ mol-1 and preexponential factors of 2×1015–7×1017 s-1 were determined. Amorphous PbTiO3, prepared from the melt by roller quenching, yields Ea¾= 633 kJ mol-1 and c0¾=4×1037 s-1. The higher activation energy of the roller quenched sample can be attributed to the stronger hindrance of movements due to the significantly higher density.The collective motion of the adjacent (Zr/Ti)O6 octahedra increases DS‡ and therefore also c0¾. d solution series P(Y)ZT(45) The samples P(Y )ZT(45), prepared in MOE with steam treatment, can be examined over the whole range 0Y100%, and additionally even with a lead excess (y= 107% and y=115%). P(5)ZT(45) and P(10)ZT(45) crystallize in the srilankite structure.At larger PbO contents, the samples form the fluorite structure with a continuous transition to the pyrochlore phase. The diVractograms are shown in Fig. 6. The Fig. 8 Apparent activation energies of PbyZr0.45Ti0.55O2+y crystallite size decreases from 350 A° [ZT(45)] and 160 A° [P(Y )ZT(45) with Y=100y=PbO content in %], prepared in 2- [P(10)ZT(45)] to 110 A° [P(40)ZT(45)] and down to 40 A° for methoxyethanol with steam treatment, with standard deviation the pyrochlore phase of P(85)ZT(45)–P(115)ZT(45). calculated from the linear regression.In Fig. 7 the DTA curves of ZT(45)–P(40)ZT(45) are depicted. With increasing PbO content, the crystallization temperature decreases and the peaks broaden. The apparent decreases with an excess of PbO to 420 kJ mol-1 for P(115)ZT(45).The transformation of the pyrochlore phase activation energies are shown in Fig. 8. A PbO content of 5% already reduces Ea¾ from 800 kJ mol-1 to below 600 kJ mol-1 to the perovskite phase, which can be observed for the samples P(85)ZT(45)–P(115)ZT(45), has a constant activation energy without changing the crystal structure.Ea¾ of the formation of the fluorite or pyrochlore phase is constant at about of 300 kJ mol-1. The crystallization enthalpy decreases strongly from 16 kJ mol-1 for ZT(45) to 8 kJ mol-1 for 530 kJ mol-1 for P(10)ZT(45)–P(100)ZT(45) and slightly J. Mater. Chem., 1998, 8(11), 2433–2439 2437Fig. 9 Schematic representation of the (Zr/Ti)MOM(Zr/Ti) bond reconstruction in the presence of PbO.M=(Zr/Ti). P(40)ZT(45) and to only 3 kJ mol-1 for the pyrochlore formation of PZT(45). The Avrami exponents nGr decrease parallel to the decrease of the crystallite size from about 4 for ZT(45)–P(10)ZT(45) to 2 for P(40)ZT(45) and about 1.5 for P(85)ZT(45)–P(115)ZT(45). The small nGr for the pyrochlore formation of P(85)ZT(45)–P(115)ZT(45) can be interpreted with a high, constant nucleation rate and subsequent growth to a maximal crystallite radius of 20 A° .21 Growth to larger Fig. 10 DiVerential thermal analysis curves of PbyTiO2+y [P(Y )T-P maximal radii corresponding to the crystallite sizes found for with Y=100y=PbO content in %], prepared in n-propanol without P(40)ZT(45) and P(25)ZT(45) yields Avrami exponents steam treatment. closer to 4.The decrease of Ea¾ and thus En and Eg can be explained by the involvement of PbO in the reconstruction of the (Zr/Ti)O6 P can be explained by a mechanism of a constant nucleation rate and subsequent crystallite growth which is more or less network. Lead has a high mobility either in the form of PbO (cf. the high vapour pressure of PbO38) or in the form of Pb2+ hindered. When the crystallite growth ceases at some large (>1000 A° ) but fixed radius, n decreases without any eVect on (cf.the high diVusion coeYcient of Pb2+ in sintered ZrTiO439). A small amount of lead is suYcient to act as a ‘catalyst’ the XRD line width. This is observed, e.g., for the perovskite formation of PZT in ref. 21. In order to explain the large and to break many bonds subsequently.The decrease of the activation energy can interpreted by the avoiding of shifts of the crystallization temperatures with increasing PbO content, a corresponding decrease of the Arrhenius pre- energetically unfavourable fragments during crystallization. The eVect of PbO is shown schematically in Fig. 9. The exponential factors p0 and k0 must also be assumed. In total, the presence of PbO in TiO2 gels seems to hinder the insertion of PbO into a OM(Zr/Ti)MOM(Zr/Ti)MO bond is energetically more favourable than the formation of crystallization.OM(Zr/Ti)MO9 |+(Zr/Ti)MO fragments which is required in the absence of lead. The PbMO bonds, which are weaker 7 Solid solution series P(Y)Z compared to (Zr/Ti)MO bonds, can be broken easily. The additional oxygen atom, introduced with the PbO, ensures The samples P(Y )Z, prepared in MOE with steam treatment with a PbO content of 0Y10%, crystallize in the tetra- that the Zr/Ti atoms are surrounded by at least 6 oxygen atoms during the rearrangement of the (Zr/Ti)O6 network.gonally distorted fluorite structure. The crystallite size decreases from over 1000 A° for ZrO2 to about 500 A° for Thus the unfavourable (Zr/Ti)MO fragments with a reduced coordination number are avoided.After the rearrangement, P(5)Z and P(10)Z. The DTA peak temperatures at a heating rate of a=3.5 K min-1 shift from 380 °C for ZrO2 to 425 °C the PbO is eliminated again, leaving the modified (Zr/Ti)O6 network. The decrease of the crystallite sizes with increasing for P(5)Z and to 455 °C for P(10)Z.All the Avrami exponents nGr have values larger than 4 due to overheating, even after lead content can also be explained by this model. If the PbO is not eliminated, then a configuration (Zr/Ti)MOM mixing with Al2O3. The apparent activation energies are constant at about 270 kJ mol-1, and the crystallization PbMOM(Zr/Ti) remains. Owing to mechanical stress caused by the volume shrinkage during crystallization, the weaker enthalpies are about 19 kJ mol-1.The eVect of the presence of PbO in ZrO2 gels is similar PbMO bonds can be broken yielding (Zr/Ti)MOM Pb+|O9 M(Zr/Ti). The continuous octahedral network is to that observed for P(Y)T-P. The activation energies of TiO2 and ZrO2 are already so low that no further decrease broken and thus the crystallite growth is stopped.The pyrochlore phase of PZT(45) is the lead rich end occurs when lead is added. It can be assumed that (Zr/Ti)MOMPbMOM(Zr/Ti) bonds are also formed in member of this solid solution series. Therefore the small crystallite size of 40 A° , the Avrami exponent of 1.5 and the these systems, and that they can be broken easily to (Zr/Ti)MOMPb+|O9 M(Zr/Ti), which stops the crystallite apparent activation energy of 500 kJ mol-1 (which is relatively high compared with Ea¾#300 kJ mol-1 for the pyrochlore to growth.Thus the crystallization is hindered by the lead addition, which is detectable in an increase of the crystalliz- perovskite transformation21) are not exceptional. ation temperatures and a decrease of the Avrami exponents and crystallite sizes. 6 Solid solution series P(Y)T The samples P(Y )T-P with 0Y15% crystallize in the 8 Summary anatase structure. The reflections are only slightly broadened, indicating a crystallite size of more than 600 A° for all samples. In the solid solution series of ZT(X), extremely high apparent activation energies, reaching the values of the sublimation The DTA curves of these samples are depicted in Fig. 10. The DTA peaks are shifted to higher temperatures and strongly enthalpies of ZrO2 and TiO2, are found for intermediate ZrO2 contents of 18–75%. The most plausible explanation is the broadened with increasing PbO content. Correspondingly, the Avrami exponents nGr decrease from 4 for TiO2-P to 1.5 for favoured formation of ZrMOMTi bonds during the gel formation. These bonds are broken with a preferred direction P(15)T-P.The apparent activation energies from the Kissinger plots are about 260 kJ mol-1, and the crystallization enthalpies yielding the fragments TiMO9 |+Zr, and thus it may be necessary to break additional bonds of a (Zr/Ti)O6 octahedron to are about 18 kJ mol-1. Both energies are independent of the PbO content. form the fragments required for the continuation of the crystallite growth.Therefore the activation energy is increased The lowering of the Avrami exponents for P(5)T-P–P(15)T- 2438 J. Mater. Chem., 1998, 8(11), 2433–243912 W. A. Johnson and R. F. Mehl, Trans. Am. Inst. Min. Eng,, 1939, strongly. A linear relation between the logarithm ln c0¾ of the 135, 416. apparent Arrhenius preexponential factor and the apparent 13 M.Avrami, J. Chem. Phys., 1939, 7, 1103. activation energy Ea¾ is found. It can be explained by the 14 M. Avrami, J. Chem. Phys., 1940, 8, 213. contribution of the activation entropy DS‡ which is increased 15 M. Avrami, J. Chem. Phys., 1941, 9, 177. parallel to the number of broken bonds and Ea¾. 16 H. E. Kissinger, Anal. Chem., 1957, 29, 1703. 17 J. W. Graydon, S.J. Thorpe and D. W. Kirk, Acta Metall. Mater., The incorporation of lead in the amorphous pyrolyzed gels 1994, 42, 3163. of P(Y )ZT(X) hinders the crystallite growth, which is mani- 18 J. W. Cahn, Acta Metall., 1956, 4, 572. fested either in a decrease of the Avrami exponent or in XRD 19 D. W. Henderson, J. Non-Cryst. Solids, 1979, 30, 301. line broadening, or both. In the samples where the correspond- 20 J.W. Graydon, S. J. Thorpe and D. W. Kirk, J. Non-Cryst. Solids, ing ZT(X) exhibits a large apparent activation energy, Ea¾ is 1994, 175, 31. 21 R. Merkle and H. Bertagnolli, Ber. Bunsenges. Phys. Chem., 1998, lowered drastically by the addition of lead and thus the 102, 1023. crystallization is accelerated. The reduction of Ea¾ can be 22 I. Manzini, G.Antoniolo, D. Bersani, P. P. Lottici, G. Gnappi explained by the stabilization of the fragments formed from and A. Montanero, J. Non-Cryst. Solids, 1995, 192–193, 519. (Zr/Ti)MOM(Zr/Ti) bonds by interaction with PbO. For 23 Bokhimi, A. Morales, O. Novaro, T. Lopez, E. Sanchez and samples where the corresponding ZT(X) already has a low R. Gomez, J. Mater. Res., 1995, 10, 2788. 24 K. Terabe, K. Kato, H. Miyazaki, S. Yamaguchi, A. Imai and value of Ea¾, the lead addition does not alter the activation Y. Iguchi, J. Mater. Sci., 1994, 29, 1617. energy, but the DTA peak temperatures are shifted strongly 25 O. Yamaguchi and H. Mogi, J. Am. Ceram. Soc., 1989, 72, 1065. to higher temperatures upon lead addition. Here the hindrance 26 A. E. McHale and R. S. Roth, J.Am. Ceram. Soc., 1986, 69, 827. of the crystallite growth is the main eVect of the PbO. 27 P. Colomban and E. Bruneton, J. Non-Cryst. Solids, 1992, 147–148, 201. 28 G. H. Aylward and T. J. H. Findlay, SI Chemical Data, J. Wiley & Sons, New York, 2nd edn., 1974. Notes and references 29 D. C. Bradley, R. Gaze and W. Wardlaw, J. Chem. Soc., 1955, 3977. 1 C. D. E. Lakeman and D. A. Payne, Mater. Chem. Phys., 1994, 30 D. C. Bradley, R. Gaze andW.Wardlaw, J. Chem. Soc., 1957, 469. 38, 305. 31 D. C. Bradley and D. G. Carter, Can. J. Chem., 1961, 39, 1434. 2 J. Gopalakrishnan, Mater. Chem., 1995, 7, 1265. 32 R. Ahlfa�nger, H. Bertagnolli, T. Ertel, U. Kolb, D. Peter, R. Naß 3 E. A. Barringer and H. K. Bowen, Langmuir, 1985, 1, 414. and H. Schmidt, Ber. Bunsenges. Phys. Chem., 1991, 95, 1286. 4 M. T. Harris and C. H. Byers, J. Non-Cryst. Solids, 1988, 103, 49. 33 W. Frank, U. Hamlescher, H. Kronmu� ller, P. Scharwaechter and 5 P. M. Smit, A. Van Zyl and A. I. Kingon, Mater. Chem. Phys., T. Schuler, Phys. Scr., 1996, T66, 201. 1987, 17, 507. 34 W. Frank, A. Ho� rner, P. Scharwaechter and H. Kronmu� ller, 6 O. Yamaguchi, T. Fukuoka and Y. Kawakami, J. Mater. Sci. Mater. Sci. Eng. A, 1994, 179/180, 36. Lett., 1990, 9, 958. 35 B. Damson and R. Wu� rschum, J. Appl. Phys., 1996, 80, 747. 36 R. W. Cahn and P. Haasen, Physical Metallurgy, North Holland 7 R. Merkle and H. Bertagnolli, J. Mater. Sci., 1998, in press. Publications, Amsterdam, 1983. 8 P. Debye and H. Menke, Phys. Z., 1930, 31, 797. 37 R. W. Schwartz and D. Payne, Mater. Res. Soc. Symp. Proc.: 9 K. A. Kerr and J. P. Ashmore, Acta Crystallogr., Sect. A, 1974, Better Ceramics through Chemistry III, 1988, 121, 199. 30, 176. 38 K. H. Ha�rdtl and H. Rau, Solid State Commun., 1969, 7, 41. 10 P. Scherrer, Go�tt. Nachr., 1918, 2, 98. 39 M. V. Slinkina and G. I. Dontsov, Inorg. Mater., 1992, 28, 429. 11 H. P. Klug and L. E. Alexander, X-ray DiVraction Procedures for Polycrystalline and Amorphous Materials, Wiley and Sons, New York, 2nd edn., 1974. Paper 8/04552F J. Mater. Chem., 1998, 8(11), 2433&n

ISSN:0959-9428    

DOI:10.1039/a804552f

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials The thio-sol–gel synthesis of titanium disulfide and niobium disulfide Part 1.—Materials chemistry Mandyam A. Sriram and Prashant N. Kumta* Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh PA 15213, USA Received 3rd April 1998, Accepted 3rd August 1998 Investigations pertaining to the synthesis of titanium disulfide (TiS2) have so far been focused on solid state reactions, low temperature chemical techniques and vapor phase reactions using titanium tetrachloride (TiCl4) as the starting material.In this paper, we have investigated the potential of titanium tetraalkoxides [Ti(OR)4], which have been widely used for the synthesis of oxides by the sol–gel approach, for the synthesis of TiS2 via the thiosol –gel process.The mechanism of the reaction of titanium isopropoxide {Ti(OPri)4} with H2S in benzene has been studied using infrared spectroscopy (FTIR), gas chromatography (GC) and chemical analysis. Based on these studies, it has been determined that the precipitate obtained from the reaction forms following a thiolysis–condensation mechanism similar to the hydrolysis–condensation mechanism that operates in the oxide sol–gel process.The precipitate, which is an alkoxysulfide, can be converted to TiS2 by heat treatments in flowing H2S. The influence of modifying agents, the role of solvents and the alkoxy group [in Ti(OR)4] on the formation of the alkoxysulfide precipitate have also been presented and discussed. Finally, the applicability of this process for the synthesis of NbS2 has also been demonstrated.by these processes with respect to control of the morphology 1 Introduction or stoichiometry of the sulfide. Considering the strong impact Titanium disulfide (TiS2) has been studied as a useful cathode of all these factors on the performance of the sulfide as a material for Li intercalation or insertion batteries.It exhibits cathode in rechargeable lithium batteries, it is useful to identify a strong potential for inserting lithium into the crystal to form a process that provides this flexibility to control both the LixTiS2, where 0<x<1, without causing any phase changes. microstructure (morphology, particle and crystallite size) and It also has a good electronic conductivity (ca. 28 S cm-1) and defect concentration of the final sulfide powder. displays a high reversibility of the intercalation reaction.1 Metal alkoxides are unique in these sense that they oVer the These properties of the material have made it a viable cathode possibility to react with nucleophilic sulfidizing agents to yield material for both bulk and thin film rechargeable lithium precursors which can be converted to form the sulfide with batteries.The performance of the sulfide as a cathode material control of defect concentration and microstructure. Metal depends on the eYcacy of the intercalation reaction, which in alkoxides have been extensively investigated for the synthesis turn depends on a number of factors such as morphology, of oxide ceramics, glasses and thin films via the sol–gel process. crystallite and particle sizes, and defect concentration.All However, there has been little work reported on the potential these factors are strongly influenced by the nature of the of these compounds for the synthesis of non-oxides.12–22 In processes and the processing conditions used to synthesize the first part of this two-part series, we report mainly the the material.solution and precursor chemistry pertaining to the synthesis There are several methods reported in the literature for of TiS2 via the thio-sol–gel process. In the second part (see synthesizing titanium disulfide in both powder and film form. following paper), the morphology, defect concentration and These methods include: (a) high temperature solid state reac- electrochemical characteristics of the thio-sol–gel synthesized TiS2 powders are presented and discussed.tions,2,3 (b) vapor phase reactions4–9 and (c) low temperature chemical precipitation techniques.10,11 The simplest method is the synthesis of the sulfide by melting the individual elements 2 Experimental procedure in an evacuated sealed quartz tube at high temperatures.Several vapor phase reactions have also been investigated to The reactions of titanium alkoxides were studied using diVerent synthesize TiS2 powders and thin films, the most common sulfidizing agents. At the same time, the influence of modifying being the reaction of TiCl4 with H2S (incidently, this was also the alkoxide, the alkyl group and the solvent system on the the first vapor phase reaction to be investigated for the sulfidization reaction were also studied.Unless otherwise mensynthesis of TiS2 powders4). Room temperature chemical tioned, the source and purity of the chemicals and the instruprecipitation routes mostly using chlorides and various inor- ments used for the analyses are: Ti(OPri)4 (as received from ganic10 and organic11 sulfidizing agents have also been success- Johnson Matthey Alfa, 97%), benzene (as received anhydrous fully implemented to synthesize fine particles of TiS2. benzene from Aldrich, 99.8%), KBr (FT-IR grade from All the methods described above result directly in the Aldrich, 99+%), CsI (Aldrich, 99.9%), H2S (Mattson gases, formation of TiS2.The low temperature precipitation routes CP grade), benzenesulfonic acid (Aldrich, tech.grade), acetoresult in a poorly crystalline sulfide which can be crystallized nitrile (anhydrous, as received from Aldrich, 99.8%), ethyl by heating the material at a higher temperature. These precipi- alcohol (absolute, McCormick Distillation Co., Pekin, IL), tates are often contaminated by chloride (Cl-) ions and by titanium tetraethoxide (Aldrich, tech.grade), titanium ninorganic by-products of the reaction. More importantly, the butoxide (Aldrich, 99%), titanium 2-ethyl hexoxide (Aldrich, composition of the material (and therefore its defect structure), 95%), dimethyl disulfide (Aldrich, 99%), hexamethyldisilthiane its morphology and particle size are all determined at the (Fluka); Gas chromatography (Hewlett Packard model 5830A), mass spectrometry (VG Analytical, model 7070), precipitate stage itself. Therefore there is little flexibility oVered J.Mater. Chem., 1998, 8(11), 2441–2451 2441does not chelate the metal atom (unlike carboxylic acids which chelate the metal atom due to the presence of the highly polar CNO group). In order to confirm the reaction of Ti(OPri)4 with BSA, the reactions were conducted in a benzene solvent (in an acid5alkoxide molar ratio, n=1510, 151 and 251). The isopropanol liberated from the reaction was azeotropically distilled under ultrahigh purity (UHP) nitrogen and quantitatively analyzed using GC. 2.2.1 Modification using the ratio n=0.1 (reaction B). The procedure adopted to study the molecular processes that occur in the reaction of modified alkoxide with H2S was very similar to that developed for the first reaction.The alkoxide was modified with BSA (n=acid5alkoxide molar ratio=0.1) in benzene and reacted with H2S. The precipitate, filtrate and distillate were analyzed as described in section 2.1. 2.2.2 Modification using the ratio n=1 and n=2. Fig. 1 Schematic flow chart showing the procedure for synthesizing Benzenesulfonic acid was added to Ti(OPri)4 in benzene in a TiS2 using the reaction of titanium alkoxide with H2S.molar ratio of 151. When H2S was passed through the solution for about 10 minutes, there was only a change in the color of scanning electron microscopy (CamScan series 4). All manipu- the solution from colorless to dark brown, without the appearlations and handling of reactants and products were executed ance of any precipitate.The vessel was left sealed for about under an argon or a nitrogen atmosphere. All reactions were 18 h after which the liquid turned black without the formation conducted in glassware dried in vacuum at 120 °C after rinsing of any precipitate. Ti(OPri)4 was also modified using 2 moles with hexane. of BSA per mole of alkoxide.When H2S was bubbled through this solution in benzene, the solution changed from colorless 2.1 Reaction of titanium isopropoxide with H2S (reaction A) to yellow without the formation of any precipitate. These reactions were not studied any further. The flow chart in Fig. 1 describes the experimental procedure. Ti(OPri)4 was dissolved in anhydrous benzene, and H2S was 2.3 Influence of solvent on the reaction of Ti(OPri)4 with H2S bubbled through it at room temperature.A black precipitate was observed within a minute, but the bubbling was continued Acetonitrile (ACN). Both reaction A (plain alkoxide+H2S) for 10 min to ensure completion of the reaction. The reaction and reaction B (modified alkoxide+H2S, using a modification vessel was sealed and isolated for 12 h.ratio n=0.1) were conducted in anhydrous ACN. In both The precipitate was then collected and washed thoroughly cases, there was immediate precipitation without any changes with anhydrous benzene in a Soxhlet extractor, and dried in in the color of the solution. The precipitates were collected vacuum at 40 °C for 3 h after which it was perceived to be using a Soxhlet extractor, washed with ACN and dried at pyrophoric and extremely air-sensitive.This precipitate formed 40 °C for 3 h. The precipitates were weighed to estimate the the precursor for synthesizing titanium disulfide (TiS2). yield of the reaction and their Ti and S contents were analyzed Chemical analysis of the precipitate was conducted by to estimate the extent of the sulfidization reaction.Galbraith Laboratories Inc. (Knoxville, TN), and an IR spectrum was collected in the 4000 to 650 cm-1 wavenumber Ethyl alcohol. Neither the plain alkoxide nor the modified range in a KBr pellet using a Fourier transform infrared alkoxide reacted with H2S in a solution of absolute ethyl spectrometer (FTIR spectrometer, Bruecker Instruments) alcohol. A slight color change to light yellow was observed equipped with an MCT detector and in the 500 to 200 cm-1 but no precipitate appeared even after allowing the reaction range in a CsI pellet using an FTIR spectrometer (Biorad) to proceed for several days.equipped with a DTGS detector and CsI optics. The filtrate obtained was dark brown indicating the presence 2.4 Influence of alkoxy group on the reaction of alkoxide with of molecular species containing TiMS bonds.The filtrate was H2S azeotropically distilled and gas chromatography (GC) per- Three other alkoxides were reacted with H2S in ben- formed on the distillate to quantify the products of the zene and in acetonitrile: titanium ethoxide {Ti(OC2H5)4}, reaction. After ensuring that all the excess benzene was distilled n-butoxide {Ti(OCH2CH2CH2CH3)4} and 2-ethylhexoxide oV, chemical analysis was performed on the dark brown liquid.{Ti[OCH2CH(C2H5)(CH2)3CH3]4}. The observations are The liquid was also analyzed employing electron impact mass shown in Table 1. The solids collected from the reactions spectroscopy. shown in Table 1 were analyzed for their Ti and S contents. The powders were also weighed to estimate the yield of the 2.2 Influence of modification on the reaction of Ti(OPri)4 with reactions.H2S Modification of titanium isopropoxide cannot be accomplished 2.5 Reaction of titanium isopropoxide with other sulfidizing by aqueous acids because of the extreme sensitivity of the agents alkoxide to water. However, it has been shown that alkoxides react with organic acids to form the metal acylates with the 2.5.1 Ti(OPri)4+dimethyldisulfide (DMDS; reaction C).release of alcohol. For example with a carboxylic acid:23 Ti(OPri)4 was dissolved in anhydrous benzene and excess dimethyl disulfide (DMDS) was added to it. The mixture was M(OR)n+m RCOOHAM(OR)n-m(OOCR)m+m ROH refluxed for 24 h at 70 °C, after which a small amount of H2S (1) gas was passed over the surface of the liquid. This resulted in the formation of a black precipitate.The precipitate was In this study, benzenesulfonic acid (BSA, C6H5SO3H) was used to modify the alkoxide for the following reasons: (a) washed with benzene and dried under vacuum at 40 °C for 2 h. The mechanism of the reaction, however, has not been BSA is a strong organic acid and (b) it is monofunctional and 2442 J.Mater. Chem., 1998, 8(11), 2441–2451Table 1 Reaction of titanium alkoxides in benzene and acetonitrile on the powders to verify the phases present and the powders were examined in an SEM. Observations Reaction of H2S with niobium(V) ethoxide modified with Alkoxy group In benzene In acetonitrile benzenesulfonic acid. Niobium ethoxide was reacted with BSA Ethoxy Orange soln., Insoluble.Introduction of in the molar ratio of 1051, alkoxide to acid and dissolved in no ppt. H2S for 10 min led to a thick acetonitrile. H2S gas was bubbled into the solution. The viscous liquid. Some solid reaction proceeded in the same way as described above. A collected by heating the black precipitate was formed, which was also dried at 40 °C liquid under vacuum at ca.for 2 h under vacuum. IR analysis and further heat treatments 60 °C. of the precipitate were conducted in the same manner as n-Butoxy Dark brown liquid, Insoluble. Introduction of no ppt. H2S for ca. 5 min led to a described above. black precipitate. The precipitate was washed with 3 Results and Discussion acetonitrile and dried at 40 °C for 3 h. In this section, the mechanisms of the reaction of titanium 2-Ethylhexoxy No reaction No reaction isopropoxide with various sulfidizing agents are presented and discussed along with the influence of solvents and the alkoxy group. This is followed by a study of the conversion of the studied.A similar scheme has been attempted by Guiton precipitates obtained from these reactions to crystalline TiS2.et al.,24 where they observed that the reaction of diethylzinc In order to facilitate the discussion, we have used the following with dibenzyl trisulfide in the presence of H2S led to an abbreviations for the various reactions. Reaction A: the reacenhancement in the kinetics of precipitation of ZnS. tion of titanium isopropoxide with H2S conducted in benzene. The same reaction conducted in acetonitrile will be termed 2.5.2 Ti(OPri)4+hexamethyldisilthiane (HMDST, reaction ‘reaction A in acetonitrile’.Reaction B: the reaction of titanium D). Owing to the extreme moisture sensitivity, coupled with isopropoxide modified with BSA using a modification ratio the toxicity and stench of hexamethyldisilthiane (HMDST), a n=0.1, conducted in benzene. Reaction C: the reaction of cannula technique was used inside a glove bag for all transfer titanium isopropoxide with dimethyldisulfide (DMDS) cataprocedures.HMDST was added to Ti(OPri)4 in anhydrous lyzed by H2S using benzene as a solvent. Reaction D: the benzene and the vessel was sealed and isolated for 12 h. A reaction of titanium isopropoxide with hexamethyldisilthiane change in the color of the mixture from colorless to bright (HMDST) conducted in benzene.yellow was observed within about 3 min and after a series of color changes, from yellow to dark brown, a black precipitate 3.1 Reaction of titanium isopropoxide with H2S (Reaction A) was finally observed after ca. 3 h. The precipitate was collected, The IR absorption spectrum collected on the precipitate washed and dried as described in previous sections.An FTIR obtained from the reaction is displayed in Fig. 2, showing the spectrum was collected on the precipitate and chemical analysis 1800 to 900 cm-1 and 500 to 200 cm-1 (inset) spectral ranges. was conducted to estimate the amounts of Ti, S, C, H and Si The doublet at 1377 and 1360 cm-1 is characteristic of the in the precipitate.gem-dimethyl structure of the isopropoxy group.25,26 Bands at 1160, 1127 and 1013 cm-1 are also characteristic of the isopro- 2.6 Conversion of the precursors to TiS2 poxy groups bonded to Ti.26 The inset shows the typical TiMS The dried powders obtained from the four reactions (reactions stretching absorption10 at 300 cm-1 (hence indicating the A–D) were then heat treated in flowing H2S at 600, 700 and formation of TiMS bonds due to the reaction of H2S with the 800 °C for 6 h (heated at 15 °Cmin-1 and furnace cooled).At alkoxide in solution), and the TiMO stretching absorption at each stage, an X-ray diVractogram was collected using Cu-Ka 450 cm-1 (owing to the unreplaced isopropoxy groups25,26). radiation from 2h= 10° to 90° (Rigaku h/h diVractometer equipped with a diVracted beam graphite monochromator), and the powders were observed under an SEM.Samples for SEM examination were prepared by ultrasonicating the powders in hexane for 5 min and placing a few drops of the suspension on a graphite surface. 2.7 Synthesis of niobium disulfide Reaction of H2S with niobium(V) ethoxide. The applicability of the thio-sol–gel process was also extended to the synthesis of NbS2. Niobium pentaethoxide [Nb(OC2H5)5; 10 g] was dissolved in 200 cm3 acetonitrile. H2S was passed through the solution for 10 min.A black precipitate was observed to form in 20 s and the vessel was left sealed for 24 h. A Soxhlet extractor was used to filter the suspension and wash the powders using acetonitrile (HPLC grade, used as received from Fisher Chemicals).In this case too, the filtrate was dark brown to black. The black precipitate obtained after filtration was dried under vacuum at 40 °C for 2 h. Absorption IR spectra were collected on the precipitate using the CsI pellet Fig. 2 The IR spectrum collected on the as-precipitated alkoxy-sulfide method (using a Mattson Galaxy Series spectrometer equipped powder (prepared using reaction A) in the wavenumber range with CsI optics) in the wavelength range from 4000 to 4000–650 cm-1 showing the presence of unreacted isopropoxy groups 200 cm-1.The precipitated powder was then heat treated at (1377, 1360, 1160, 1122, 1013 cm-1) bonded to titanium. The inset 700 and 800 °C for 6 h in flowing H2S (heated at 15 °Cmin-1 shows Ti–O and Ti–S absorptions in the far IR range.Note: the absorbance axis has arbitrary units. and furnace cooled). X-Ray diVraction patterns were collected J. Mater. Chem., 1998, 8(11), 2441–2451 2443Table 2 Chemical analyses and quantitative GC (reaction A) with SH, (b) GC in conjunction with chemical analyses confirm condensation of thiols to be the dominant mechanism. (a) Chemical analyses of liquid and solid products Chemical analysis of the liquid product (Table 2) showed a S/Ti molar ratio of about 0.06.In addition, the mass fragmen- Ti S C H O (diVerence) Weight/g tation pattern of the liquid was identical to that of titanium Solid (wt%)a 33.6 32.3 17.3 3.5 13.3 0.754 isopropoxide, implying that the dark liquid was in fact Ti Solid (mol%) 9.4 13.5 19.3 46.8 11 isopropoxide, which had not undergone any significant reac- Liquid (wt%)b 7.3 0.3 76.8 8.1 7.5 30.088 tion with H2S.Some of the alkoxide molecules remained Liquid (mol%) 1 0.06 42.3 53.5 3.1 unreacted, possibly due to (a) association in inert solvents like (b) Correlation of isopropanol replacement from chemical analyses benzene (titanium isopropoxide is known to have an average and GC molecular complexity of 1.3 in benzene28), or (b) due to the formation of partially reacted soluble oligomers which do not Moles of titanium isopropoxide used in the reaction 0.0504 undergo any further reaction in benzene because of steric Moles of isopropanol detected in the distillate 0.025 hindrance to nucleophilic attack by H2S.Experiments conduc- Moles of isopropanol that should be liberated, calculated 0.021 from chemical analyses and assuming reaction (4) for all ted in acetonitrile (a coordinating solvent which is known to thiol groups be a better medium for the dissociation of H2S), have shown more than a five-fold increment in product yield, providing aThe solid precipitate was analyzed after drying at 40 °C for 2 h in vacuum.bThe liquid was analyzed after removing benzene by support for this hypothesis.Increase in the product yield when distillation. the reaction was conducted in ACN led us to explore the eVects of solvent and alkoxy groups on the yield of the reaction. These results are presented in the following sections. 3.2 Influence of modification Based on the chemical analysis of the precipitate (shown in Table 2) and the IR results, it can be concluded that the solid Titanium isopropoxide dissolved in benzene was modified is an alkoxysulfide, containing (a) unreacted isopropoxy groups using BSA in the modification ratios (n=molar ratio of acid and (b) sulfur bonded to Ti.to alkoxide) of 0.1, 1 and 2. The isopropanol liberated from Isopropanol was also detected in the distillate implying the the reaction was azeotropically distilled and quantitatively replacement of isopropoxy groups (OPri) by thiol groups (SH) analyzed using gas chromatography.Based on the number of from H2S. This attack of the alkoxy groups would form the moles of isopropanol liberated, we could confirm that the basis of a thiolysis reaction very similar to the hydrolysis following modification reaction went to completion: reactions in the sol–gel process, as illustrated by Livage:27 Ti(OC3H7)4+n C6H5SO3HATi(OC3H7)4-n(C6H5SO3)n Pri +nC3H7OH (n=0.1, 1, 2) (5) | H2S+Ti(OPri)4 A H2SATi(OPri)4 A (HS) (OPri)3TiBOH The alkoxides, modified to diVerent extents, were reacted with H2S using anhydrous benzene as a solvent.A Ti(OPri)3SH+PriOH (2) 3.2.1 Modification using the ratio n=0.1 (reaction B) The The overall reaction can therefore be written as infrared spectrum of the precipitate obtained from the reaction Ti(OPri)4+nH2S A Ti(OPri)4-n(SH)n+nPriOH (n<4) of the alkoxide (partially modified by benzenesulfonic acid in (3) the molar ratio of 10 to 1, alkoxide to acid) with H2S is shown in Fig. 3. The most important feature of the spectrum is the After thiolysis, the precipitates could have formed due to existence of absorptions at 1000, 1020 and 1040 cm-1 all of condensation–polymerization of the alkoxythiols by the liberwhich correspond to the sulfonyl vibrations of the acid.29,30 ation of H2S as explained in the following paragraphs: In addition to this, other vibrations due to the alkoxy groups 2p {Ti(OPri)4-n(SH)n} A {(PriO)4-n(SH)n-1Ti-S-Ti(SH)n-1(OPri)4-n}p+pH2S (n<4) (4) The condensation of alkoxy groups is less likely due to the highly polar character of the TiMO bonds as well as steric hindrance caused by the bulky isopropoxy groups.As shown in Table 2, 0.025 moles of alcohol per 0.0504 moles of starting Ti isopropoxide, was detected by quantitative GC. It should be noted that the alcohol released is a result of the alkoxythiol (OPri<SH) replacement reaction responsible for the formation of both the liquid and the solid products.In order to confirm the formation of the precipitate by the above mentioned condensation mechanism [reaction (4)], the isopropanol liberated was calculated from the chemical analysis of the liquid and solid products. Assuming that the precipitates are formed as a result of condensation of all the thiol groups leaving sulfur formally bonded to two titanium atoms, calculations indicate an expected release of 0.021 moles of isopropanol, which is in good agreement with the isopropanol content estimated by quantitative GC.This, therefore, vali- Fig. 3 The IR spectra of the powders obtained from the reaction of dates the assumed condensation mechanism.Two facts, there- modified alkoxide with H2S (reaction B). In comparison to Fig. 5 the fore, become clear from the above analysis: (a) infrared additional vibrations at 1000, 1020 and 1040 cm-1 are all due to the spectroscopy and GC have indicated that the thiolysis reaction sulfonyl groups of the acid. Note: the absorbance axis has arbitrary units.as shown in reaction (3) causes partial replacement of OPri 2444 J. Mater. Chem., 1998, 8(11), 2441–2451Table 3 Chemical analyses and quantitative GC (reaction B) sation of which could be sterically hindered due to the presence of the bulky benzenesulfonyl groups. (a) Chemical analysis of liquid and solid products 3.2.2 Modification using the ratio n=1 and n=2. When H2S Ti S C H O (diVerence) Wt./g gas was bubbled into a solution of Ti(OPri)4 in benzene Solid (wt%)a 32.3 30.3 20.8 3.5 13.1 1.234 modified using a modification ratio of n=1, a series of color Solid (mol%) 8.8 12.4 22.7 45.3 10.8 changes was observed, from colorless to yellow to dark brown, Liquid (wt%) 16.4 1.8 48.5 8.4 24.9 12.3b without the formation of any precipitate.The change in color Liquid (mol%) 2.4 0.4 28.0 58.4 10.8 indicated the possibility of some thiolysis reaction occurring (b) Correlation of replacement of isopropanol from chemical analysis in solution leading to the formation of some TiMSH linkages and GC without any condensation.Similarly, the alkoxide solution modified using the ratio n=2 changed from colorless to yellow Moles of titanium isopropoxide used in the reaction 0.0504 when reacted with H2S, again without the formation of any Moles of isopropanol detected in the distillate ignoring the 0.016 precipitate. This occurs mainly due to steric hindrance oVered isopropanol liberated from the modification reaction Moles of isopropanol that should be liberated, calculated 0.027 by the bulky acid groups.In the case of H2S, even though from chemical analysis assuming complete thiol based thiolysis may occur, condensation does not occur because of condensation mechanism steric hindrance.These observations are similar to the oxide aThe solid precipitate was analyzed after drying at 40 °C for 2 h in sol–gel process, wherein the hydrolysis reaction of water with vacuum. bCalculated by conserving titanium.a modified alkoxide is considerably slower than that with an unmodified alkoxide. 3.3 Influence of solvent bonded to Ti can be observed (CMO vibrations at 1120 and To evaluate the eVect of solvent, reactions A and B were 1160 cm-1) indicating also the existence of unreacted alkoxy conducted in a non-polar solvent (anhydrous benzene, groups in the solid precipitate from the sulfidization reaction.described above), a polar protic solvent (absolute ethyl The replacement of some alkoxy groups by SH is of course alcohol ) and a polar aprotic solvent (anhydrous acetonitrile). again confirmed by the presence of the TiMS vibration at The yield of each of the reactions was measured along with 300 cm-1 (see inset in Fig. 3).10 This replacement has been the S/Ti molar ratios in the precipitates obtained from the verified in a manner similar to reaction A, namely by the reactions (see Table 4).identification of isopropanol as a by-product of the reaction It is well known that the HS- ion is a stronger nucleophile using a combination of quantitative GC and chemical analysis than H2S. A simple calculation of partial charges on the (see Table 3).electronegative species shows that S in H2S has a partial The observations discussed above imply that: (a) the acid charge dS=-0.13, while in HS-, dS=-0.6. Therefore, a reacts to completion with the alkoxide to modify one tenth of larger concentration of HS- ions in solution should strongly the titanium alkoxide molecules as shown in reaction (5) for aVect the thiolysis reaction.The dissociation of H2S to H+ n=0.1 and (b) the sulfonyl group is not replaced by H2S and HS- is higher in solvents with a higher relative permit- during the sulfidization reaction, but the molecules of alkoxide tivity. In benzene, molecular H2S acts as the nucleophile in its that are modified do participate in the thiolysis reactions (no reaction with the alkoxide.sulfonyl group vibrations could be detected in the IR spectrum It is likely that during thiolysis in benzene, intermediate of the liquid obtained from the reaction) as do those that are oligomers form, which are resistant to nucleophilic attack not modified: because of steric hindrance hence lowering the overall yield. H2S+Ti(OPri)3(C6H5SO3) A H2SATi(OPri)3(C6H5SO3) A Electron impact mass spectrometry of the liquid product from the first reaction indeed showed that the mass fragmentation (HS) (OPri)2(C6H5SO3)TiBOH A Ti(C6H5SO3)(OPri)2SH pattern of the liquid fraction from the reaction was similar to that of the plain alkoxide.In acetonitrile, the dissociation of | +PriOH H2S causes the formation of the stronger nucleophile, HS-, Pri (6) which can react even with the intermediate species leading to and condensation.An interesting point to note is that although ethyl alcohol has a relatively large relative permittivity, com- Pri parable to that of acetonitrile, (implying the formation of | more HS- species), the reaction of titanium isopropoxide does H2S+Ti(OPri)4 A H2SATi(OPri)4 A (HS) (OPri)3TiBOH not proceed to any significant extent in this solvent.This is A Ti(OPri)3SH+PriOH (7) because in protic solvents, the anions are solvated by hydrogen bonding from the solvent.31 Therefore in alcohols, the HS- After thiolysis, the precipitates form due to the condensationions are solvated by alcohol molecules, which prevent them polymerization of the alkoxythiols by the liberation of H2S in the same way as shown in reaction (4). The reaction mechanism can be elucidated further by (a) Table 4 Molar yield of titanium from reactions A and B in benzene, ethyl alcohol and acetonitrile performing quantitative GC analysis of the benzene–isopropanol azeotrope obtained upon distillation of the filtrate from Modified the reaction and (b) a correlation of this analysis to the results Plain isopropoxide isopropoxidea of chemical analysis in a manner similar to that described in (reaction A) (reaction B) the previous section.These results are displayed in Table 3. Relative Solvent permittivity Yield (%) S/Tib Yield (%) S/Tib The isopropanol observed in the distillate is lower than that calculated from chemical analysis (assuming complete conden- Benzene 2.3 ca. 15 1.4 ca. 15 1.3 sation, i.e. the condensation of all the TiMSH groups to form Ethyl alcohol 32.7 — — — — TiMSMTi bonds). This implies that part of the sulfur (approxi- Acetonitrile 37.5 ca. 90 1.3 ca. 90 1.5 mately one third of the number of moles of S) in the solid aModified in a ratio of 1051, isopropoxide to benzenesulfonic acid. and the liquid are still not condensed completely leading to bMolar ratio of sulfur to titanium in the precipitated powder.the presence of some TiMSH linkages in the solid, the conden- J. Mater. Chem., 1998, 8(11), 2441–2451 2445Table 6 Chemical analysis of precursors obtained from reactions C from attacking the Ti center of the alkoxide (again by steric and D hindrance). On the other hand, in aprotic solvents like acetonitrile, the cation (H+) is strongly solvated by solvent mol- Reaction Ti S C H Si O (diVerence) ecules because the exposed N atom in acetonitrile carries a significant negative partial charge.31 The positive end of the Reaction C 34.9 28 15.5 3.3 — 18.3 (DMDS), wt.% dipole is buried in the alkyl group which makes it ineVective Reaction D 32 22.5 20 3.3 4.3 17.9 in solvating the negative ions (HS-).This leaves the anions (HMDST), wt.% free to participate in the reaction. Therefore, the best solvent system for the thio-sol–gel process involving H2S as a sulfidizing agent is an aprotic polar solvent. reaction of H2S with alkoxides. Alkoxides with the smallest alkoxy group on the metal center and having the least molecu- 3.4 Influence of alkoxy group lar complexity exhibit the lowest steric hindrance to nucleophilic attack. Such an alkoxide would be most likely to react A total of four diVerent alkoxides of titanium were reacted with H2S and undergo thiolysis and condensation reactions to with H2S in benzene and in acetonitrile: ethoxide, isopropoxform solid precursors useful for the synthesis of sulfides.ide, n-butoxide and 2-ethylhexoxide. The yields of the reactions Similarly, the most appropriate solvent system is a polar in benzene and in acetonitrile are shown in Table 5, along with aprotic solvent having a large relative permittivity (for their molecular complexities.The molecular complexity of example, acetonitrile) which is an eVective medium for the titanium ethoxide is high because of the small size of the dissociation of H2S to form HS- species, which are stronger ethoxy group. The larger alkoxy groups show a lower molecunucleophiles in comparison to H2S.lar complexity, because complexity itself is reduced by steric hindrance. If we consider the reactions conducted in benzene, 3.5 Reactions of titanium isopropoxide with DMDS and the isopropoxide is the only alkoxide of titanium which shows HMDST any reaction with H2S.The ethoxide shows no reaction because of steric hindrance caused by a higher molecular complexity The precipitates that resulted from reactions C and D were and the larger alkoxides (n-butoxide and ethylhexoxide) show very air sensitive and extra precautions were taken while no reaction because of hindrance from the bulky alkyl groups, handling these powders.Table 6 shows the results of chemical although the larger alkoxides tend to exist as monomers in analysis performed on these precursors. It can be observed benzene. In the case of titanium, the isopropoxy group oVers that the sulfur contents in these precipitates are considerably the best combination of size, alkyl group and molecular lower, indicating a lower extent of replacement of the alkoxy complexity.Hence, the sulfidization reaction is seen to occur groups. In addition, the reaction of the alkoxide with HMDST to considerable extents using Ti isopropoxide in both polar results in a powder contaminated with Si. aprotic and in non-polar solvents. The as-precipitated powders were amorphous, while the IR The ethoxide, n-butoxide and ethylhexoxide do not dissolve spectra of the powders showed the same characteristic in acetonitrile. However, when H2S is bubbled through the vibrations corresponding to the presence of isopropoxy groups suspension, the ethoxide and the n-butoxide do react. The (in the region of 1800 to 900 cm-1) as seen in the powders ethoxide forms a thick viscous liquid precursor, which forms prepared using the first two reactions.TiMO and TiMS a glassy solid when dried under vacuum at 60 °C. The solid vibrations were also seen at 450 and 300 cm-1 respectively. In shows a S to Ti molar ratio of 1.5, which implies a significant addition to these absorptions, the powder prepared using extent of the reaction with H2S (similar to reaction A in HMDST as the sulfidizing agent also showed nSiMO and benzene or ACN using titanium isopropoxide).It was also dSi(CH3)3 vibrations at 933 and 757 cm-1 respectively.33 observed that the liquid precursor could be easily coated on In reaction C (using DMDS as a sulfidizing agent) there is glass (by dip-coating or spin-coating) to form films which a possibility of heterolytic cleavage of the SMS bond due to appeared to be quite stable in air.These preliminary obser- the presence of H2S, causing the formation of RS- species vations indicate a potential application of this process for the which attack the alkoxide.34 In reaction D the following formation of TiS2 thin films. The reaction of H2S with titanium sequence could operate: n-butoxide also proceeds to a significant extent, the precipitates OTiOPri+R3Si-S-SiR3A OTi-S-SiR3+R3Si-OPri (8) showing a molar ratio of S to Ti of 1.5 (comparable to reaction A using titanium isopropoxide).The yield from this reaction OTi-S-SiR3+OTi-OPriAOTi-S-TiO+R3Si-OPri (9) is also high, although not as high as what is observed in the (R=methyl group) case of the reaction of H2S with titanium isopropoxide in the same solvent. Titanium 2-ethylhexoxide does not react At present, the proposed reactions paths are hypothetical and with H2S either in benzene or in acetonitrile because of steric have been proposed based on the properties of the sulfidizing hindrance from the bulky ethylhexoxy group.agents and the IR results of the solid precipitates obtained In summary, steric hindrance plays a significant role in the from the reactions.More detailed studies to elucidate the exact reaction mechanisms were not conducted owing to the benefits exhibited by the use of H2S in reactions A and B. Table 5 Reactions of various titanium alkoxides with H2S in benzene and in acetonitrile 3.6 Conversion of precursors to TiS2 Reaction in Reaction in Reaction A. The precipitates obtained from reactions A–D benzene acetonitrile Molecular were amorphous to X-rays.The X-ray traces obtained on the Alkoxide complexitya Yield (%) S/Tib Yield (%) S/Tib precipitate obtained from reaction A after conducting diVerent heat treatments are shown in Fig. 4 indicating the formation Ethoxide 2.5 — — —c 1.5c of crystalline TiS2 with increasing temperature. At 600 °C, Isopropoxide 1.3 15 1.4 90 1.3 crystalline TiS2 was observed along with the presence of rutile n-Butoxide 1.4 — — 80 1.5 and anatase phases of TiO2.The formation of TiO2 could be 2-Ethylhexoxide 1 — — — — explained by the favored high temperature condensation reac- aReferences 28,32. bMolar ratio of sulfur to titanium in the solid. cA tion of the unreacted isopropoxy groups: thick viscous liquid was obtained from the reaction.A solid was obtained by heating the liquid under vacuum at 60 °C. Ti-OPri+PriO-Ti A Ti-O-Ti+hydrocarbons (10) 2446 J. Mater. Chem., 1998, 8(11), 2441–2451Fig. 4 The X-ray diVractograms of the powders prepared from reaction A, heat treated at 600, 700 and 800 °C for 6 h respectively. The TiS2 peaks appear at 600 °C along with the rutile and anatase peaks of TiO2 marked by ‘O’.The oxide peak (rutile) is of low intensity at 700 °C and is eliminated at 800 °C. This process could have occurred at the drying stage as well as during the initial stages of heat treatment. At 700 °C after 6 h in H2S, only a small amount of oxide is present as Fig. 5 SEM micrographs of the powders; (a) as-prepared powders evidenced by the relatively low intensity peak visible at 2h= from reaction A, and heat treated to (b) 600 °C, (c) 700 °C and 27.5, implying the reduction of the TiMO bonds by H2S in (d) 800 °C for 6 h each in flowing H2S.The micrographs (b)–(d) the vicinity of this temperature. We have observed that con- clearly indicate changes in morphology occurring with the crystalliztinued heat treatment at 700 °C for about 10 h can eliminate ation of TiS2.the oxide phase. At 800 °C, the oxide is completely eliminated, yielding single phase TiS2. Based on these results, a mechanism 15 m2 g-1 makes it more sensitive to handling in air, hence for the formation of the sulfide could be postulated as follows. several precautions were taken to minimize its exposure to air. (1) The sulfur containing thiol groups attached to titanium in At 600 °C, as shown in Fig. 5(b), small platelets of TiS2 are the amorphous precursor transform in the presence of H2S to seen to form, which are separated from the spherical particles. form crystalline TiS2 at 600 °C, while the unreacted alkoxy At 700 and 800 °C however, there are very few spherical groups attached to titanium undergo condensation reactions particles and some sintering has occurred between the platelets to form the oxide.(2) The oxide then reacts with H2S at as indicated in the micrographs in Fig. 5(c), (d). The size of 700 °C to form the sulfide. (3) It is also possible that some of the platelets is not uniform and they range in width from 0.5 the sulfur remains bonded to Ti as an oxysulfide. The presence to 1 mm.Surface area measurements of the powder heat treated of such oxysulfides of titanium has been reported.35 Thus, at 700 °C have indicated a more than threefold increase to a although a portion of the unreacted alkoxide transforms to value of about 52 m2 g-1. the sulfide via the gas–solid reaction, the formation of single phase TiS2 at 800 °C in 6 h is an indication of the enhanced Reaction B.Conversion of the precipitates obtained from kinetics of the sulfidization reaction due to the incorporation reaction B to TiS2 was again accomplished by heat treatment of sulfur in the initial stages leading to the formation of the in flowing H2S. The X-ray diVraction patterns of the powders alkoxysulfide precursor. The formation of titania led us to heat treated to various temperatures are shown in Fig. 6. At conduct some control experiments employing fine particles of 600 °C the peaks are broad (in comparison to the correspond- TiO2 obtained by hydrolyzing titanium isopropoxide in a ing pattern shown in Fig. 4 for the products of reaction A) solution of acetonitrile via a sol–gel reaction. The powders indicating the presence of fine crystallites.Peaks characteristic were heat treated in H2S employing the same conditions of rutile and anatase also appear at this temperature, however (800 °C for 6 h) and the results showed TiO2 to be the major heat treatments at higher temperatures of 700 and 800 °C yield phase. Similar observations have been reported on the formasingle phase hexagonal TiS2. tion of cubic La2S312–15 from the sulfidization of alkoxides.The SEM micrographs of the precipitates and heat treated The partially sulfidized lanthanum oxysulfide precursors transform to the crystalline sulfide (La2S3) at reduced temperatures and in much shorter reaction times than those reported for the conventional routes involving high temperature sulfidization of sol–gel derived oxide gels in H2S. The alkoxide approach therefore oVers a novel route to the synthesis of sulfide ceramics of reactive metals.The evolution of morphology of the powders appears to follow an interesting path as displayed in Fig. 5(a)–(d). The as-precipitated powder is spherical and monodispersed with a particle size of about 0.5 mm. The monodispersed particle size distribution points to the fact that the rate of thiolysis is much slower than the rate of condensation.36 This helps maintain the concentration of the condensing species [shown in reaction (3)] below the critical concentration required for homogenous nucleation.Therefore any new species formed merely condense on the particles already formed during the initial burst of Fig. 6 Powders derived from reaction B heat treated at 600, 700 and nucleation, hence causing growth.In addition to its molecular 800 °C for 6 h each in flowing H2S. Peaks marked ‘O’ are due to TiO2 (anatase and rutile). structure, the high surface area of the precursor of about J. Mater. Chem., 1998, 8(11), 2441–2451 2447Fig. 7 Morphology of the powders obtained from reaction B (modified alkoxide+H2S), (a) precipitated powder and the powder heat treated at (b) 600 °C, (c) 700 °C and (d) 800 °C for 6 h each in flowing H2S.powders are shown in Fig. 7(a)–(d). The as-precipitated powder consists of agglomerated spherical particles about 2 mm in diameter which are about four times as large as the Fig. 8 (a) X-Ray diVractograms of the powders obtained from reaction particles obtained from reaction A. On heat treatment at C (alkoxide+H2S+DMDS) heat treated at 600, 700 and 800 °C for 600 °C, it can be seen that the TiS2 crystallites begin to form 6 h each in flowing H2S.TiS2 is formed at 600 °C with relatively small from individual spherical particles with their prismatic planes amounts of oxide (peak marked ‘O’), (b) X-ray diVractograms of the directed radially outwards. In this transformation, the spherical powders obtained from reaction D (alkoxide+HMDST) heat treated shape of the original particles, however, is retained.At 700 °C, in flowing H2S under the same conditions. the platelets grow further along both basal and prismatic planes while maintaining the overall shape of the agglomerates. At 800 °C, the platelets seem to have grown in both the prismatic and basal plane directions. The morphologies that have formed in this case are distinctly diVerent from those observed in reaction A due to the pseudomorphous transformation of the spherical precipitates to form TiS2.There is therefore a strong influence of modification of the alkoxide on the evolved morphology of the TiS2 platelets. Reactions C and D. X-Ray diVractograms of the precipitates subjected to heat treatments in flowing H2S are shown in Fig. 8(a), (b). In the case of reaction C (Ti isopropoxide+DMDS+H2S), the conversion of the alkoxysulfide precipitate to TiS2 is close to completion upon heat treatment at 600 °C for 6 h (note the low relative intensity of the peak characteristic of titania). The broad TiS2 peaks grow and sharpen at higher temperatures while the oxide peaks are completely eliminated.On the other hand, the powder obtained from reaction D (Ti isopropoxide and HMDST) shows sharp TiS2 peaks at 600 °C superimposed on an amorphous back- Fig. 9 SEM micrographs of the powders from reaction C; (a) as ground in addition to the oxide (rutile and anatase) peaks of precipitated, showing monodispersed spherical particles, (b) heat TiO2 [see Fig. 8(b)]. At 700 and 800 °C, however, single phase treated at 600 °C, showing the formation of TiS2 platelets, (c) heat TiS2 is formed. treated at 700 °C and (d) at 800 °C. All heat treatments done for 6 h. The SEM micrographs of the precipitated precursor and the heat treated powders (obtained from reaction C) are displayed in Fig. 9(a)–(d). The precursor powder shows spherical mono- particles are spherical, but are also polydispersed.They range in size from 0.5 to about 2 mm. At 600 °C, TiS2 platelets dispersed particles of about 0.5 mm in diameter, similar to the powders obtained from reaction A [compare with Fig. 5(a)]. measuring 1 to 2 mm along the basal plane can be seen along with spherical particles which exhibit rough surface features.At 600 °C, however, there are no spherical particles remaining, since most of them have transformed (probably along a path At 700 °C, however, the morphology is similar to Fig. 7(c), where platelets (about 0.5 to 1 mm) have grown from the similar to that observed in the case of reaction B) to form agglomerates of fine platelets about 0.3 to 0.5 mm along the spherical precursor particles.On further heat treatment at 800 °C the morphology of the powders show a controlled basal plane. At higher temperatures (700 and 800 °C) there is random growth of the platelets which have basal plane dimen- growth of the platelets in both the basal and prismatic plane directions similar to the characteristic morphology exhibited sions ranging from a fraction of a micrometer to several micrometers.The as-precipitated precursor powders from reac- by the powders obtained from reaction B using BSA [see Fig. 7(d)]. tion D [shown in Fig. 10(a)], however, are quite diVerent; the 2448 J. Mater. Chem., 1998, 8(11), 2441–2451Fig. 11 IR spectra of precipitates obtained by the reaction of (a) Nb(OEt)5 with H2S in acetonitrile and (b) Nb(OEt)5 modified with BSA reacted with H2S in acetonitrile.ref. 29): MMO stretching vibration at 585 cm-1; terminal Fig. 10 SEM micrographs of the powders obtained from reaction D; (a) as-precipitated and heat treated for 6 h at (b) 600 °C, (c) 700 °C CMO vibrations at 925, 1264 and 1100 cm-1; vibrations and (d) 800 °C. characteristic of the CH3 group at 1376, 1440, 2872 and 2962 cm-1; and vibrations characteristic of the CH2 group at 1464, 2853 and 2926 cm-1.In addition, the precipitate The SEM analysis of the precursors and the transformed obtained from the modified alkoxide [Fig. 11(b)] shows crystalline powders obtained from reactions A–D reflect the additional absorptions at 1000 (sharp), 1020 and 1040 consequences of the diVerent reaction mechanisms in solution (shoulders) cm-1 which are characteristic of the sulfonyl group instrumental for the formation of the precursor itself.Thus present in BSA.29,30 From these observations, it is clear that the four diVerent reactions exhibit diVerences in chemical the precipitates are formed by a partial thiolysis reaction of composition and structure, in addition to the various particle Nb(OEt)5 with H2S and condensation of the thiol groups, a sizes and their distributions (due to the competing rates of the mechanism similar to what has been determined in the case of formation of condensable species and their subsequent condentitanium isopropoxide.In this case the extent of the conden- sation reactions). These structural and compositional varisation reaction has not been determined, but complete conden- ations could provide diVerent reaction pathways for the sation is not expected due the five-fold coordination of Nb formation of the crystalline sulfide during the H2S treatments.which could oVer steric hindrance to condensation. The contrast is clearly seen in the intermediate morphology of When the precipitates from both the reactions were heat the powders for example, obtained from reactions A and B at treated in flowing H2S at 700 and 800 °C for 6 h, they showed 600 °C and at higher temperatures.The composition and the formation of single phase NbS2 (Fig. 12). At 700 °C, both structure allow diVerent responses to the sulfur potentially powders show the presence of small crystallites of NbS2. leading to variations in the morphology of the product, which SEM micrographs of the precipitates and the heat treated is clearly unique to the thio-sol–gel process.The heat treatment powders are shown in Fig. 13 and 14. In both cases, the conditions can be used to control the defect structure and precipitates consist of spherical particles ranging in size from morphology of the TiS2 powders, which in turn strongly 0.75 to 1 mm in diameter. At 700 °C, the powders derived from influence the electrochemical properties of the material.These the modified alkoxide [Fig. 14(b)] clearly show a pseudo- aspects are further discussed in the second part of this morphous transformation, causing the formation of NbS2 two-part series. platelets growing from the spherical particles. In the plain alkoxide derived powder, the platelets are not clearly dis- 3.7 Synthesis of niobium disulfide tinguishable [Fig. 13(b)]. The particles have retained their Nb(OEt)5, plain (or unmodified) as well as modified with spherical shape with increased roughness on the surface of the BSA (in a molar ratio of 1510, acid to alkoxide) were particles. At 800 °C [Fig. 13(c) and 14(c)], however, both separately dissolved in acetonitrile and reacted with H2S at powders show the presence of platelets ranging in size from a room temperature, causing the formation of a black precipi- fraction of a micrometer to several micrometers.This is also tate. Chemical analysis of the precipitate (for Nb and S) is approximately reflected in the extensive broadening at the base shown in Table 7.Infrared spectra collected on these precipi- of the diVraction peaks in the XRD patterns obtained for the tates in the range of 4000 to 200 cm-1 are shown in Fig. 11. powders heat treated at 800 °C. Both spectra show the formation of NbKS bonds as evidenced by the characteristic absorption at 357 cm-1.10 These bonds 4 Summary and conclusions could have formed through a thiolysis–condensation mechanism similar to what was observed in the case of titanium.The reaction of Ti(OPri)4 with H2S results in an alkoxysulfide There are also unreplaced alkoxy groups in the precipitates as precipitate through a thiolysis–condensation mechanism simievidenced by the following absorptions (ref. 23, pp. 114–122; lar to the hydrolysis–condensation mechanism seen in the oxide-sol–gel process.The unique features of this reaction {Ti(OPri)4+H2S} are that the condensation reaction is rapid Table 7 Chemical analysis of precursors obtained by the reaction of niobium ethoxide with H2S in acetonitrile and all thiol groups that are bonded to titanium during the thiolysis of the isopropoxide condense to form TiMSMTi Reaction Nb(wt.%) S(wt.%) S/Nba linkages in the solid.Modification of the alkoxide by benzenesulfonic acid, the Plain alkoxide+H2S in acetonitrile 43.6 29.6 2.0 solvent system and the alkoxy group bonded to titanium all Modified alkoxide+H2S in acetonitrile 44.2 24.7 1.6 significantly influence the reaction of the alkoxide with H2S. aMolar ratio of S to Nb. In the case of Ti(OPri)4 modified with benzenesulfonic acid, J.Mater. Chem., 1998, 8(11), 2441–2451 2449Fig. 14 SEM micrographs of (a) precipitates obtained by the reaction Fig. 12 X-Ray diVraction patterns of precipitates obtained by (a) the of modified niobium ethoxide with H2S in acetonitrile, (b) the reaction of niobium ethoxide with H2S in acetonitrile and (b) the precipitates heat treated at 700 °C for 6 h in flowing H2S and (c) the reaction of niobium ethoxide modified with benzenesulfonic acid (in precipitates heat treated at 800 °C for 6 h in flowing H2S.a molar ratio of n=1510, acid to alkoxide), heat treated in flowing H2S at 700 and 800 °C for 6 h respectively. chemical and spectroscopic analysis of the solid precipitate and of the liquid obtained from the reaction has revealed that the condensation reaction does not go to completion, possibly due to steric hindrance by the bulky benzenesulfonyl groups.The solvent used for the reaction has a significant influence on the yield of the reaction. A comparison of the reaction of titanium isopropoxide and H2S in diVerent solvents has shown that acetonitrile is the best solvent system for the reaction because it facilitates the decomposition of H2S to form highly nucleophilic SH- species in solution.A comparison of the reaction of various alkoxides with H2S has shown that steric hindrance plays a significant role in the reaction. Consequently, alkoxides with smaller alkoxy groups having lower molecular complexities in solution are more likely to undergo any significant thiolysis and condensation reactions with H2S.Preliminary investigations have shown that the viscous liquid that forms from the reaction of titanium ethoxide with H2S can be spin coated to form thin films. Titanium isopropoxide has also been reacted with dimethyldisulfide and hexamethyldisilthiane. Both reactions also lead to the formation of an alkoxysulfide precipitate although with a lower extent of sulfidization in comparison to H2S.All the reactions yield spherical alkoxysulfide precipitates that exhibit significant variations in particle size indicating diVerences in the rates of thiolysis and condensation reactions. Heat-treatment in flowing H2S results in the formation of crystalline TiS2 powder at diVerent temperatures exhibiting striking variations in crystallite morphology.In particular, distinct diVerences can be observed between TiS2 formed using plain Ti(OPri)4 and that formed using Ti(OPri)4 modified with benzenesulfonic acid. In the former, a wide platelet size distribution with random orientations have been observed. In the latter, a pseudomorphous transformation from the spheri- Fig. 13 SEM micrographs of (a) precipitates obtained by the reaction cal precursor particles has been observed, leading to a platelet of niobium ethoxide with H2S in acetonitrile, (b) the precipitate heat morphology which retains the overall spherical shape of the treated at 700 °C for 6 h in flowing H2S and (c) the precipitate heat treated at 800 °C for 6 h in flowing H2S.precursor particles. 2450 J. Mater. Chem., 1998, 8(11), 2441–245115 P.N.Kumta, V. P. Dravid and S.H. Risbud, Philos. Mag. B, 1993, Finally, the thio-sol–gel process has been used to synthesize 68, 67. NbS2 in order to demonstrate the applicability of this process 16 Y. Han and M. Acink, J. Am. Ceram. Soc., 1991, 74, 2815. for the synthesis of other transition metal sulfides. In this 17 M. A. Sriram and P. N. Kumta, Mater. Res. Soc. Symp. Proc., system too, alkoxysulfide precipitates form, which can be 1993, 327, 15–22.converted to NbS2 upon heat treatment in H2S. NbS2 synthe- 18 M. A. Sriram and P. N. Kumta, J. Am. Ceram. Soc., 1994, 77, 1381. sized by this method also exhibits variations in morphology 19 M. A. Sriram, K. S. Weil and P. N. Kumta, Appl. Organomet. similar to that observed in TiS2. Chem., 1997, 11, 163. 20 M. A. Sriram and P. N. Kumta, Ceram. Trans., 1996, 65, 163. 21 J. Y. Kim, M. A. Sriram, P. H. McMichael, P. N. Kumta, This work has been supported by the US National Science B. L. Phillips and S. H. Risbud, J. Phys. Chem. B, 1997, 101, 4689. Foundation Grant DMR 9301014 and a Research Initiation 22 V. Stanic, A. C. Pierre, T. H. Etsell and R. J. Mikula, J. Mater. Award (RIA) from the US National Science Foundation Res., 1996, 11, 363. Grant CTS 9309073, and CTS 9700343. The authors would 23 D. C. Bradley, R. C. Mehrotra and D. P. Gaur, Metal Alkoxides, Academic Press, London, 1978. also like to acknowledge the technical support of Dr. George 24 T. A. Guiton, C. L. Checkaj and C. G. Pantano, J. Non-Cryst. E. Blomgren of Eveready Battery Co. Solids, 1990, 121, 7. 25 J. V. Bell, J. Heisler, H. Tannenbaum and J. Goldenson, Anal. Chem., 1953, 25, 1720. References 26 C. T. Lynch, K. S. Mazdiyasni, J. S. Smith and W. J. Crawford, Anal. Chem., 1964, 36, 2332. 1 S. D. Jones, J. R. Akridge and F. K. Shokoohi, Solid State Ionics, 27 J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 1994, 69, 357. 1988, 18, 259. 2 M. S. Whittingham and J. A. Panella, Mater. Res. Bull., 1981, 28 D. C. Bradley, R. C. Mehrotra and W. Wardlaw, J. Chem. Soc., 16, 37. 1952, 5020. 3 R. P. Clement, W. B. Davies, K. A. Ford, M. L. H. Green and 29 L. J. Bellamy, The Infra-red Spectra of Complex Molecules, John A. J. Jacobson, Inorg. Chem., 1978, 17, 2754. Wiley and Sons Inc., New York, 1958, pp. 364–367. 4 V. W. Biltz and P. Ehrlich, Z. Anorg. Allg. Chem., 1937, 234, 97. 30 R. N. Haszeldine and J. M. Kidd, J. Chem. Soc., 1954, 4228. 5 K. Kanehori, K. Miyauchi and T. Kudo, Hitachi Ltd., Tokyo, 31 T. H. Lowry and K. S. Richardson, Mechanism and Theory in Japan, US Patent 4572873, Feb. 25, 1986. Organic Chemistry, Harper Collins Publishers, New York, 1987, 6 K. Kanehori, F. Kirino, Y. Ito, K. Miyauchi and T. Kudo, pp. 182–183. J. Electrochem. Soc., 1989, 136, 1265. 32 D. C. Bradley, R. C. Mehrotra, J. D. Swanwick and W.Wardlaw, 7 K. Kanehori, Y. Ito, F. Kirino, K. Miyauchi and T. Kudo, Solid J. Chem. Soc., 1953, 2025. State Ionics, 1986, 18–19, 818. 33 L. S. Jenkins and G. R. Willey, J. Chem. Soc., Dalton Trans., 1979, 1697. 8 S. Kikkawa and M. Miyazaki, J. Mater. Res., 1990, 5, 2894. 34 W. A. Pryor, Mechanisms of Sulfur Reactions,McGraw-Hill Book 9 A. A. van Zomeren, J.-H. Koegler, J. Schoonman and P. J. v.d. Co., New York, 1962, p. 59. Put, Solid State Ionics, 1992, 53–56, 333. 35 G. Meunier and R. Bormoy, in Microionics, Solid State Integrable 10 R. R. Chianelli and M. B. Dines, Inorg. Chem., 1978, 17, 2758. Batteries, ed. N. Balkanski, North Holland, New York, 1991, 11 A. Bensalem and D. M. Schleich, Mater. Res. Bull., 1988, 23, 857. pp. 73–95. 12 P. N. Kumta and S. H. Risbud, Mater. Sci. Eng., B, 1989, 2, 281. 36 E. A. Barringer and H. K. Bowen, Langmuir, 1985, 1, 414. 13 P. N. Kumta and S. H. Risbud, Mater. Sci. Eng., B, 1993, 18, 260. 14 P. N. Kumta and S. H. Risbud, Prog. Crystal Growth Charact., 1991, 22, 321. Paper 8/02564I J. Mater. Chem., 1998, 8(11), 2441–2451 2451

ISSN:0959-9428    

DOI:10.1039/a802564i

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Extended defect structures in zinc oxide doped with iron and indium Tom Ho� rlin,*a Gunnar Svenssonb and Eva Olssonc† aDepartment of Inorganic Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden. E-mail: tom@inorg.su.se bDepartment of Structural Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden cDepartment of Physics, Chalmers University of Technology and Gothenburg University, SE-41296 Gothenburg, Sweden Received 8th July 1998, Accepted 26th August 1998 The eVects of iron- and indium oxide doping on the structure and magnetic susceptibility of ZnO have been studied.The nominal compositions were InxFe2-xO3(ZnO)n with 0x1 and n=23, 48 and 98. Magnetic measurements showed the iron-doped samples to be paramagnetic, with a behaviour indicating antiferromagnetic coupling between the iron ions.HREM studies showed that indium and iron are incorporated as layer defects of two kinds. One type forms cubic close packed (ccp) planes perpendicular to the c axis, and the other appears as corrugated layers inserted between the former. The folds in the corrugated layers consist of alternating (114) and (1149) planes.Analytical transmission electron microscopy studies revealed that indium prefers the ccp layers, whereas iron can be found in both types of defects. Structural models based upon the experimental results are presented. The In2O3–ZnO system was first studied by Kasper,5 who Introduction found an homologous series with the general formula Zinc oxide is one of the most important semiconductors used In2ZnnO3+n.Cannard and Tilley6 later reinvestigated the in large quantities. Although ZnO devices are quite diVerent system using X-ray powder diVraction and transmission elecfrom components made of e.g. silicon and gallium arsenide, tron microscopy. They suggested an intergrowth structure there are similarities that can be utilised.Examples are the consisting of slabs of wurtzite-type ZnO separated by bixbyiteenhancement of conductivity by doping and the creation of type layers of In2O3. The structures suggested by Cannard non-linear junctions. The project described in this article was and Tilley have recently been confirmed by McCoy et al.7 initiated by a study of the possibility of using ZnO as electrode The phase compsitions in the In2O3–Fe2O3–ZnO system material in the smelting of aluminium.have been investigated by Nakamura et al.,9,15 Kimizuka It is easy to promote the electric conductivity of zinc oxide et al.8,10,11 and Siratori and Kimuzuka.13 They found a number by doping it with normally trivalent metals. Some of these of phases which can be described by the formula ions may enter as divalent, with one rather loosely bound InxFe2-xO3(ZnO)n were n varies between 1 and 13 and x as electron that may dissociate and participate in n-type conduc- 0.72x1.00 for n=1 and as 0x2.0 for n=13.They tion. Only n-conduction has been confirmed in zinc oxide; suggest that the compounds with n=1–6 and x=1 are isostrucp- conduction has not yet been observed. tural with their lutetium analogues LuFeO3(ZnO)n, which It is known1–3 that the addition of gallium or indium ions have been determined by X-ray single-crystal diVraction techenhances the electric conductivity of ZnO, which soon becomes niques.23 The structures consist of wurtzite-type ZnO slabs metallic.For gallium a maximum of the conductivity has been intergrown with single layers of indium atoms in octahedral observed around 0.4 atom% of the dopant.3 This result is in cavities formed by cubic close packed oxygen. The iron atoms agreement with what we have found for both gallium and form a dilute solution in the ZnO slabs.For structural reasons indium.4 Iron-doped ZnO stays semiconducting, on the other the ZnO tetrahedra have to be connected with their apices to hand, with a maximum iron content of 0.3–0.5 atom% at the ccp layers.The ZnO slabs have diVerent polarity on either 1000 °C. The reason for this is that Fe2+ has a higher ionisation side of the indium-containing layers. Consequently, an inverenergy than In2+. Simultaneous doping with 1 atom% indium sion of the polarity must occur within the ZnO slabs between and iron does not markedly increase the conductivity at low these layers, which can be achieved by a gradual displacement temperatures, since at this doping level there is enough trivalent of the metal atoms from one tetrahedron to the neighbouring iron to trap all loosely bound electrons according to the one via the shared triangular face.reaction:4 HREM studies on InFeO3(ZnO)n, however, revealed a more complex picture.19–22 At least for n6, wave-like defects were In2++Fe3+�In3++Fe2+ (1) found between the indium oxide layers.EDS spot analysis If doped ZnO is prepared in air, only a fraction of the trivalent showed an increased signal from iron in these defects. dopant ions will be reduced to divalent. Divalent dopants may In this article we will describe how low concentrations of substitute for zinc without structural consequences, but the iron and indium dopant, lower than those reported in the trivalent ions entering ZnO must be accompanied by structural literature, are structurally accommodated in ZnO.We will defects such as metal vacancies or other alien structure elements discuss what structure elements are formed and how are they in order to conserve charge neutrality.Sooner or later these related to the phases found at higher dopant concentrations, defects may start to order in the host structure, eventually and how this doping influences the magnetic properties of the leading to new phases. compounds. There have been numerous studies of binary and ternary systems with ZnO and one or two trivalent metal oxides.2–22 Experimental Powders of ZnO (Fotofax), Fe2O3 (Merck p.a.) and In2O3 †Present address: Analytical Materials Physics, The A° ngstro�m Laboratory, Uppsala University, Box 534, SE 75121 Uppsala, Sweden.(Aldrich 4N) were milled with SIALON balls in high-density J. Mater. Chem., 1998, 8(11), 2465–2473 2465polyethylene bottles. The powder mixtures were pressed to pellets, which were heated at 1400 °C for 3 days, embedded in loosely packed ZnO powder.After the heating period the furnace was switched oV and the door was opened. Earlier work9 indicates that very long heating periods (weeks) are required to obtain ordered phases. Our dopant (In+Fe) compositions—2, 4 and 8% of the total metal content—were more dilute, so we did not expect to reach complete threedimensional ordering. The formula can be written as InxFe2-xO3(ZnO)n where n=98, 48, 23, and x=0.0, 0.5, 1.0.The inner parts of the pellets were used for characterisation. Solid pieces were cut for the magnetic susceptibility measurements and for the electric measurements. Fine powders were ground for X-ray and HREM investigations. X-Ray powder diVraction patterns were recorded with a focusing camera of Guiner–Ha�gg type, using Cu-Ka1 radiation and with silicon added as internal standard (a=5.431 A° ).The Fig. 1 The XRD pattern for the n=23, x=1 sample. The additional films were evaluated with a scanner system.24 lines not indexed with a ZnO type unit cell are marked with arrows. For high-resolution electron microscopy studies (HREM) we used a JEOL JEM 200CX operated at 200 kV, capable of 2.4 A° point resolution, and a JEOL JEM 3010 operated at 300 kV, capable of 1.7 A° point resolution; and for the analytical TEM studies a field-emission gun Philips CM200 supertwin microscope operated at 200 kV, equipped with an Oxford LINK ISIS EDX system and a Gatan imaging filter for electron energy loss spectroscopy (EELS) and electron spectroscopy imaging (ESI).For high-resolution and analytical transmission electron microscopy studies, dispersions in nbutanol of finely crushed samples were put on perforated carbon films supported by a copper grid. A model 7000 AC susceptometer from Lake Shore, equipped with a ‘Cryodyne’ closed-cycle helium refrigerator from CTI Cryogenics, was used for the measurements of magnetic susceptibility.We have also aomputer-controlled diVerential transformer to the system, because the excellent balance of the pick-up coils at room temperature and moderate frequencies declines at low temperatures and high frequencies, and it is therefore necessary to have a device that can restore the balance when the temperature and frequency vary. The system is controlled by an in-house developed computer program.The samples were pieces around 0.7 g in weight. The magnetising field (H) ranged from 177 A m-1 up to 707 A m-1; the stronger fields were used for samples with less iron. The frequency was 2000 Hz. The temperature was varied from ca. 11.3 K up to 320 K. The susceptibilities of the samples were found to be independent of frequency and magnetising field.Fig. 2 (a) Unit cell volume and (b) c/a ratio versus 1/n and x, for InxFe2-xO3(ZnO)n, n=23, 48, 98, 2 and x=0, 0.5, 1.0. Results 1.0 samples, as shown in Fig. 2(b). In all samples c/a was less X-Ray powder diVraction than the ideal value for hcp packing: c/a=2(2/3)1/2#1.630. All the X-ray powder patterns showed strong lines from a HRTEM studies hexagonal unit cell closely similar to that of ZnO.The XRD pattern for the n=23 and x=1.0 sample is shown in Fig. 1. We first looked for crystal fragments oriented along a [100] (i) The XRD patterns of the n=98 and x=0.0 samples direction of ZnO, since the two shortest projected distances revealed no additional lines, and only one faint extra line was between zinc atoms (2.82 and 2.77 A° ) in the corresponding seen for the x=0.5 and 1.0 samples.(2910)-projection are well within the resolution of our HRTEM. (ii) The XRD patterns of all the n=48 samples contained [The diVerence between these distances is due to a slight the same extra line mentioned above, and for the x=0.5 and distortion of the tetrahedra (c/a<1.630).] We then looked 1.0 samples of n=48, five faint additional extra lines were parallel to possible planar defects perpendicular to the c-axis, observed.such as those reported by Uchida et al.21 Fig. 3 shows the (iii) The X-ray powder pattern of the n=23, x=0.0 sample HREM image and the corresponding electron diVraction showed three weak extra lines, and those of the x=0.5 and pattern taken along [100] of a crystal found in a sample of 1.0 samples seven and five weak extra lines, respectively.the nominal composition InFeO3(ZnO)23. A magnification of The unit cell volume for the ZnO phase in the n=98 samples the edge is shown in Fig. 3(c). In the images we see two types was the same as for undoped ZnO, while it increased for n= of irregularities in the ZnO sructure, similar to those reported 48 and 23 samples as shown in Fig. 2(a).The increase was by Uchida et al.21 One type is a sharp uninterrupted defect, largest for samples containing both indium and iron. The c/a unwrinkled and perpendicular to the c-axis. Most probably these defects are extended in depth, forming layers parallel to ratio increased with decreasing n value, but less so for the x= 2466 J. Mater. Chem., 1998, 8(11), 2465–2473Fig. 3 (a) An electron diVraction pattern and (b) corresponding lattice image taken along [100] of a crystal found in a sample with nominal composition InFeO3(ZnO)23. The width of the bouncing defect at the arrows corresponds to a crystal thickness of 60 A° , see text. (c) Magnification of a part of the edge in (b). The shift of 0.9 A° along [100] when crossing the (001) defect (marked with an arrow) is emphasised with a line.The shift when crossing the bouncing defects (marked with arrows) is emphasised with two lines. the (001) planes of the ZnO network. In the higher magnifi- cation one can see that the dark spots ascribed to zinc atoms at this defocus (ca. -400 A° ) are shifted 0.9 A° along [120] when crossing this defect. This distance corresponds to a third of the height of a tetrahedral face.The second type of defect was not always seen in the crystallites viewed along [100] or its equivalents. These defects have a more diVuse contrast, bouncing back and forth between the (001) defect layers forming zigzag patterns. In this projection (2910), they have the shape of ribbons with more or less pronounced rims, increasing in width with the distance from the edge of the crystallites.(They appear as inclined layers when viewed along the [100] axis.) A shift of the contrast along the c-axis is observed in Fig. 3 when crossing a defect of the second type, which might be interpreted as a small shift of the zinc atoms along the c-axis. Weak streaking along the c-axis is seen in the corresponding ED patterns in Fig. 3(a), caused by the variation of the distance between the defects parallel to the (001) planes of ZnO. It has to be mentioned that anomalies of the same type were found in samples doped with less indium: x=0 and 0.5. In order to determine the orientation of the defects in three dimensions, some crystallites were tilted 30° around the c-axis to the [1190] or the equivalent [210] directions.After this operation the bouncing defects were still seen in the HREM images in some cases, while in others they had disappeared. In both cases the first type of defect parallel to (001) remained clearly visible. An HREM image with bouncing defects visible in the new direction, and the corresponding electron diVraction pattern, are shown in Fig. 4. In accordance with reports on highly doped compounds in this system,19–21 we suggest that one half of the impurity atoms (indium or iron) are located in the plane defect layers.The number m of ‘ZnO’-planes between the defect planes would Fig. 4 (a) HREM image of a crystallite viewed along [1190] found in then be m=n+1 [n from the formula InxFe2-xO3(ZnO)n]. As a sample with nominal composition InFeO3(ZnO)48.(b) The correexpected due to the short firing period, this number m varies sponding electron diVraction pattern, and magnification of a diVraction spot exhibiting a star of intensity along [114] and [1149]. markedly in the crystallites. However, the average of m for J. Mater. Chem., 1998, 8(11), 2465–2473 2467Table 1 Average distance between defect layers in the HREM images electron beam will thus be either approximately 60 or 0°, and of crystallites studied in InxFe2-xO3(ZnO)n samples, m=n+1 the bouncing defects will be visible in one case out of three.Clearly this defect structure breaks the hexagonal wurtzite m x Observed ma symmetry. We can therefore introduce an orthorhombic cell with reduced symmetry, related to the hexagonal wurtzite cell 24 0.0 24 0.5 30 by: 1.0 24 49 0.0 55 1.0 51 base(orth)=A1 19 0 1 1 0 0 0 1Bbase(hex) (2) 99 1.0 90 aApproximate value.This is of course a subcell of the structures that can occur in this system. However, for simplicity of description we will the crystallites studied corresponds well with the amount of retain the hexagonal cell of ZnO. doping, as shown in Table 1.The planar defects seem to be The bounds of the ribbons formed by the bouncing defects, very sturdy and without faults. This indicates that ordering seen in the (2910) projection, are the intersections between the towards a smaller spread in m is very sluggish; otherwise we bouncing defect layers and the surfaces (top and bottom) of would see that some layers were ‘on the move’ by showing the crystal fragment.The width of the ribbons b is related to steps. A fast structural rearrangement is not to be expected, the thickness t of the crystal as b=(t sin a)/Ó3 (a=38.7°, the since it must involve interchange of atoms. angle between (114) and (001)), assuming the crystallites to When viewed along [1190] or equivalent axes in ZnO, the be perpendicular to the beam.It is thus possible to determine shortest distances between zinc atoms in the projection are the thickness of the crystals by measuring the width of the c/2=2.60 A° and a/2=1.625 A° . We can therefore only expect ribbons: for example the crystallite shown in Fig. 3(b) would them to be resolved in the c direction. In the crystallite shown then have a thickness t#60 A° at the position marked with the in Fig. 4(a), both types of extended defects are very clearly two arrows. distinguished, the bouncing defects being more narrow and Our conclusions on the orientation of the defect planes are distinct in this projection than when viewed along [100]. This shown in a drawing. In Fig. 5(a) we look straight down on gives the impression that the defect planes are now oriented the (114) and (1149) defects along [1190] in ZnO.The resem- parallel to the beam. The angle between the two types of blance to corrugated cardboard is striking. In Fig. 5(b) the defect planes is approximately 39°. The estimated value of this model is tilted 30° with respect to in ZnO. The model is wedge- angle does not vary much within diVerent parts of the crystal shaped from the edge (at the bottom) to the position marked or between diVerent crystals.If we define the direction in ZnO with arrows, and then has constant thickness. The ribbons where the bouncing defects are sharply visible to be [1190], then the bouncing defects will be layers alternately parallel to (114) and (1149). The angle between (114) or (1149) and (001) planes in ZnO should be arctan(c/2a)#arctan(1.60/2)#38.7°, which is well in accord with the observed value.In the ED-pattern taken along [1190] there is streaking along the c axis as in the [100] patterns. The (114) or (1149) boundary defects are also seen in Fig. 4(b) as a faint streaking in the shape of a weak star around the diVraction spots in the ED pattern. An enlargement of a diVraction spot is inserted in Fig. 4(b). The directions of the star points are close to the [114] and [1149] directions in reciprocal space, in agreement with the interpretation of the HREM images. The distinct 39° angle found between the planar defects and the bouncing defects when viewing along [1190] decreases a few degrees, to ca. 35°, when viewing along [100], with a larger spread in observations.The preferred cleavage surface for ZnO is reported to be (2910).25,26 It is therefore not unreasonable that the crystallites studied here are flakes extended in the equivalent (2910)-planes—perpendicular to the [100]-axis. The angle between the defect types projected on a (2910)-plane should be arctan[(c/2a)cos(30°)]#34.72°. The spread of observations is caused by the fact that the crystallites are wedge-shaped at the edges and increase in thickness with the distance from the edge, and they may not necessarily be exactly extended in the (2910)-plane.The structure of the bouncing defects together with the planar (001) defects very much resembles corrugated cardboard. The corrugated defect layers are not always seen, since in both viewing directions, [100] and [1190], there are three equivalent cases in the hexagonal setting of the sublattice.In the first view along [100] the planes of corrugated defects make a 30° angle with the incident beam, but they may also be perpendicular to the incident beam and thus not seen. In the other projection, [1190] and [210], the corrugated planes are either aligned with or 60° oV the beam.In the latter case Fig. 5 Reconstruction of the (114), (1149) and (001) defect planes. the angle may be too large to allow the observation of the (a) Viewed along [1190] and (b) along [100] in ZnO. The model is wedge-shaped from the edge to the position marked with arrows. defect layer. The angle between the corrugated layer and the 2468 J. Mater. Chem., 1998, 8(11), 2465–2473slabs remains to be answered, but it seems probable that some iron is incorporated there.The sample just described did not contain indium, so a crystallite found in a sample with nominal composition InFeO3(ZnO)48 was investigated to locate this element and the zinc, indium and iron elemental maps are shown in Fig. 8. This crystallite is viewed along an equivalent to the [1190], zone axis, in a direction where the bouncing defects are not viewed edge on, e.g.[210]. The indium map reveals a preference for the planar defects, whereas the iron and zinc signals indicate deficits in these defects. A uniform iron signal is found between the planar defects, as expected for a crystallite of finite thickness. The conclusions from this elemental mapping are (i) indium prefers the planar defect layer.Whether it is possible for indium to enter the corrugated Fig. 6 HREM image of a crystallite found in a sample with nominal layers is not clear. (ii) Iron may enter both types of defects. composition In0.5Fe1.5O3(ZnO)23, showing a well developed net of corrugated defects. Magnetic susceptibility The magnetic susceptibility measurements revealed all doped found in the HREM images are clearly seen in this orientation.samples to be paramagnetic. The diamagnetic contributions Their width increases with the thickness of the ‘crystal,’ exactly from zinc and oxygen have been removed in presented data. as observed in the HREM images. For this we used the measured susceptibility of ZnO, which An impact of a corrugated layer on one side of a plane was in excellent agreement with Pascal’s constants tabulated layer is frequently matched with an impact on the other side in ref. 26. For the various species we expect the mB2 to be 35 at almost the same place (exceptions occur). This means that for Fe3+, 24 for Fe2+ and 3 for electrons or In2+, assuming the corrugated layers are mirrored through the planar layers, spin-only contribution and high-spin configuration.(In oxides i.e. the waves of adjacent corrugated layers are 180° out of the ligand field is usually not strong enough to induce a lowphase. The traces of the corrugated layers form a diamond spin state.) One may expect divalent indium and free electrons net. This net is of course only well developed when there is to be trapped by iron according to a prior study:4 some order in the c direction.Fig. 6 (of In0.5Fe1.5O3(ZnO)23 In2++Fe3+�In3++Fe2+ e-+Fe3+�Fe2+ (3) [100]) illustrates a rather well developed net with one planar and two corrugated layers are joined at each node. This is not There are two kinds of trivalent iron ions and a small fraction always the case, as shown by the HREM image in Fig. 4(a), of divalent in the compounds studied.The squared eVective where both crossing and in-phase patterns are seen. There are Bohr magneton number per iron atom is shown in Fig. 9–11 also cases where corrugated defects turn before reaching the for n=98, 48 and 23 respectively. The curves marked A and planar defect. The general appearance of our images is slightly B are from the samples with x=0.0 and 1.0, respectively.The diVerent to that reported by Uchida et al. and by Bando and results of the analytical TEM show that iron is located in both coworkers.19–21 In the (2910) projection they observed sinus- types of defects when no indium is present (x=0.0). For x= oidal contrast waves, in phase with each other and thus not 1.0 the iron has a preference for the corrugated layers.A forming a net. Uchida et al. also report the period lengths of simple estimation of meff2 for iron in the planar defects can be the waves for two compositions [InFeO3(ZnO)6 and calculated from InFeO3(ZnO)13] in the (2910) and the (1190) projection. They meff2 (C)=2meff2 (x=0)-meff2 (x=1) (4) found the length shorter in the latter, which is consistent with our observations that the projected angle between the two which is shown in the curves marked C.At low temperatures types of defects is larger in the latter projection. The ratio the calculated value of the iron meff2 value in the planar defects between the lengths in the two projections is, however, not in becomes negative for two of the samples (n=23 and 48). This accordance with our findings, and is not the same for the two indicates of course that the model represented by eqn.(4) is compositions. rather crude, but it can be used for a qualitative discussion. The low meff2 values, and the appearance of the curves in Analytical TEM Fig. 9–11, clearly indicate an antiferromagnetic interaction between Fe3+ ions in both types of defect layers. This antiferro- Spot analysis using EDX was performed on a crystallite magnetic interaction is more pronounced in the planar defects.oriented along [100], found in the InFeO3(ZnO)48 sample. Wea hexagonal layer of edge-sharing FeO6 octahedra there are made point analyses of the planar defects, the bouncing defects six nearest neighbours. Only four of these yield favourable and the intervening triangular ZnO parts.The beam size used antiferromagnetic interactions in an ordered structure, and the was 10 A° . Although the low signal-to-noise ratio was unsatisother two contacts will be unfavourable. In a hexagonal lattice factory, the analysis gave a clear indication: indium prefers there is no preference for the orientation of these interactions, the planar defects while iron seems to be enriched in the but if the hexagonal symmetry is distorted this degeneracy is bouncing defects in this compound.The result is in agreement broken. The corrugated cardboard structure with orthorhom- with the report of Bando and coworkers.19 To obtain a better bic symmetry may at least to some extent stabilise and orient signal-to-noise ratio we used EELS to perform elemental an antiferromagnetic structure by giving a stronger interaction.mapping. Two samples were investigated: (i) Fe2 O3(ZnO)48 The sharp bend of the C-curves at low temperatures may be and (ii ) InFeO3(ZnO)48. The results confirmed the impression an indication of a transition to a state where the magnetic from the EDS analysis. A crystallite in the Fe2O3(ZnO)48 structure of the planar defect is lined up with the corrugated sample was oriented along [1190], of which an image using the layer structure.zero beam is shown in Fig. 7(a). Two images using electrons that have lost energy due the K-absorption edges of iron and zinc are shown in Fig. 7(b) and (c). The iron map clearly Discussion shows that iron is enriched in the defects, and a corresponding dark contrast due to depletion of zinc in the planar defects is The structure of ZnO, wurtzite, can be described as a hexagonal close packing (hcp) of oxygen atoms, containing two diVerent seen in the zinc map.To what extent there is iron in the ZnO J. Mater. Chem., 1998, 8(11), 2465–2473 2469Fig. 7 Elemental distribution map, using EELS, of a crystallite in the Fe2O3(ZnO)48 sample oriented along [1190] to give the best contrast of the defects.(a) An image using the zero beam. The resolution is rather low due to the limitations of the EELS system, but the defects are clearly resolved. (b), (c) Images using electrons that have lost energy due to the (b) Fe-K absorption edges and (c) Zn-K absorption edges. The light contrast in the iron map clearly shows that iron is enriched in the defects, while a corresponding dark contrast due to deficiency is seen in the zinc map. types of tetrahedral voids and one octahedral.The zinc atoms atoms. The insertion of the extra close-packed layer is such that a portion of the oxygen layers are cubic close packed, occupy only one type of the tetrahedral voids. The empty and the occupied tetrahedra share a basal plane, so as to form a and the voids with octahedral coordination are filled with trivalent ions.The formation of such a ccp slab shifts the zinc trigonal bipyramid (in contrast to ccp). All the filled tetrahedra point their apices in the same direction, resulting in a polar atoms in the ZnO layers on opposite sides through a/2Ó3 A° (ca. 0.9 A° ) relative to each other.The indium and/or iron structure. By shifting the zinc atoms through the equatorial plane of the bipyramids, the structure is mirrored and the atoms are situated in octahedral cavities in these ccp layers. This model is in agreement with results of single-crystal studies polarity is reversed. The (001)-plane defects in the doped structures are caused of LuFeO3(ZnO)n (n=1, 4, 5 and 6) reported by Isobe et al.23 The directions of the shift of the zinc atoms on opposite sides by insertion of an extra close-packed oxygen layer plus a reversal of the polarity of ZnO to one side.The result is that of the ccp layers will depend on the number of ‘ZnO’ planes, m=n+1, between the defect planes, as observed in the HREM the apices of the ZnO4 tetrahedra from each side of the (001) defects point towards each other without sharing oxygen images.In the ordered structures an even number results in a Fig. 8 Elemental distribution mapping, using EELS, of a crystallite oriented along [1190] to give the best contrast of the defects found in a sample with nominal composition InFeO3(ZnO)48. (a) Zero-beam image. Images using electrons that have lost energy due to the (b) In-K absorption edges and (c) Fe-K absorption edges and (d) the Zn-K absorption edges.The orientation of the crystallite is such that the bouncing defects are inclined to the beam. The image shows that indium prefers the planar defects, which are avoided by the zinc and iron atoms. 2470 J. Mater. Chem., 1998, 8(11), 2465–2473Fig. 9 The squared eVective Bohr magneton number per iron ion for Fig. 11 The squared eVective Bohr magneton number per iron ion for n=98. A, The value for the samples x=0.0 with iron in both types of n=23. A, The value for the samples x=0.0 with iron in both types of layer defects. B, The contribution from samples x=1.0 with iron layer defects. B, The contribution from samples x=1.0 with iron atoms only in the bouncing defect layers, since there is indium in the atoms only in the bouncing defect layers, since there is indium in the planar defects.C, The magnetic moment from iron in the planar planar defects. C, The magnetic moment from iron in the planar defects is calculated from eqn. (4) in the text. defects is calculated from eqn. (4) in the text. of these compounds revealed no preference of iron for the layer of trigonal prisms halfway between the ccp layers.(One would expect the zinc atoms to avoid the trigonal bipyramidal positions, as this co-ordination is very unusual for zinc, while there are some compounds known with iron in trigonal bipyramids, see e.g. ref. 28. The lutetium atoms were found in the ccp layers, as expected. As already mentioned, Uchida et al.21 observed the wave structures discussed above when investigating InFeO3(ZnO)n (n6), but they did not present any structural explanation for the phenomenon.If similar waves also occur in the structure of the lutetium compound with n=6, which was determined by single crystal diVraction, it could explain why no preference for the trigonal bipyramids was found for iron.The single-crystal studies give the average structure, and this partly disordered modulation therefore cannot be seen. As a consequence, there are not necessarily any perfect (001) layers Fig. 10 The squared eVective Bohr magneton number per iron ion for of trigonal bipyramids half-way between the (001) layers n=48. A, The value for the samples x=0.0 with iron in both types of when n>0.layer defects. B, The contribution from samples x=1.0 with iron In dilute structures, n>6, most of the material is ZnO which atoms only in the bouncing defect layers, since there is indium in the planar defects. C, The magnetic moment from iron in the planar naturally relaxes to the wurtzite structure. Therefore the defects is calculated from eqn. (4) in the text.reversal of polarity must be confined to rather narrow ranges, less than the width of the slabs between the planar defect layers. The corrugated layers may contain such a transition. hexagonal space group, P6/3mcm, while odd numbers yield the rhombohedral symmetry R3m, see Appendix. We have It is diYcult, however, to explain why such a domain boundary should be locked to certain crystallographic planes (114) or simulated HREM images of the ccp layers, using the coordinates from the ordered structures with n=5 and 6.These show (1149) if the transition is gradual. Moreover, a gradual transition does not fit in with the fact that the electric charges are good agreement with our observed HREM images and also with published calculated and observed images.21 The diVer- local.The charge required for polarity reversal is provided by proper localisation of trivalent iron. This idea is supported by ence between the simulated images having indium or iron in the ccp layers is rather small. the observation that the iron is enriched in the bouncing defects. It is very reasonable to anticipate that the these defects As mentioned above, the ZnO4 tetrahedra are always oriented with their apices towards the ccp layers.If the polarity cause the shift in polarity in ZnO necessary to fit the ccp layers. The small shift in contrast in the HREM images when were to be reversed by letting three oxygen atoms of the ZnO4 tetrahedra be shared by the octahedra, then the metal ions crossing these bouncing defect layers corroborates this idea, see Fig. 3. Since these structures are disordered, detailed would come to close to each other. The polar axis of the ZnO slabs thus must change direction somewhere between the ccp information cannot be obtained by using X-ray diVraction techniques. Still, there is some information that can be used layers. The single-crystal structure studies23 of the ordered lutetium-containing compounds [LuFeO3(ZnO)n] with small n to construct a structural model for the defects: (i) There is a diVerence in polarity between the ZnO slabs values, n6, show that this change of polarity is accommodated by a gradual shift of the metal atoms from one tetra- on opposite sides of the ccp layers. Consequently there has to be a reversal of polarity somewhere between these layers. hedron to the neighbouring empty one.As a result the metal atoms have trigonal bipyramidal coordination halfway (ii) There must be reasonable charge balances, interatomic distances and coordination spheres around the atoms. Fe3+ between the ccp layers. The detailed crystal structure analysis J. Mater. Chem., 1998, 8(11), 2465–2473 2471second, with octahedra, for higher n-values. The most important argument for the octahedral model is that there are rather few oxides known with iron in trigonal bipyramidal coordination except the lutetium compounds.23 One such compound is InFeO3.28 To finally settle which model is correct, one of our suggestions or some other, additional information is needed from, for example EXAFS and Mo�ssbauer studies.Appendix The common chemical formula for these types of compounds is: MM¾O3(ZnO)n where M is the metal residing in the plane defect layers (marked below with dots in the layer sequences).In the ordered compounds there are then n+3 close packed oxygen layers within each subperiod (distance between the plane defect layers). If n is an integer there are two cases: Fig. 12 A structural model showing the bouncing defects as formed (i) n is odd and the sequence of oxygen layers is: by trigonal bipyramids viewed along [1190].| subperiod | •(A B)k • (C A)k • (B C)k • (A B)k • (C A)k • can occupy tetrahedra, trigonal bipyramids and octahedra, whereas Zn2+ prefers tetrahedra. | period | (iii) EDS and EELS investigations clearly show an Here: k=(n+3)/2 and period=3×subperiod; enrichment of indium or iron in the ccp layers and of iron in the bouncing defect layers ((114) and (1149) planes).the symmetry is trigonal; (iv) Magnetic data show a coupling between the iron atoms. (ii) n is even and the sequence of oxygen layers is: They are located close to each other in both types of defects (if not diluted by indium in the (001) defect). | subperiod | (v) The defects are narrow and distinct in the crystallites •(A B)k A• (C B)k C• (A B)k A• (C B)k C• viewed along [1190].In accordance with the list above, two structural models | period | have been constructed to describe the bouncing defect layers. Here: k=(n+2)/2 and period=2×subperiod; In the first model we suggest that the change in polarity occurs via double chains of trigonal bipyramids, with trivalent ions the symmetry is hexagonal.in the equatorial plane. These chains run along [1190] and are linked to form planes parallel to (114) and (1149) in ZnO. This This work originates as a spin-oV from a multi-national structural model viewed along [1190] is shown in Fig. 12. The industrial project concerning high-temperature electrode mate- second model comprises a movement of the cations from the rials.In the Northern countries it was co-ordinated by Michael equatorial positions in the bipyramids to neighbouring octa- Hatcher at ‘Permascand’. Our work was later supported by hedral cavities. The latter model is shown in Fig. 13. The NUTEK (Swedish Board for Industrial and Technical octahedra share edges as in a-PbO2, forming chains along Development) and NFR (Swedish Natural Science Research [1190] in (114) and (1149) planes in ZnO.None of the models Council ). We are in debt to the participants in the early stages includes large movements of the oxygen atoms. The hcp net of this work: Mats Nygren, Jekabs Grins, and the late Thomasz of oxygen atoms is in principle preserved, and only the cations Niklewski.are moved. These two models conform to the list of observations and requirements above and are simple enough to be tangible, but it is not obvious which is the correct one, if any. References One may also consider the possibility that the first, with 1 G.Heiland, E.Mollwo and F.Sto�ckmann, Solid State Phys., 1959, bipyramids, is appropriate for lower n-values and that the 8, 193. 2 R.Wang, A. W. Sleight and D. Cleary, Chem.Mater., 1996, 8, 433. 3 R. Wang, A. W. Sleight, R. Platzer and J. A. Gardner, J. Solid State Chem., 1996, 122, 166. 4 T.Ho�rlin and J. Grins, unpublished results. 5 H. Kasper, Z. Anorg. Allg. Chem. 1967, 349, 113. 6 P. J. Cannard and R. J. D.Tilley, J. Solid State Chem., 1988, 73, 418. 7 M. A. McCoy, R. W. Grimes and W. E. Lee, Philos. Mag. A, 1997, 76, 1187. 8 N. Kimizuka, T. Mohri, Y. Matsui and K. Siratori, J. Solid State Chem., 1988, 74, 98. 9 M. Nakamura, N. Kimizuka and T. Mohri, J. Solid State Chem., 1990, 86, 16. 10 N. Kimizuka, M. Isobe, M. Nakamura and T. Mohri, J. Solid State Chem., 1993, 103, 394. 11 N. Kimizuka and T. Mohri, J. Solid State Chem., 1989, 78, 98. 12 M. Nakamura, N. Kimizuka and T. Mohri, J. Solid State Chem., 1991, 93, 298. 13 K. Siratori and N. Kimizuka, J. Solid State Chem., 1992, 99, 243. 14 M. Nakamura, N. Kimizuka, T. Mohri and M. Isobe, J. Solid State Chem., 1993, 105, 535. Fig. 13 A structural model showing the bouncing defects as formed 15 M. Nakamura, N. Kimizuka, T. Mohri and M. Isobe, J. Alloys Comp., 1993, 192, 105. by a-PbO2 type chains of octahedra viewed along [1190]. 2472 J. Mater. Chem., 1998, 8(11), 2465–247316 T. Tsubota, M. Ohtaki, K. Eguchi and H. Arai, J. Mater. Chem., 23 M. Isobe, N. Kimizuka, M. Nakamura and T. Mohri, Acta Crystallogr., Sect. C, 1994, 50, 332. 1997, 7, 85. 17 H. Ohta, W-S. Seo and K. Koumoto, J. Am. Ceram. Soc., 1996, 24 K. E. Johansson, T. Palm and P.-E. Werner, J.Phys. E, 1980, 13, 1289. 79, 2193. 18 N. Kimizuka, M. Isobe and M. Nakamura, J. Solid State Chem., 25 V. D. Frechette and C. F. Cline, Am. Mineral., 1963, 48, 1381. 26 R. A. Powell, W. E. Spicer and J. C. McMenamin, Phys. Rev. B, 1995, 116, 170. 19 N. Uchida, Y. Bando and N. Kimizuka, 13th Int. Conf. Electron. 1972, 6, 3065. 27 F. E. Mabbs and D. J. Machin, Magnetism and Transition Metal Microsc., Paris, 1994, vol. 2, p. 891. 20 Y. Bando, 13th Int. Conf. Electron. Microsc. Paris, 1994, vol. 1, Complexes, Chapman and Hall, London, 1973. 28 D. M. Giaquinta, W. M. Davis and H.-C. Zur Loye, Acta p. 591. 21 N. Uchida, Y. Bando, M. Nakamura and N. Kimizuka, Crystallogr., Sect. C, 1994, 50, 5. J. Electron Microsc., 1994, 43, 146. 22 E. Olsson, G. Svensson and T. Ho� rlin, Eur. Meet. Electron. Micr. Paper 8/05291C Dublin, 1996. J. Mater. Chem., 1998, 8(11), 2465–2

ISSN:0959-9428    

DOI:10.1039/a805291c

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Attempted preparation of diamond-like carbon nitride by explosive shock compression of poly(methineimine) Tamikuni Komatsu National Institute of Materials and Chemical Research, High Density Energy Laboratory, 1-1 Higashi, Tsukuba-shi, Ibaraki 305, Japan Received 2nd June 1998, Accepted 6th August 1998 Explosive shock compression of poly(methineimine) produced a large portion of amorphous graphitic carbon nitride of composition CN0.2 and small amounts of diamond-like carbon containing a slight amount of nitrogen.Codeposition of the two materials suggests the possibility of cubic phase transformation of heterocyclic C–N compounds under well-designed shock-compression conditions. Introduction Experimental Sample preparation Diamond and related materials combined or doped with hetero elements such as B, N, Si and P are expected to be promising The starting material was prepared by ring opening polymerizmaterials in mechanics and electronics for their potential uses ation of s-triazine with excess ZnCl2 at 250 °C for 5 h in an due to their outstanding hardness, thermal conductivity and autoclave, according to ref. 16. The product, which was wide bandgap semiconductivity. The presentations of cubic obtained as a black powder, was heated with conc. HCl in a C–BN by Badzian1 and b-C3N4 by Liu and Cohen2 have water bath, filtered with a glass filter, washed with distilled stimulated numerous investigators to search for diamond-like water and vacuum-dried at 200 °C. The starting material thus hetero compounds.Cubic C–BN compounds were synthesized obtained was mixed with small copper balls in a polymer:copin small amounts by Wedlake,3 Badzian,1 Nakano,4 and per=2:98 mass ratio, filled into a steel capsule, and pressed Knittle5 under static high pressure and high temperature into a disc. The bulk density of the disk was set at 70% of the conditions from 1979 to 1995, and in the next year a consider- theoretical value.The disc was shock compressed using a able amount of purified heterodiamond of composition BC2.5N shock compression apparatus shown in Fig. 1. The apparatus was synthesized by Komatsu et al.6 using an explosive shock- was constructed with a detonator, a sheet explosive discharger compression technique. For b-C3N4, a great number of papers (the shock wave velocity is 6 km s-1), a high melting point have been reported, but only a few studies claim the existence explosive (the shock wave velocity is 9 km s-1), a copper of b-C3N4 so far.The crystalline carbon nitrides which were flying plate, a brass-vessel containing the sample disc, and a synthesized by the groups of Niu,7 Yu,8 Li,9 and Sung10 using steel momentum trap surrounding the sample vessel.The net various chemical and physical vapor deposition methods shock pressure and temperature applied to the sample through agreed fairly well with the predicted interplanar spacings of b- the sample vessel compacted by the explosively accelerated C3N4, however in a strict sense the materials were not com- copper plate was estimated to be 15 GPa and 3500 K, respectpletely proved because of unidentified chemical structures and ively.The recovered sample was machined, immersed in 30% stoichiometry. This essential problem seems to arise from the HNO3 to remove the copper matrix, heated with conc. HCl thermodynamically more unstable b-structure compared to the to remove trace amounts of metallic contaminants, washed a-structure.11,12 There is room for reviewing whether the CVD with distilled water, and vacuum-dried at 200 °C.A fine black and PVD methods are suitable for the synthesis of b-C3N4 powder was obtained at a yield of 36.5%. The material and whether the deposition is quantitatively enough for identification and purification. An explosive shock-compression technique usually used for diamond synthesis is able to supply an extremely high pressure and high temperature to the sample in microseconds and therefore is desirable to obtain non-equilibrium materials like heterodiamond as a bulk form.Wixom attempted to prepare b-C3N4 by shock compression of several pyrolyzed C–H–N compounds under a shock pressure of 30–50 GPa, however he obtained a mixture of amorphous graphitic material and small amounts of diamond.13 Previously, we studied the phase transformation of poly(aminomethineimine) under an explosive shock pressure of about 40 GPa, but unexpectedly no diamond-like materials were produced except for sp2-bonded amorphous carbon nitrides having several diVerent morphologies. 14 As one of the causes, a high atomic ratio of hydrogen included as amino groups of the starting material was anticipated, because a large amount of hydrogen seems to terminate a growth of active species into diamond.15 In this work, the preparation of diamond-like carbon nitride Fig. 1 Section of a plain shock-compression apparatus: 1, detonator; was attempted by shock compression of poly(methineimine) 2, sheet explosive discharge; 3, HMX explosive; 4, copper flying plate; 5, brass vessel containing the sample disk; 6, steel momentum trap.having no functional groups except for theMCHNNMchains. J. Mater. Chem., 1998, 8(11), 2475–2478 2475Table 1 Elemental analysis of the starting material and shock-compression product Starting material Shock-compression product Elemental analysis (%) Atomic ratio Elemental analysis (%) Atomic ratio C H N C H N C H N C H N 49.2 3.9 41.2 1.0 0.9 0.7 41.9 0.1 11.5 1.0 0.0 0.2 obtained was checked in terms of the presence or absence of molecular reconstruction has been induced by shock compression.The following IR and XRD results suggest that this a diamond-like material with an analytical transmission electron microscope. change is possibly due to complicated cyclization of the starting material accompanying elimination of p-bonding nitrogen atoms.Measurements The chemical compositions of the samples were determined IR spectra using a Holiba CHN analyzer. The crystalline structures were determined using a Rigaku RINT-2500 X-ray powder Carbon nitrides are IR active owing to an increased polarity diVractometer (XRD) equipped with a position-sensitive pro- and breaking of symmetric vibrations by incorporation of portional counter and graphite monochromater on the detec- nitrogen with carbon. The materials show generally two tor.Ni-filtered Cu-Ka radiation generated at 50 kV and vibrational modes in the range 1600–1300 cm-1 which are 200 mA was used. The crystalline structure, elemental composi- related to the Raman G and D bands of amorphous carbon.tion, and chemical bonding nature in the microscopic regions Fig. 2(a) shows the IR spectrum of the starting material. The of the samples were investigated using a Hitachi HF-2000 spectrum was simple and broad and resembled the IR spectrum field-emission transmission electron microscope (FETEM) of paracyanogen17 and those of several amorphous CNx thin equipped with an electron diVractometer (ED), a parallel films.18–20 The middle absorption band at 3322 cm-1 was recording electron energy-loss spectroscope (PEELS), and an assigned to NMH stretching of imine, the middle band at energy-dispersive X-ray analyzer (EDX).These measurements 3125 cm-1 to NCMH stretching, the strong bands at were made at 200 kV accelerating voltage.The infrared (IR) 1612 cm-1 and 1549 cm-1 to CNN and CNC stretching spectra of the samples were measured in KBr disk form using (related to the G band), the middle plateau band ranging from a Perkin-Elmer FTIR-1640 spectrometer. 1260 cm-1 to 1450 cm-1 (showing a peak at 1387 cm-1) to ring stretching and CMN stretching (related to the D band), and the small band at 927 cm-1 to NCMH bending.The Results and discussion small peak at 616 cm-1 was unassigned. The chemical structure Chemical analysis of the starting material is an incomplete poly(methineimine) structure including sp3 carbon and nitrogen possibly Table 1 gives the elemental analysis of the starting material and the shock-compression product. The chemical composition of the starting material was approximately CH0.9N0.7 and that of the shock-compression product was close to carbon nitride of composition CN0.2 in which a large portion of hydrogen atoms and significant amounts of nitrogen atoms of the starting material were lost.Such a remarkable change in composition after shock compression indicates that some Fig. 2 IR spectra showing the chemical structure change before and Fig. 3 FETEM image and ED pattern of a large portion of particles after shock compression: (a) the starting material, (b) the shockcompression product. included in the shock compression product. 2476 J. Mater. Chem., 1998, 8(11), 2475–2478cross-linked with adjacent polymer chains, as depicted on the next page. CH C N N) n N CH N (CH CH On the other hand, the shock-compression product showed a more simple broad spectrum as several peaks of the starting material disappeared, as shown in Fig. 2(b). The broad band centered at 1592 cm-1 was assigned to a superposition of CNN stretching and CNC stretching and the plateau band around 1344 cm-1 was assigned to ring stretching. The band at 3442 cm-1 is due to moisture contained in the IR specimen. The simple IR bands and decreased IR activity of the shockcompression product must be related to the graphitic ring formation and the remarkable decrease in nitrogen content compared to the starting material.Fig. 6 PEEL spectrum of the particle observed in Fig. 5. Note the presence of a large amount of carbon and a small amount of nitrogen and the carbons having a large s-bonding feature relative to the p feature.An expanded N-K edge shows only a s* peak. X-Ray diVraction The XRD patterns of the samples before and after shock compression were almost identical: the pattern of the starting material consists of two broad peaks centered at 2h=26.08° (strong peak, d=0.350 nm) and 42.63° (weak, 0.227 nm), and that of the shock-compression product consists of two broad peaks at 2h=26.21° (strong, 0.349 nm) and 42.75° (weak, 0.227 nm) but no patterns of diamond-like materials.These patterns, which are similar to the pattern of amorphous carbon, indicate amorphous structures. TEM analysis Fig. 3 and 4 show a representative TEM image and ED pattern and a PEEL spectrum of the shock-compression product, Fig. 4 PEEL spectrum of the particle observed in Fig. 3. Note the presence of carbon and nitrogen both having p features. respectively.The TEM image and ED pattern show a dis- Fig. 5 FETEM image and ED pattern of small amounts of nanoparticles microdiVused in the matrix of the shock-compression product. J. Mater. Chem., 1998, 8(11), 2475–2478 2477ordered structure peculiar to amorphous carbon, and the Conclusions PEEL spectrum indicates the material is composed of a large Explosive shock compression of poly(methineimine) was amount of carbon and a small amount of nitrogen both having carried out under 15 GPa, 3500 K conditions in order to clear 1s�p* and 1s�s* transitions at the carbon and nitrogen prepare diamond-like carbon nitride.A large portion of K-edges. This means that the grain is combined with sp2- amorphous graphitic CN0.2 and small amounts of diamond- bonding carbon and nitrogen.Mixed with this grain, small like carbon combined with a slight amount of nitrogen were amounts of microdiVused nanoparticles showing a diVerent confirmed by analytical TEM. The low nitrogen concentration bonding nature were observed. The particle was amorphous of the shock-compression product compared to the starting or slightly crystalline, from the TEM image and ED pattern material may be due to phase separation of an unidentified shown in Fig. 5.Considering the influence from the graphitic intermediate material. The finding of diamond-like and gra- material underneath, the measurements of the TEM and PEEL phitic carbon nitrides in the shock-compression product sug- spectrum of this particle were made as nearly as possible on gests the possibility of producing diamond-structured carbon such thin fringes.From the PEEL spectrum shown in Fig. 6, nitrides by shock compression of heterocyclic C–N it was found that the material is composed of a large portion compounds. of carbon and trace amounts of nitrogen and that the carbon K-edge shows a large s* feature relative to the p* feature.This indicates a mixture of sp3- and sp2-bonding carbons. The References ratio of sp3-carbon to total carbons, sp3/(sp2+sp3), was 1 A. R. Badzian, Mater. Res. Bull., 1981, 6, 1385. estimated from the following equation according to ref. 21: 2 A. Y. Liu and M. L. Cohen, Science, 1989, 245, 841. 3 De Beers Industrial Diamond Division Ltd., Ger.Pat., 2806070, sp3/(sp2+sp3)=3(cstd-cexp)/(3cstd+cexp) 1979; R. J. Wedlake and A. L. Penny, Chem. Abstr., 1979, 90, 42865 Z. cstd=(Ip*/Is*)std, cexp=(Ip*/Is*)exp 4 S. Nakano, M. Akaishi, T. Sasaki and S. Yamaoka, Chem.Mater., 1994, 6, 2246. 5 E. Knittle, R. B. Kaner, R. Jeanloz and M. L. Cohen, Phys. Rev. where (Ip*/Is*) is the intensity ratio of the p* and s* peaks B, 1995, 51, 12149.at the carbon K-edge, and the subscripts std and exp, respect- 6 T. Komatsu, M. Nomura, Y. Kakudate and S. Fujiwara, J. Mater. ively, refer to the standard and experimental, and graphite Chem., 1996, 6, 1799. was used as the standard. The result ranged from 40% to 60% 7 C. Niu, Y. Z. Lu and C. M. Lieber, Science, 1993, 261, 334. for many particles. In order to clarify the diVerence in the 8 K.M. Yu, M. L. Cohen, E. E. Haller, W. L. Hansen, A. Y. Liu and I. C.Wu, Phys. Rev. B, 1994, 49, 5034. bonding nature of nitrogen in this material and the earlier 9 Y. A. Li, S. Xu, H. S. Li and W. Y. Luo, J. Mater. Sci. Lett., 1998, graphitic material, the nitrogen K-edges in Fig. 6 and 4 were 17, 31. expanded and compared with each other. The p* and s* peak 10 S.L. Sung, T. G. Tsai, K. P. Huang, J. H. Huang and H. C. Shih, positions in the N–K edge of the graphitic material appeared Jpn. J. Appl. Phys., 1998, 37, L148. at 401.7 and 408.4 eV. On the other hand the noted material 11 Y. Gou and W. A. Goddard III, Chem. Phys. Lett., 1995, 237, 72. showed an appreciable s* peak but an insignificant p* peak 12 D. M. Bhusari, C. K. Chen, T.J. Chuang, L. C. Chen and M. C. Lin, J. Mater. Res., 1997, 12, 322. in these positions. The nitrogen content was too small to 13 M. R. Wixom, J. Am. Ceram. Soc., 1990, 73, 1973. estimate the C/N ratio of the material by PEELS analysis. 14 T. Komatsu and M. Samejima, J. Mater. Chem., 1998, 8, 193. The PEEL spectrum resembled those of amorphous CNx thin 15 R. H. Wentorf, J. Phys.Chem., 1965, 69, 3063. films22,23 and diamond-like carbon.21 Therefore, the material 16 D. Wohrle, Tetrahedron Lett., 1971, 22, 1969. was assigned as amorphous diamond-like CNx. 17 L. Maya, J. Polym. Sci., A: Polym. Chem., 1993, 31, 2595. 18 J. H. Kaufman, S. Metin and D. D. Saperstain, Phys. Rev. B, Although the content of the diamond-like CNx was slight 1989, 38, 13053. in comparison with the formation of graphitic carbon nitride 19 X.A. Zhao, C. W. Ong, Y. C. Tsang, Y. W. Wong, P. W. Chan CN0.2, codeposition of the two materials suggests the synthesis and C. L. Choy, Appl. Phys. Lett., 1995, 66, 2652. of diamond-structured carbon nitrides to be possible by shock 20 J. Hartmann, P. Siemroth, B. Schultrich and B. Raushenbach, compression of heterocyclic C–N compounds. The large nitro- J. Vac. Sci. Technol. A, 1997, 15, 2983. gen loss after shock compression is possibly due to phase 21 D. L. Pappas, K. L. Saenger, J. Bruley, W. Krakow, J. J. Cuomo, T. Gu and R. W. Collins, J. Appl. Phys., 1992, 71, 5675. separation of an unidentified intermediate material because 22 L. A. Bursill, P. Julin, V. N. Gurarie, A. V. Orlov and S. Prawer, the high nitrogen ratio of heterocyclic C–N compounds means J. Mater. Res., 1995, 10, 2277. they tend to lose nitrogen at mild temperatures.17,24 Balance 23 J. Hu, P. Yang and C. M. Lieber, Phys. Rev. B, 1998, 57, R3185. of the applied shock pressure and temperature to the sample, 24 L. Maya, D. R. Cole and E. W. Hagamo, J. Am. Ceram. Soc., preferably high pressure and low temperature conditions, must 1991, 74, 1686. be important for creation of a C–N heterodiamond which is supposed to be kinetically much more unstable than diamond. Paper 8/04 2478 J. Mater. Chem., 1998, 8(11), 2475–2478

ISSN:0959-9428    

DOI:10.1039/a804137g

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Structural complexity and metal coordination flexibility in two acetophosphonates Aurelio Cabeza, Miguel A. G. Aranda and Sebastian Bruque* Departamento de Quý�mica Inorga�nica, Cristalografý� a y Mineralogý�a, Universidad de Ma�laga, 29071 Ma�laga, Spain. E-mail: bruque@uma.es Received 18th June 1998; Accepted 29th July 1998 Two divalent metal acetophosphonates, Pb6(O3PCH2CO2)4 and Mn3(O3PCH2CO2)2, have been synthesised hydrothermally.They crystallise in the triclinic system, space group P19, a=11.0064(1), b=12.3604(1), c=8.9783(1) A° , a=98.632(1), b=90.474(1), c=75.629(1)°, Z=2, for M=Pb, and a=10.0146(5), b=6.3942(4), c=8.4796(6) A° , a=101.452(4), b=106.254(2), c=96.431(4)°, Z=2, forM=Mn. The structures were solved ab initio using direct methods from synchrotron powder diVraction data (l#0.4 A° ) for M=Pb and from laboratory X-ray data for M=Mn.The crystal structure of the Pb compound is very complex with 38 non-hydrogen atoms in general positions (114 refined positional parameters), it had been refined by Rietveld method using soft constraints, and converged to RWP=6.8% and RF=1.6%. The structure forM=Mn has a moderate complexity with 19 nonhydrogen atoms (57 refined positional parameters) which was also refined with soft constraints to RWP=8.3%, RF=3.9%. Both compounds show a framework built of alternate metal oxide inorganic layers, pillared by the organic groups.The metal environments in these materials are very distorted. Manganese atoms present three diVerent distorted oxygen environments: four-, five- and six-coordinate.Thermal and IR data are also reported and discussed. tetrahedral and one octahedral sites for the zinc atoms. As Introduction yet, the structure of Mn3(O3PC2H4CO2)2 has not been solved, The interest in the chemistry of phosphonates has drastically although it seems to be isostructural to the Zn analog.20b The increased in the last twenty years.1 Initially, these compounds synthesis and structure of Co3(O3PC2H4CO2)2·6H2O has very (with phosphonic acid H2O3PR, R=alkyl or aryl group) were recently been reported exhibiting a 3D ‘open’ framework.21 mainly layered with structures very related to that of the In this paper, we report the synthesis, characterisation parent zirconium hydrogen phosphate a-Zr(HPO4)2·H2O.2 and crystal structure of two acetophosphonates, The ability to design structures with specific properties that Pb6(O3PCH2CO2)4 and Mn3(O3PCH2CO2)2.these materials show, as well as their unusual compositional and structural diversity varying from one-dimensional arrange- Experimental ments3,4 to three-dimensional microporous frameworks,5–7 via the most common layered frameworks,8–10 have stimulated Synthesis of Mn3(O3PCH2CO2)2 extensive exploration of their chemistry.In fact, the importance of such systems in several research areas such as electrochemis- Chemicals of reagent quality were obtained from Aldrich and try,11,12 microelectronic,13 photochemical mechanisms14 and used without purification. Manganese(II) acetophosphonate catalysis15,16 has been widely recognised.was synthesised by adding 1.43 mmol manganese(II) acetate Metal phosphonates with 3D frameworks can be synthesised tetrahydrate dissolved in 10 ml distilled water to an aqueous as nanotubular phosphonates or alternatively as pillared lay- solution (10 ml ) containing 7.14 mmol acetophosphonic acid; ered structures (PLS) by using, for example, diphosphonic the resulting Mn5P molar ratio was 155 and the solution has acids H2O3P–R–PO3H2 as pillaring agent.17 Hence, it is poss- a pH of 1.4.No precipitate is formed even on hydrothermally ible to design the interlayer spacing (and chemistry) through heating at 150 °C for one week thus the pH of the solution the shape, size and nature of the organic spacer R. Several was increased by adding 30% aqueous NaOH dropwise up to other ways to obtain PLS materials have also been adopted, pH 4.3.At this point, a precipitate started to develop. e.g. use of carboxyphosphonates,18 or through the reaction of This suspension was heated again in a Teflon-lined autoclave free hydrogen carboxyphosphonate groups with intercalated at 150 °C for 5 days. A single powdered phase was filtered, alkyl diamines H2N–R–NH2 at high temperature which yields washed with water and with acetone, and dried under vacuum.covalent amide links.19 The synthesis and structures of several 2-carboxyethyl- Synthesis of Pb6(O3PCH2CO2)4 phosphonates of divalent and trivalent metals have Lead acetophosphonate was also prepared hydrothermally. been reported. For example, Fe phosphonates,20a 4.08 mmol acetophosphonic acid were dissolved in 10 ml dis- FeIII(HO3PR)3(H2O3PR), FeII(HO3PR)2, FeIII(HO3PR) tilled water.A second solution containing 0.816 mmol of lead (O3PR)·H2O and FeIIIO(HO3PR)·H2O, with R=C2H4CO2H; acetate trihydrate dissolved in 15 ml of water was added slowly Bi phosphonates,18c Bi(O3PC2H4CO2)·H2O and and with constant stirring. The resulting solution has a Pb5P Bi(HO3PC2H4CO2H)(O3PC2H4CO2H); and even bimetalmolar ratio of 155 and a pH of 1.4.Under these conditions, lic phosphonates,20b MnZn2(O3PC2H4CO2)2, as well as no precipitate formed; thus as described above, the pH was Mn(O3PC2H4CO2H)·H2O and Mn3(O3PC2H4CO2)2. The increased up to 1.8, leading to the formation of a white structures of most of these compounds exhibit inorganic layers precipitate.This mixture was heated in a Teflon-lined autoclave formed by the metal cations and the PO3 and CO2 moieties, at 150 °C for 6 days. A single powdered phase was isolated by pillared by the organic groups to yield 3D frameworks. The filtration, washed with water and acetone, and finally dried metal environments are very versatile in these materials as has been shown for Zn3(O3PC2H4CO2)2,18b where there are two under vacuum.J. Mater. Chem., 1998, 8(11), 2479–2485 2479Elemental analysis. Carbon and hydrogen contents were thermal decomposition reactions: determined by elemental chemical analysis on a Perkin-Elmer Mn3(O3PCH2CO2)2+4O2�Mn3(PO4)2+4CO2+2H2O 240 analyser. Analytical data for Mn3(O3PCH2CO2)2: C, Pb6(O3PCH2CO2)4+8O2�2Pb3(PO4)2+8CO2+4H2O 10.54; H, 0.95.Calc.: C, 10.94; H, 0.91%. Analytical data for Pb6(O3PCH2CO2)4: C, 5.20; H, 0.45. Calc.: C, 5.35; H, 0.45%. The thermal decomposition products were identified through the powder patterns collected for the samples heated at 1000 °C. This high temperature was used to increase crystal- Thermal analysis. TGA and DTA data were collected on a linity which helps in the identification procedure.The patterns Rigaku Thermoflex apparatus at a heating rate of 10 K min-1 matched with those present in the PDF database: no. 31-0827 in air with calcined Al2O3 as an internal reference standard. for Mn3(PO4)2 and 24-0585 for Pb3(PO4)2. IR study. IR spectra were recorded on Perkin Elmer 883 IR spectroscopy study spectrometer in the spectral range 4000–400 cm-1, using dry KBr pellets containing 2% of sample.The IR spectra of both compounds are shown in Fig. 2. There are no bands in the O–H stretching region (3500–3000 cm-1), X-Ray powder diVraction. The powder diVraction pattern which is consistent with the absence of water molecules or for Mn3(O3PCH2CO2)2 were collected on a Siemens D-5000, hydrogen phosphonate/carboxylate groups in the structures. automated diVractometer using graphite-monochromated Cu- As it can be observed in Fig. 2, no band is seen at ca. Ka radiation. The sample was diluted and blended with 1715 cm-1 corresponding to n(CNO) for the free acid spherical particles of Cab-O-Sil M-5 (Fluka), to reduce pre- (–COOH). However, there are two pairs of strong bands ferred orientations.22 The angular range scanned was 7–80° centred at 1610, 1550 and 1425, 1380 cm-1, for (2h), with a step size of 0.02° and counting time of 20 s per step.Mn3(O3PCH2CO2)2, and at 1550, 1505 cm-1 and 1420, For Pb6(O3PCH2CO2)4, high resolution synchrotron 1370 cm-1, for Pb6(O3PCH2CO2)4, which are assigned to the powdlected on the diVractometer of the BM16 antisymmetrical and symmetrical stretching vibrations of C–O line of ESRF (Grenoble, France).The sample was loaded in groups when present as COO- moieties.24 There are two set a borosilicate glass capillary (diameter=0.5 mm) and rotated of bands probably due to crystallographically diVerent during data collection. The pattern was collected with l= carboxylic groups coordinated to the metal atoms, as has been 0.399 89(2) A° , in the angular range 1–30° in 2h, for an overall confirmed by XRD.Other bands characteristic of the count time of 10 h. Raw data were normalised and reduced to phosphonate groups are also present in the IR spectra. a constant step size of 0.003° with local software. Further experimental details about data collection and analysis of this Structure determination type of data have been already reported.23 The X-ray laboratory powder pattern for Mn3(O3PCH2CO2)2 was auto-indexed using the TREOR90 program25 giving a Results and discussion triclinic unit cell with a=10.001, b=6.379, c=8.478 A° , a= 101.39, b=106.32, c=96.38°, V=500.8 A° 3, Z=2, Vat (non-H Thermal study atoms)=13.2 A° 3 atom-1, M20=3326 and F20=55 (0.0085, TGA–TDA curves for Mn3(O3PCH2CO2)2 and 43).27 The X-ray synchrotron powder pattern for Pb6(O3PCH2CO2)4 are shown in Fig. 1. Only one exothermic Pb6(O3PCH2CO2)4 was auto-indexed by the TREOR9025 proe Vect, with an abrupt change in the DTA curve, was observed gram in a triclinic unit cell with dimensions: a=11.002, for both compounds. For Mn3(O3PCH2CO2)2 the exotherm b=12.365, c=8.984 A° , a=98.68, b=90.49, c=75.64°, V= takes place at higher temperature (570 °C) than for 1170.0 A° 3, Z=2, Vat=15.38 A° 3 atom-1, M20=4826 and F20= Pb6(O3PCH2CO2)4 (418 °C).This eVect is due to the combus- 161 (0.0048, 26).27 Both crystal structures were solved by ab tion of the acetocarboxylic groups and it has an associated initio procedures. The pattern decomposition option of the mass loss of 18.5 and 11.0%, for M=Mn and Pb, respectively.GSAS package28 was used to extract corrected structure These values are in good agreement with theoretical values factors, using the Le Bail method,29 from a limited region of (19.14 and 9.38%, respectively) calculated for the following the pattern, 14<2h<62° for Mn compound (650 reflections) and 1.5<2h<20.5°, for Pb compound (1500 reflections).The Fig. 1 TGA-DTA curves for (a) Mn3(O3PCH2CO2)2 and (b) Fig. 2 IR spectra for (a) Mn3(O3PCH2CO2)2 and (b) Pb6(O3PCH2CO2)4. Pb6(O3PCH2CO2)4. 2480 J. Mater. Chem., 1998, 8(11), 2479–2485Fig. 3 Observed, calculated and diVerence X-ray powder diVraction profiles for Mn3(O3PCH2CO2)2. The tick marks are calculated 2h angles for Bragg peaks.patterns were fitted without any structural model by refining P–C [1.80(1) A° ], O,O [2.55(1) A° ], O,O [2.73(1) A° ], C–Ccarb [1.50(1) A° ], Ccarb–Ocarb [1.23(1) A° ], C,Ocarb the overall parameters: background, zero-point error, unit cell and peak shape values. A pseudo-Voigt peak shape function30 [2.36(1) A° ] and Ocarb,Ocarb [2.15(1) A° ], to retain a reasonable geometry for the tetrahedral O3PC and carboxylic groups.The corrected for asymmetry31 was used. SIRPOW9232 gave the positions of three manganese atoms and two phosphorus final weights for the soft constraints were -10. The powder pattern collected in h/2h geometry for M=Mn showed a atoms by direct methods. SHELXS8633 gave the positions of the six lead atoms by both Patterson map and direct methods.strong preferred orientation along the [010] and [100] directions, which were corrected using the March–Dollase34 func- For both compounds, the found atoms were included in the Rietveld refinements using the overall parameters obtained in tion with coeYcients of 1.147(7) for [010] and 0.666(6) for [100]. The synchrotron powder pattern collected on a capillary the last cycle of the ab initio refinements. RwP dropped to 23.6% for M=Pb and to 32.0% for M=Mn by refining only for M=Pb did not show preferred orientation.The final refinement for Mn3(O3PCH2CO2)2 converged to RwP=8.29%, the scale factors. Successive diVerence Fourier maps and soft constrained refinements led to the atomic positions of the RP=6.43% and RF=3.91%; and for Pb6(O3PCH2CO2)4 to RwP=6.76%, RP=5.22% and RF=1.64%; R factors are defined remaining atoms.It is worthy to underline that due to the complexity of these structures, the atomic positions were by Rietveld,35 and Larson and Von Dreele.28 The Rietveld plots for Mn and Pb compounds are shown in Fig. 3 and 4, refined using the following soft constraints, P–O [1.53(1) A° ], Fig. 4 Observed, calculated and diVerence synchrotron X-ray (l#0.4 A° ) powder diVraction profiles for Pb6(O3PCH2CO2)4 between 1.5 and 30° (2h).The tick marks are calculated 2h angles for Bragg peaks. J. Mater. Chem., 1998, 8(11), 2479–2485 2481Table 1 Positional parameters for Mn3(O3PCH2CO2)2 in space group respectively. Atomic parameters are presented in Table 1 and P19 bond lengths in Table 2 for M=Mn, and in Table 3 and 4 for M=Pb, respectively.Atom x y z Uiso/A° 2 Attempts to solve the structure of Pb6(O3PCH2CO2)4 from laboratory X-ray powder data were unsuccessful. Thus, a Mn(1) 0.4490(4) 0.0986(9) 0.8271(6) 0.017(2) Mn(2) 0.1209(5) 0.8001(10) 0.5498(8) 0.032(2) synchrotron pattern was collected owing to the high quality Mn(3) 0.3827(5) 0.6098(11) 0.3819(6) 0.022(2) of diVraction data, utilising the very high angular resolution P(1) 0.6856(7) 0.9316(14) 0.6480(8) 0.044(4) and the absence of preferred orientation. Under these con- P(2) 0.3651(7) 0.5789(13) 0.7877(10) 0.027(3) ditions, with better structure factors, such a complex structure O(1) 0.6212(11) 1.1134(19) 0.7346(13) 0.014(2) (38 non-hydrogen atoms in the asymmetric part of the unit O(2) 0.5839(11) 0.7120(17) 0.5985(15) 0.014 cell including six crystallographically independent lead atoms) O(3) 0.7177(13) 0.9879(23) 0.4927(12) 0.014 O(4) 0.2851(11) 0.6070(23) 0.6098(12) 0.014 could be successfully solved from powder diVraction data.O(5) 0.4682(10) 0.4203(18) 0.7756(17) 0.014 O(6) 0.4398(10) 0.8000(15) 0.9083(15) 0.014 Structure description O(7) 0.9902(12) 0.6850(23) 0.6842(22) 0.014 O(8) 0.8228(15) 0.5306(21) 0.7581(22) 0.014 The crystal structure of Mn3(O3PCH2CO2)2 contains 19 O(9) 0.2318(15) 0.0852(20) 0.8128(18) 0.014 non-hydrogen atoms in the asymmetric unit of the unit cell, O(10) 0.0533(12) 0.1953(24) 0.6658(18) 0.014 C(1) 0.8496(10) 0.9099(20) 0.7968(15) 0.008(5) Table 3 Positional parameters for Pb6(O3PCH2CO2)4 in space group C(2) 0.8891(20) 0.6936(19) 0.7397(33) 0.008 P19 C(3) 0.2329(11) 0.4619(18) 0.8705(15) 0.008 C(4) 0.1619(15) 0.2302(19) 0.7876(22) 0.008 Atomx y z Uiso/A° 2 Pb(1) 0.33231(30) 0.29920(27) 0.91369(32) 0.0111(9) Pb(2) -0.00392(28) 0.49459(25) 0.23407(31) 0.0081(9) Pb(3) 0.17993(28) 0.69240(27) 0.50670(31) 0.0055(8) Pb(4) 0.50768(29) 0.50721(25) 0.73390(30) 0.0059(9) Pb(5) 0.39799(28) 0.87470(26) 0.77513(33) 0.0113(9) Pb(6) 0.11856(29) 0.11283(27) 0.08619(31) 0.0204(10) P(1) 0.2555(14) 0.4298(12) 0.5390(15) 0.006(2) P(2) 0.7553(14) 0.4389(12) -0.0656(14) 0.006 P(3) 0.5803(14) 0.0665(13) 0.8220(15) 0.006 P(4) 0.0900(13) 0.0029(11) 0.7295(14) 0.006 O(1) 0.2434(27) 0.4115(21) 0.7048(15) 0.006 O(2) 0.3625(20) 0.4870(22) 0.5195(29) 0.006 O(3) 0.1299(18) 0.5001(20) 0.4887(30) 0.006 O(4) 0.3979(24) 0.3001(33) 0.1981(29) 0.006 O(5) 0.2017(26) 0.3019(34) 0.1846(29) 0.006 O(6) 0.8684(21) 0.4867(23) -0.0129(31) 0.006 O(7) 0.6337(20) 0.5102(20) 1.0190(28) 0.006 O(8) 0.7415(26) 0.4337(20) 0.7620(16) 0.006 O(9) 0.8882(26) 0.3109(33) 0.2086(30) 0.006 O(10) 0.6940(25) 0.3074(34) 0.2191(29) 0.006 O(11) 0.5839(28) 0.1399(21) 0.9763(20) 0.006 O(12) 0.3439(24) 0.0569(14) 0.1757(28) 0.006 O(13) 0.5568(16) 0.9301(26) 0.2210(30) 0.006 O(14) 0.5240(33) 0.3048(22) 0.7667(31) 0.006 O(15) 0.6004(34) 0.2736(22) 0.5403(27) 0.006 O(16) 0.0417(15) 0.9513(23) 0.1959(27) 0.006 O(17) 0.8087(20) 0.0264(23) 0.1525(25) 0.006 O(18) 0.9049(28) 0.1020(15) 0.3916(23) 0.006 O(19) 0.8962(33) 0.7474(24) 0.1525(25) 0.006 O(20) -0.0199(33) 0.6963(21) 0.3555(33) 0.006 C(1) 0.2925(30) 0.2924(14) 0.4223(20) 0.006 C(2) 0.2963(25) 0.305(4) 0.2586(24) 0.006 C(3) 0.7845(29) 0.2963(13) 0.9779(22) 0.006 C(4) 0.7901(27) 0.302(5) 0.1457(23) 0.006 C(5) 0.6523(26) 0.1229(19) 0.6810(30) 0.006 C(6) 0.595(4) 0.2444(21) 0.6650(28) 0.006 C(7) 0.1257(30) 0.1115(15) 0.6344(26) 0.006 Fig. 5 [001] View of the crystal structure of Mn3(O3PCH2CO2)2. C(8) 0.082(5) 0.2308(21) 0.7132(30) 0.006 Table 2 Bond lengths (A° ) for Mn3(O3PCH2CO2)2.Long Mn–O interactions are given in italics Mn(1)MO(1) 2.085(12) Mn(1)MO(6) 2.156(12) Mn(1)MO(9) 2.135(15) Mn(1)MO(5) 2.185(11) Mn(1)MO(6) 2.134(12) Mn(1)MO(3) 2.667(10) Mn(2)MO(3) 2.147(11) Mn(2)MO(7) 2.123(14) Mn(2)MO(9) 2.465(14) Mn(2)MO(4) 2.173(13) Mn(2)MO(10) 2.153(13) Mn(2)MO(10) 2.753(14) Mn(3)MO(1) 2.190(13) Mn(3)MO(4) 2.399(11) Mn(3)MO(3) 2.975(15) Mn(3)MO(2) 2.240(11) Mn(3)MO(5) 2.265(11) Mn(3)MO(2) 2.152(11) Mn(3)MO(8) 2.060(15) P(1)MO(1) 1.550(5) P(2)MO(4) 1.554(5) P(1)MO(2) 1.547(5) P(2)MO(5) 1.535(5) P(1)MO(3) 1.538(5) P(2)MO(6) 1.538(5) C(1)MC(2) 1.506(6) P(1)MC(1) 1.808(5) P(2)MC(3) 1.820(5) C(2)MO(7) 1.232(5) C(2)MO(8) 1.232(5) C(3)MC(4) 1.505(6) C(4)MO(9) 1.240(6) C(4)MO(10) 1.235(6) 2482 J.Mater. Chem., 1998, 8(11), 2479–2485Table 4 Bond lengths (A° ) for Pb6(O3PCH2CO2)4.Long Pb–O interactions are given in italics and the average Pb–O distance for each polyhedron (coordination number as subscript) are also given Pb(1)MO(1) 2.55(2) Pb(1)MO(7) 2.46(3) Pb(1)MO(19) 2.76(4) Pb(1)MO(4) 2.65(3) Pb(1)MO(11) 3.08(3) Pb(1)MO(8) 3.98(2) Pb(1)MO(5) 2.83(3) Pb(1)MO(13) 2.88(3) Pb(1)MO(20) 4.18(3) Pb(1)MO(6) 3.02(3) Pb(1)MO(14) 2.52(3) <Pb(1)MO9> 2.75 Pb(2)MO(1) 2.63(3) Pb(2)MO(6) 2.62(2) Pb(2)MO(8) 3.22(3) Pb(2)MO(3) 2.71(2) Pb(2)MO(6) 2.57(3) Pb(2)MO(9) 3.24(3) Pb(2)MO(3) 2.84(3) Pb(2)MO(20) 2.53(3) Pb(2)MO(19) 4.19(4) Pb(2)MO(5) 2.83(4) Pb(2)MO(9) 2.79(3) <Pb(2)MO8> 2.69 Pb(3)MO(2) 2.84(3) Pb(3)MO(10) 2.82(3) Pb(3)MO(1) 4.04(2) Pb(3)MO(3) 2.55(3) Pb(3)MO(15) 2.60(4) Pb(3)MO(8) 2.69(2) Pb(3)MO(18) 2.51(2) Pb(3)MO(9) 2.68(3) Pb(3)MO(20) 2.57(3) <Pb(3)MO8> 2.66 Pb(4)MO(2) 2.51(3) Pb(4)MO(7) 2.79(2) Pb(4)MO(15) 3.08(3) Pb(4)MO(2) 2.72(2) Pb(4)MO(8) 2.53(3) Pb(4)MO(1) 3.39(3) Pb(4)MO(4) 2.81(4) Pb(4)MO(10) 2.75(4) Pb(4)MO(15) 3.89(2) Pb(4)MO(7) 2.90(3) Pb(4)MO(14) 2.53(3) <Pb(4)MO9> 2.73 Pb(5)MO(4) 2.74(3) Pb(5)MO(17) 2.35(2) Pb(5)MO(12) 3.91(1) Pb(5)MO(10) 2.69(4) Pb(5)MO(15) 3.13(3) Pb(5)MO(11) 2.27(2) Pb(5)MO(12) 3.17(3) Pb(5)MO(13) 2.58(3) Pb(5)MO(18) 3.61(3) <Pb(5)MO5> 2.53 Pb(6)MO(5) 2.74(4) Pb(6)MO(19) 2.93(3) Pb(6)MO(18) 3.67(3) Pb(6)MO(12) 2.56(3) Pb(6)MO(9) 3.13(4) Pb(6)MO(11) 3.91(3) Pb(6)MO(16) 2.67(3) Pb(6)MO(16) 3.19(3) Pb(6)MO(17) 2.54(3) Pb(6)MO(17) 3.67(3) <Pb(6)MO5> 2.69 P(1)MO(1) 1.551(7) P(2)MO(6) 1.547(7) P(3)MO(11) 1.543(7) P(1)MO(2) 1.540(7) P(2)MO(7) 1.541(7) P(3)MO(12) 1.550(7) P(1)MO(3) 1.543(7) P(2)MO(8) 1.548(7) P(3)MO(13) 1.546(7) P(1)MC(1) 1.810(8) P(2)MC(3) 1.813(8) P(3)MC(5) 1.809(7) P(4)MO(16) 1.538(7) C(1)MC(2) 1.502(8) C(3)MC(4) 1.499(8) P(4)MO(17) 1.546(7) C(2)MO(4) 1.233(8) C(4)MO(9) 1.236(8) P(4)MO(18) 1.553(7) C(2)MO(5) 1.235(8) C(4)MO(10) 1.233(8) P(4)MC(7) 1.815(8) C(5)MC(6) 1.506(8) C(6)MO(14) 1.231(8) C(8)MO(19) 1.230(7) C(6)MO(15) 1.233(8) C(7)MC(8) 1.498(8) C(8)MO(20) 1.231(7) Fig. 6 [010] View of the crystal structure of Mn3(O3PCH2CO2)2 with atoms labeled. J. Mater. Chem., 1998, 8(11), 2479–2485 2483be schematically summarised as ,Mn(3)O(1)O(5)- Mn(1)O(6)O(6)Mn(1)O(1)O(5)Mn(3)O(2)O(2)Mn(3),. It is of interest that all these oxygen atoms belong to the phosphonate groups.These chains are interconnected along the a-axis through the carboxy groups and the Mn(2)O4 groups. Fig. 6 shows the crystal structure down to the b-axis from which the links of the chains along the c-axis through the oxygens of the phosphonates defining a layer can be seen (in the bc plane). The structure can be depicted as inorganic layers in the bc-plane formed by the manganese polyhedra sandwiched by acetophosphonate groups in an ordered way such that phosphonate heads always point to the more coordinated manganese groups and the carboxylate tails always point to the four-coordinate manganese atoms.In this sense, it can be conceived as a PLS but with no space between the inorganic layers. There are small cavities where the hydrogens of the CH2 groups [C(1) and C(3)] are located (Fig. 6); there is not enough empty space even for water molecules. The crystal structure of Pb6(O3PCH2CO2)4 contains 38 atoms in the asymmetric of the unit cell, all in general positions and there are six crystallographically independent lead atoms. To define the oxygen polyhedra around these lead atoms is more diYcult than in the Mn case owing to the irregular geometry around Pb2+.It is also important to keep in mind the possible lone-pair eVect of PbII which has very important implications in coordination environment. It has been assumed that Pb–O interactions occur for distances >15% of the Shannon average PbII–O bond distance in an eight-fold oxygen coordination resulting in a limiting Pb–O bond distance of 3.09 A° .Shannon average Pb–O bond distances in 5, 6, 7, 8, 9 and 10 oxygen coordinations are: 2.59, 2.61, 2.63, 2.69, 2.75 and 2.80 A° , respectively. The Pb–O bond distances are given in Table 4. With this criterion, there are two five-, two eightand two nine-coordinated lead atoms. It is of note that both five-coordinated lead atoms have two oxygens at quite short interacting distances of ca. 3.15 A° so they may be conceived as six- or even seven-coordinated (see Table 4). As for the manganese compound, the four phosphonate groups are tetra- Fig. 7 Crystal structure of Pb6(O3PCH2CO2)4 down to the a-axis with hedral and the four carboxy groups are trigonal. the numbering scheme used in Table 3. Only Pb–O bonds shorter than The structure of Pb6(O3PCH2CO2)4 (Fig. 7) is fairly similar 2.95 A° are shown for clarity.to that of the manganese analogue. This is expected as both compounds have the same stoichiometry and the same organoall in general positions. There are three crystallographically inorganic covalent building block [O3PCH2CO2]3-. For M= independent manganese atoms. Shannon average MnII-O bond Pb, there are also two types of lead layers with higher and distances are 2.15 A° for four-fold oxygen coordinations, and lower oxygen coordination numbers.One layer is formed by 2.23 A° for six-fold oxygen coordinations. If it is assumed that Pb(1)MPb(4) which are eight and nine-coordinated. The other Mn–O interactions take place within 15% of the reported type of layer is built up of Pb(5) and Pb(6) which are fiveaverage MnII–O bond distances, then, three types of manganese coordinate.However, the arrangements of the carboxy phoscoexist in this structure. Although somewhat arbitrary, this phonate chains between these layers are diVerent in both assumption allows us to define the coordination polyhedra. materials. To satisfy the coordination requirement around the Hence, Mn(1) is surrounded by five oxygens with bond lead layer with lower coordination number, two carboxy distances ranging between 2.09 and 2.19 A° , with a long inter- phosphonate chains point with the phosphonate heads towards action to a sixth oxygen at 2.68 A° .Mn(2) is surrounded by this layer and the tails to the other type of lead layer. four oxygens with bond distances between 2.12 and 2.17 A° , To summarise, we have studied two acetophosphonates, with two long interactions at 2.47 and 2.75 A° . Mn(3) is six- Mn3(O3PCH2CO2)2 and Pb6(O3PCH2CO2)4, which show high coordinate with bond distances between 2.06 and 2.40 A° .The thermal stability. Although the syntheses were carried out at two phosphonate groups are tetrahedral and the two carboxy low pH (4.3 and 1.8, respectively) they do not result in free groups are trigonal.carboxylic groups which would presumably result in more The crystal structure of Mn3(O3PCH2CO2)2 viewed down open structure. These structures are diVerent and clearly the c-axis is displayed in Fig. 5. Infinite chains of oxide more compact than those shown by their analogous 2-carbmanganese polyhedra [Mn(1) and Mn(3)] run parallel to the oxyethylphosphonates, i.e.Zn3[O3P(CH2)2CO2]218b and b-axis, and share edges. The chains can be described as Co3(O3PC2H4CO2)2·6H2O.21 This is mainly due to the pres- Mn(1)2O8 dimers linked to Mn(3)2O8 dimers by a common ence of an extra methylene group which leads to a more edge formed by O(1) and O(5). The Mn(1)MO(1)MMn(3) hydrophobic region which pillars the metal layers.and Mn(1)MO(5)MMn(3) angles are 106.5 and 100.7°, respectively. Mn(1)2O8 dimers form by sharing of an edge This work was supported by the research grants FQM-113 of with two symmetry equivalent Mn(1)MO(6)MMn(1) angles Junta de Andalucý�a (Spain). We thank Drs. E. Dooryhee, G. of 103.2(4)°. Dimers Mn(3)2O10 also form by sharing an Vaughan and A.Fitch for assistance during data collection on edge with two symmetry equivalent Mn(3)MO(2)MMn(3) angles of 96.4(4)°. These edge-sharing infinite chains can BM16 and to ESRF for the provision of synchrotron facilities. 2484 J. Mater. Chem., 1998, 8(11), 2479–24851993, 32, 1357; (c) G. Alberti, F. Marmottini, S. Murcia-Mascaros References and R. Vivani, Angew. Chem., Int.Ed. Engl., 1994, 33, 1594; (d) M. E. Thompson, Chem.Mater., 1994, 6, 1168; (e) D.M. Poojary, 1 A. Clearfield, Prog. Inorg. Chem., 1998, 47, 371; S. Drumel, B. Zhang, P. Bellinghausen and A. Clearfield, Inorg. Chem., 1996, V. Penicaud, D. Deniaud and B. Bujoli, Trends Inorg. Chem., 35, 5254. 1996, 4, 13. 18 (a) G. Cao, L. K. Rabenberg, C. M. 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Chem., 1985, 18, 367. 1988, 27, 2781; (b) G. Cao, H. Lee, V. M. Lynch, J. S. Swinnea 26 P. M.WolV, J. Appl. Crystallogr., 1968, 1, 108. and T. E. Mallouk, Inorg. Chem., 1990, 29, 2112; (c) A. Cabeza, 27 G. S. Smith and R. L. Snyder, J. Appl. Crystallogr., 1979, 12, 60. M. A. G. Aranda, M. Martinez-Lara, S. Bruque and J. Sanz, Acta 28 A. C. Larson and R. B. von Dreele, Program version: PC, summer Crystallogr., Sect. B, 1996, 52, 982. 96, Los Alamos National Lab. Rep. No. LA-UR-86-748, 1994. 10 (a) D. M. Poojary, B. Zhang, P. Bellinghausen and A. Clearfield, 29 A. Le Bail, H. Duroy and J. L. Fourquet, Mater. Res. Bull., 1988, Inorg. Chem., 1996, 35, 4942; (b) A. Cabeza, M. A. G. Aranda, 23, 447. S. Bruque, M. D. Poojary, A. Clearfield and J. Sanz, Inorg. Chem., 30 P. Thompson, D. E. Cox and J. B. Hasting, J. Appl. Crystallogr., 1998, 37, 4168. 1987, 20, 79. 11 R. W. Murray, Acc. Chem. Res., 1980, 13, 135. 31 L. W. Finger, D. E. Cox and A. P. Jephcoat, J. Appl. Crystallogr., 12 J. S. Facci, Langmuir, 1987, 3, 525. 1994, 27, 892. 13 G. G. Roberts, Adv. Phys.,1985, 34, 475. 32 A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, 14 (a) M. Gratzel, Pure Appl. Chem., 1982, 54, 2369; (b) J. K. M. Burla, G. Polidori and M. Camalli, SIRPOW92, J. Appl. Thomas, Acc. Chem. Res., 1988, 21, 275. Crystallogr., 1994, 27, 435. 15 M. A. Richard, J. Deustsch and G. M. Whitesides, J. Am. Chem. 33 G. M. Sheldrick, SHELXS86. Program for the Solution of Crystal Soc., 1979, 100, 6613. Structures, University of Go� ttingen, Germany, 1985. 16 H. Byrd, A. Clearfield, D. Poojary, K. P. Reis and 34 W. Dollase, J. Appl. Crystallogr., 1986, 19, 267. M. E. Thompson, Chem. Mater., 1996, 8, 2239. 35 H. M. Rietveld, J. Appl. Crystallogr., 1969, 2, 65. 17 (a) M. B. Dines, R. E. Cooksey, P. C. GriYth and R. H. Lane, Inorg. Chem., 1983, 22, 1003; (b) G. Alberti, R. Costantino, F. Marmottini and Z. P. Vivaniv, Angew. Chem., Int. Ed. Engl., Paper 8/04626C J. Mater. Chem., 1998, 8(11), 2479–248

ISSN:0959-9428    

DOI:10.1039/a804626c

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Structural investigations of new copper fluorides NaRECu2F8 (RE3+=Sm3+, Eu3+, Gd3+, Y3+, Er3+, Yb3+) C. De Nadaý�, A. Demourgues,* L. Lozano, P. Gravereau and J. Grannec ICMCB, Avenue du Dr A. Schweitzer, 33608 Pessac cedex, France. E-mail: demourg@icmcb.ubordeaux.fr Received 22nd April 1998, Accepted 30th June 1998 A series of copper(II ) and rare earth fluorides has been prepared.The crystal structure of NaGdCu2F8 has been solved from single crystal data and refined to conventional R=0.028 (wR=0.048) for 585 independent reflections. The symmetry is tetragonal (space group I422) with a=5.407(1) and c=10.382(1) A° (Z=2). A Rietveld analysis of NaYCu2F8 and refinements of the powder X-ray diVraction data for the other compounds of the series confirmed the I422 space group.The structure consists of layers of CuF4 square-planes perpendicular to the c-axis, interleaved alternatively by Na+ or RE3+ cations located in antiprisms. 4/mmm with systematic extinctions h+k+l=2n+1, consistent Introduction with space groups I422, I4mm, I492m, I49m2, I4/mmm. During recent years a wide range of ternary fluorides with Intensity data were collected on an Enraf Nonius CAD4 general formula AIMIIIF4 or AIIMIIF4 have been isolated: their four-circle automatic diVractometer using graphite-monochroinvestigation has led to the characterization of various struc- mated Mo-Ka radiation.Corrections were applied for Lorentz tural types given in ref. 1 and 2 (and references cited therein). and polarization eVects, followed by an empirical absorption When M is a d-transition element, the KBrF4-type3–5 is often correction using y-scan technique.For extinction corrections found in the presence of Jahn–Teller M2+ or M3+ cations. an empirical coeYcient e was used in the expression Fc(corr)= This is particularly true for CaCuF4 which exhibits a tetragonal Fc (1+10-3 eFc2 l3/sin2h).Intensity treatment and refinement symmetry6,7 (space group I4/mcm). This structure is related to calculations were performed using the SHELXL93 program.10 the fluorite-type by cationic ordering, yielding a doubling of Atomic scattering factors and anomalous dispersion paramthe c parameter value. The whole of the cations shows a fcc eters were taken from ref. 11. Crystal data and experimental stacking, but an important shift of the anionic sublattice leads conditions are listed in Table 1.The quality of the acquisition to define two types of environment for the cations: MF4 and refinement was based on the conventional reliability square-planes at z=0 and 1/2 which are isolated one from factors Rint and R1, wR2 respectively. another. Each layer of (MF4)2- polyhedra are shifted relative Full crystallographic details, excluding structure factors, to the other ones and the larger cations A are located between have been deposited at the Cambridge Crystallographic Data these layers at the center of AF8 antiprisms.Centre (CCDC). See Information for Authors, J. Mater. Some studies devoted to quaternary fluorides belonging to Chem., 1998, Issue 1.Any request to the CCDC for this the scheelite family8,9 have shown that it was possible to material should quote the full literature citation and the substitute the M2+–M4+ couple for trivalent ions in LiREF4. reference number 1145/108. In view of these results, it seemed worthwhile to investigate Complementary studies of NaGdCu2F8 and NaYCu2F8 appropriate substitutions in ternary fluorides belonging to the phases were carried out by the Rietveld method.12 Powder XKBrF4- type, which could induce a cationic ordering in eight- ray diVraction (XRD) profiles were recorded on a Philips PW fold coordination.The present paper reports the structural determination of Table 1 Crystal data and experimental conditions for data collection several quaternary NaRECu2F8 fluorides.formula NaGdCu2F8 symmetry tetragonal Experimental space group I422 (no. 97) Preparation of the compounds unit-cell parameters/A° a=5.407(1) c=10.382(1) Polycrystalline samples were synthesized by solid state reac- volume/A° 3 303.5(1) tions from stoichiometric mixtures of the binary fluorides. The Z 2 starting materials were mixed under a dry argon atmosphere Dc/g cm-3 5.03 crystal size/mm 0.19×0.14×0.21 in a glove box due to oxygen and moisture sensitivity.The radiation Mo-Ka (l=0.71073 A° ), reactions were carried out in sealed platinum tubes for 15 h at graphite monochromator 550 °C. The reaction mixture was annealed several times in absorption coeYcient, m/mm-1 18.0 the same conditions with intermediate grindings in order to F(000) 410 obtain pure powders.All reactions were followed by a tempera- measuring range/degrees hmax=45 ture quenching. scan type v–2h index ranges -10h10, -10k10, 0l20 reflections collected 2578 X-Ray diVraction analysis independent reflections 636 [585 Fo2>2s(Fo2)] extinction coeYcient, e 0.06723 Single crystals were obtained by melting about 2 g of Rint 0.0826 NaGdCu2F8 at 700 °C followed by cooling to room tempera- final R indices [I>2s(I )] R1=0.0279; wR2=0.0484 ture at a rate of 3 °C h-1.goodness of fit, S 1.201 Weissenberg and precession photographs of a single crystal final Fourier residuals/eA° -3 -1.57, 1.33 showed a tetragonal symmetry belonging to the Laue class J. Mater. Chem., 1998, 8(11), 2487–2491 2487Table 2 Rietveld data and experimental conditions for data collection Table 3 Comparison of refined isotropic atomic displacements (A° 2) in NaGdCu2F8 formula NaGdCu2F8 NaYCu2F8 symmetry tetragonal tetragonal Atom Ueq (I4/mmm) Ueq (I422) twinned single crystal space group I422 (no. 97) I422 (no. 97) unit-cell parameters/A° a=5.408(1) a=5.370(1) Gd (2a) 0.0063(2) (2a) 0.0066(1) Na (2b) 0.0231(11) (2b) 0.0107(5) c=10.390(1) c=10.291(1) volume/A° 3 303.9(1) 296.8(1) Cu (4d) 0.0023(2) (4d) 0.0099(1) F (32o) 0.0163(7) (16k) 0.0161(4) Z 2 2 Dc/g cm-3 5.02 4.37 radiation Cu-Ka (l=1.5418 A° ), graphite monochromator peak shape function g=0.58 g=0.25 PV=gL+(1-g)G crystallizes in the I422 space group with unit-cell parameters FWHM function H2=Utan2h+Vtanh+W a=5.407 and c=10.382 A° .The final values of atomic coordimeasuring range/degrees 15<2h<100 16<2h<100 nates and anisotropic atomic displacements are listed in reflections collected 126 126 Table 4.Table 5 gives the main interatomic distances and parameters used 29 24 angles. The bond lengths obtained are close to the sum of the in refinement Shannon radii18 (for Na+ and Gd3+ with C.N.=8 and for cRp 0.190 0.152 cRwp 0.150 0.118 Cu2+ with C.N.=4; in each case F- is threefold coordinated RI 0.042 0.038 to cations as in CaCuF4).In addition, piezoelectric measurements were carried out in order to show the occurrence of a non-centrosymmetrical group. Confirmation of the postulated space group was not 1050 diVractometer in Bragg–Brentano geometry, using graphpossible using this technique.ite-monochromated Cu-Ka radiation. The sample was set into Finally, a powder diVraction study seemed necessary to an air-tight cell, filled in a dry atmosphere, by dusting the avoid the twinning eVect observed in the single-crystal investi- powder through a 20 mm sieve in order to minimize orientation gation. The structure of NaGdCu2F8 was refined by the eVects. Owing to the Bragg–Brentano geometry of the equip- Rietveld method.12 However, during the synthesis it was ment, the mylar windows of the cylindrical cell are crossed by impossible to avoid traces of hexagonal NaGdF4;19 this phase the X-ray beam over a quasi-constant thickness.Data were has been taken into account in the refinement on the basis of collected over 152h100°, in 0.02° steps, with integration the structural hypothesis of NaNdF4.20 One should note that times of 20 s.the RI factor relating to this impurity remains high (ca. 30%) The refinements were performed with the FULLPROF owing to its rather small quantity. Moreover the peaks of program package.13 The background level was optimized with NaGdF4 are not well separated from the main diVraction lines a polynomial function and the peak shape s fitted by a of the new compound.The I422 and I4/mmm space groups pseudo-Voigt function. The change in the peak full-width at were tested, following exactly the same procedure. The com- half-maximum (FWHM) across the diVraction pattern was parison of the reliability factors and of the isotropic thermal defined by the function determined by Caglioti et al.14 The displacements showed that the best results were obtained with reliability factors were the usual ones in Rietveld method (RI, I422 space group (Table 6).In addition, the negative value of Rp and Rwp).15 The powder data and experimental conditions Biso found for the sodium led us to assume a small exchange are given in Table 2. between Gd3+ and Na+ ions in the 2a and 2b sites respectively.As is shown in Table 6, the best refinement corresponds to an Structural determination exchange rate equal to 3.5%. This is consistent with the similarity between both these sites. Nevertheless such a result NaGdCu2F8 was not observed in the single crystal study. It could be explained considering the mode of cooling in each case : the First, the structure was solved in the I4/mmm space group, by conventional Patterson method for the gadolinium and copper single crystal was subjected to a very slow cooling which favours a complete ordering, whereas the powder was cooled atoms positions; sodium and fluorine atoms were located from a diVerence-Fourier synthesis.The results were in good agree- by a temperature quenching which could generate an exchange between two similar sites.However this exchange rate of 3.5% ment with a previous structural hypothesis concerning NaEuCu2F8.16 The refinement led to R1=0.0341 and wR2= has been tested for the single crystal, and the results of refinement are quite acceptable. Thus, the structural param- 0.0786 with anisotropic atomic displacements, but required to take into account a random distribution of fluorine atoms eters of powder XRD are in good agreement with the crystal study, and the reduced coordinates correspond as near as one around copper, the site (32o) being 50% occupied.Secondly, calculations were carried out in the other space groups; the and the half standard deviation. Moreover the best argument in favour of I422 space group best residual R values were obtained for the I422 space group if considering a twinned single crystal (R1=0.0279, wR2= was the determination of the structure factors for the (211) and (213) reticular planes.Indeed the intensity of the XRD 0.0484 with anisotropic thermal factors). The value of Flack’s index17 equal to 0.5 for the refinement in the I422 space group lines according to the Debye law [F(hkl )=A+iB]21 with hk0 and l0 diVers strongly depending on the space could reveal the occurrence of a twin in our crystal; a and baxes inversion (50%) brought the value of Flack’s index close group: the centrosymmetric space group I4/mmm leads to B= 0 whereas in the I422 space group B diVers from 0.The to zero and could explain the results obtained in the I4/mmm space group.Furthermore the refined atomic displacements U comparison of X-ray diVraction patterns for both hypotheses in Fig. 1 and the values of structure factors in Table 7 showed considering the I4/mmm space group or the hypothesis of a twinned single crystal with the I422 space group have been clearly these diVerences and a best fit in I422 space group. Finally the powder diVraction study confirms the single compared for each element (see Table 3).As far as the I4/mmm hypothesis is concerned, owing to the results found for Gd crystal structure determination and one can conclude that NaGdCu2F8 crystallizes in the I422 space group. In order to and F atoms, the Ueq value of Na atoms appeared too large whereas that of Cu atoms was too small.The refinement in verify if all the compounds of the NaRECu2F8 series exhibit exactly the same structure, another phase involving a smaller the I422 space group gives more suitable Ueq values. According to the crystallographic data, NaGdCu2F8 rare earth ion such as Y3+ was studied by powder XRD. 2488 J. Mater. Chem., 1998, 8(11), 2487–2491Table 4 Atomic coordinates and displacements (A° 2) in NaGdCu2F8 [estimated standard deviations (esds) in parentheses] Atom Site x y z U11 U22 U33 U23 U13 U12 Ueq Gd 2a 0 0 0 0.0065(1) 0.0065(1) 0.0069(2) 0 0 0 0.0066(1) Na 2b 0 0 0.5 0.0102(7) 0.0102(7) 0.0116(11) 0 0 0 0.0107(5) Cu 4d 0 0.5 0.25 0.0099(2) 0.0099(2) 0.0097(2) 0 0 0 0.0099(1) F 16k 0.1872(5) 0.3303(5) 0.3763(2) 0.0158(8) 0.0132(8) 0.0191(10) 0.0025(6) -0.0083(7) -0.0018(9) 0.0160(4) Table 5 Interatomic distances (A° ) and angles (°) in NaGdCu2F8 (esds in parentheses) (4×) 1.893(2) CuMF (4× ) 2.785(2) GdMF (8× ) 2.314(2) NaMF (8× ) 2.422(2) FMCuMF 87.81(13) FMGdMF 72.05(4) FMNaMF 70.42(8) 92.34(13) 74.24(9) 73.66(4) 175.95(13) 86.02(12) 84.93(11) 112.56(9) 115.94(8) 133.29(13) 130.61(12) 152.61(13) 153.85(13) Table 6 Isotropic thermal displacements for diVerent structural hypotheses and corresponding R factors for NaGdCu2F8 (esds multiplied by Berar’s factor in parentheses) Biso/A° 2 Space group 2a 2b 4d 32o/16k RI cRp cRwp I4/mmm 0.07(16) 0.9(9) 0.9(3) 2.4(7) 0.070 0.214 0.176 I422 without exchange 0.50(13) -1.6(5) 0.6(2) 1.2(3) 0.044 0.195 0.154 I422 with exchange 96.5%/3.5% 0.31(12) 0.5(6) 0.9(2) 1.4(3) 0.042 0.190 0.150 Table 7 Comparison of the observed and calculated structural factors in arbitrary units for the two reticular planes (211) and (213) in the case of the single crystal study F(hkl )=A+iB I422 with twinned I422 single crystal I4/mmm hkl Fo Fc Fo Fc Fo Fc 211 8691 8690 8663 8684 8046 6466 213 9127 9230 9104 9254 8581 7244 NaYCu2F8 The structure of NaYCu2F8 was refined by the Rietveld method.The previous structural results of NaGdCu2F8 were used as a starting model. The I422 and I4/mmm space groups were considered as structural hypothesis; we did not succeed in obtaining good results with the latter (cRp=0.200, cRwp=0.184, RI=0.0875). On the contrary, the refinement in the I422 space group rapidly converged to cRp=0.162, cRwp=0.127 and RI=0.050. A negative thermal displacement on the sodium site and a large one on the yttrium site led us to consider an exchange rate of 8% between Y3+ and Na+ ions in the 2a and 2b sites respectively (cRp=0.152, cRwp=0.118, RI=0.038).The refined atomic coordinates and isotropic thermal displacements are listed in Table 8. The main interatomic distances and angles are given Table 8 Atomic coordinates and isotropic thermal displacements (A° 2) in NaYCu2F8 (esds multiplied by Berar’s factor in parentheses) Occupancy Atom Site x y z Biso (%) Y 2a 0 0 0 0.6(2) 92 Na 2a 0 0 0 0.6(2) 8 Y 2b 0 0 0.5 1.1(7) 8 Fig. 1 X-Ray diVraction patterns of NaGdCu2F8: (a) observed (,), Na 2b 0 0 0.5 1.1(7) 92 calculated (——) and (b) diVerence; the tick marks labelled (c) repre- Cu 4d 0 0.5 0.25 1.0(2) 100 sent the position of the diVraction lines for NaGdF4 (top) and for F 16k 0.1897(14) 0.3332(14) 0.3775(12) 1.2(3) 100 NaGdCu2F8 (bottom).J. Mater. Chem., 1998, 8(11), 2487–2491 2489Table 9 Interatomic distances (A° ) and angles (°) in NaYCu2F8 (esds multiplied by Berar’s factor in parentheses) (4×) 1.887(10) CuMF (4× ) 2.775(9) YMF (8× ) 2.274(9) NaMF (8× ) 2.414(9) FMCuMF 88.3(8) FMYMF 72.1(4) FMNaMF 69.4(5) 91.9(6) 74.3(6) 74.2(4) 174.7(9) 85.7(6) 84.3(6) 112.6(5) 117.0(5) 133.6(7) 130.1(6) 152.2(7) 153.9(7) Table 10 Unit-cell parameters of the NaRECu2F8 series Compounds a/A° c/A° V/A° 3 NaSmCu2F8 5.426(1) 10.430(3) 307.1(2) NaEuCu2F8 5.412(2) 10.398(2) 304.5(3) NaGdCu2F8 5.407(1) 10.382(1) 303.5(1) NaErCu2F8 5.360(1) 10.270(2) 295.0(1) NaYbCu2F8 5.354(2) 10.225(2) 293.1(3) NaYCu2F8 5.370(1) 10.290(1) 296.8(3) in Table 9.The NaMF and CuMF bond lengths are of the same order of magnitude as those obtained in NaGdCu2F8. The YMF distance is shorter than GdMF for the same site 2a, in good agreement with the smaller ionic radius of Y3+.18 The NaRECu2F8 series This series has been studied for several rare earths.As far as the largest rare earth ion is concerned, only the phase containing Sm3+ has been isolated when SmF3 was stabilized in the variety which crystallizes in orthorhombic symmetry. The hexagonal form of SmF3 did not react with the other binary fluorides to give the quaternary compounds. Therefore the series exists only with rare earth fluorides which adopt the YF3-type structure (space group Pnma).22 The Lu3+ ion is expected to be the lower limit of the series.The phases containing the cations Sm3+, Eu3 +, Gd3 +, Er3+, Yb3+ and Y3+ have been studied. For each compound, the unit-cell parameters have been determined from slow powder diVraction pattern (angle step: 0.02° and counting time: 10 s) using a silicon standard, followed by least-squares refinement.All parameters are listed in Table 10. The unit-cell volume decreases gradually as a function of the rare earth size. Fig. 3 (001)-projection of NaGdCu2F8 structure: (a) Gd environment, (b) Na environment (surrounded fluorine atoms are located below the Na or RE atom plane). The respective distances are indicated Fig. 4 (001)-projection of CaCuF4 structure (surrounded fluorine Fig. 2 Perspective view of NaGdCu2F8 structure. atoms are located below the Ca atom plane). 2490 J. Mater. Chem., 1998, 8(11), 2487–2491square-planes is smaller compared to NaGdCu2F8 compound Description of the structure and discussion for instance. Thus the raising of ion exchange rate could be The structure of NaRECu2F8 can be related to that of CaCuF4 associated with the decreasing of the [CuF4]2- square-plane (space group I4/mcm)6,7 which crystallizes in the KBrF4-type.5 distortion, and also are related to the electronic configuration A perspective view is represented in Fig. 2.Projections of of the rare earth. NaGdCu2F8 and CaCuF4 on the (001) plane are shown in Fig. 3 and 4. References The CaMF bond length in CaCuF4 is 2.349 A° .7 Owing to the cationic ordering in NaGdCu2F8, two distinct GdMF and 1 D. Babel, Struct.Bonding (Berlin), 1967, 3, 32. NaMF bond distances are found at 2.314 and 2.422 A° , 2 D. Babel and A. Tressaud, in Inorganic Solid Fluorides, Chemistry and Physics, Academic Press Inc., 1985, ch. 3, Crystal Chemistry respectively. of Fluorides. In the fluorite-type structure, the Ca2+ ions are eight-fold 3 S.Siegel, Acta Crystallogr., 1956, 9, 493. coordinated to fluorine and occupy a site of cubic symmetry. 4 W. G. Sly and R. E. Marsh, Acta Crystallogr., 1957, 10, 378. In NaRECu2F8, the occurrence of Cu2+ cations in square 5 A. J. Edwards and G. R. Jones, J. Chem. Soc. A, 1969, 1936. planar environment constrains the larger cations (Na+ and 6 D. Dumora, J. Ravez and P. Hagenmuller, Bull.Soc. Chim. Fr., RE3+) to be in more or less distorted tetragonal antiprisms 1970, 4, 1301. 7 H. G. von Schnering, B. Kolloch and A. Kolodziejczyk, Angew. depending on their size and their charge. Chem., 1971, 83, 440. The four short CuMF distances (1.893 A° ) are of the same 8 A. Vedrine, L. Baraduc and J-C. Cousseins, Mater. Res. Bull., order of magnitude, but slightly longer, than those encountered 1973, 8, 581.in CaCuF4 (1.880 A° ) and SrCuF4 (1.858 A° ).7 All these struc- 9 A. Vedrine, D. Trottier, J-C. Cousseins and R. Chevalier, Mater. tures are constructed from layers of isolated [CuF4]2- units Res. Bull., 1979, 14, 583. perpendicular to the c-axis and shifted with respect to each 10 G. M. Sheldrick, SHELXL93, A Program for Refinement of Crystal Structure, University of Go� ttingen, Germany, 1993.other. In each layer a [CuF4]2- square-plane is oriented at 90° 11 International Tables for X-ray Crystallography, Kynoch Press, from each adjacent one. The Na+ and RE3+ ions are located Birmingham, 1974, vol. 4 (present distributor: Kluwer Academic between these layers. Publishers, Dordrecht). In this structure, compared to the I4/mcm space group of 12 H.M. Rietveld, J. Appl. Crystallogr., 1969, 2, 65. CaCuF4, the disappearance of the sliding plane c leads to a 13 J. Rodriguez-Carvajal, FULLPROF, ver. 3.2, jan. 1997, LLBCEA, Saclay, France. small tilting of the [CuF4]2- square-planes from the bisecting 14 G. Caglioti, A. Paoletti and F. P. Ricci, Nucl. Instrum. Methods, plane [(a+b)/2, c] in contrast with CaCuF4, inducing a 1958, 3, 223. FMCuMF angle slightly diVerent from 90° (Table 5, Fig. 3). 15 R. J. Hill and R. X. Fisher, J. Appl. Crystallogr., 1990, 23, 462. Such a distortion of the square-planes has already been 16 N. Ruchaud, Ph. D. Thesis, University of Bordeaux I, France, observed in KAuF4 5 where it could be due to the strong 1991. Jahn–Teller eVect of AuIII in low-spin state. In the case of 17 H. D. Flack, Acta Crystallogr., Sect. A, 1983, 39, 876. 18 R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751. NaRECu2F8, such a tilting must be related to the cationic 19 R. E. Thoma, H. Insley and G. M. Hebert, Inorg. Chem., 1966, ordering which takes into account the diVerent size and charge 5, 1222. of Na+ and RE3+. 20 J. H. Burns, Inorg. Chem., 1965, 4, 881. However the higher ion exchange rate equal to 8% has been 21 M. VanMeersche and J. Feneau-Dupont, Introduction a` la cristalfound surprisingly for NaYCu2F8 where the diVerence of ionic lographie et a` la Chimie Structurale, Vander Ed., 1973. radii between Na+ and Y3+ is the largest. Nevertheless one 22 O. Greis and T. Petzel, Z. Anorg. Allg. Chem., 1974, 403, 1. should notice that in this latter phase the FMCuMF angle is closer to 90° and consequently the distortion of [CuF4]2- Paper 8/03015D J. Mater. Chem., 1998, 8(11), 2487–2491 24

ISSN:0959-9428    

DOI:10.1039/a803015d

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Determination of the fluorite related structure of Mn3Ta2O8 using synchrotron X-ray powder and electron diVraction data Saeid Esmaeilzadeh,a Jekabs Grinsa and Andy Fitchb aDepartment of Inorganic Chemistry Arrhenius Laboratory Stockholm University SE-106 91 Stockholm Sweden bESRF BP 220 F-380 43 Grenoble Cedex France Received 29th June 1998 Accepted 4th August 1998 The Mn2+-containing oxide Mn3Ta2O8 has been synthesised at 1200 °C in Ar atmosphere and its structure has been solved from X-ray synchrotron powder data (l#0.65 A° ) by direct methods. The structure was refined by the Rietveld method to RF=6.3% with space group I41/a a=11.2728(2) c=9.8030(3) A° V=1243.47 A° 3 from 1190 reflections with d0.65 A° . It is related to the fluorite structure with a#Ó5af and c#2af.The Ta atoms are octahedrally coordinated by oxygen atoms and the three crystallographically diVerent Mn atoms by 7 4 +4 and 4 oxygen atoms. Electron diVraction patterns show the presence of weak superstructure reflections corresponding to a primitive unit cell with a¾=a and c¾=6c. The melting point of Mn3Ta2O8 is 1470 °C in Ar atmosphere. It is a semiconductor with an activation energy of 1.2 eV and a conductivity s=3.7×10-5 S cm-1 at 600 °C. The magnetic susceptibility shows a maximum at 23 K and a Curie–Weiss behaviour at higher temperatures with meff=5.7(1) mB per Mn atom.When Mn3Ta2O8 is oxidised at 1100 °C in air an Mn–Ta oxide forms which has a wolframite type structure with unit cell a=4.7574(5) b=5.7296(6) c=5.1133(4) A° and b=91.202(9)°. 900 °C for 12 h and then at 1200 °C for 12 h with intermediate Introduction grinding and re-pelleting.The obtained sintered pellets were The phase relations of Mn-Ta oxides have been investigated brown. Higher and lower Mn contents yielded materials with by Turnock,1 at 1200 °C and partial pressures of oxygen from Mn4Ta2O9 and MnTa2O6 respectively as secondary phases. 10-17 to 1 atm. The oxides observed were the orthorhombic Corresponding syntheses above 1350 °C yielded partially columbite-type MnTa2O6 Mn4Ta2O9 with a corundum-related melted materials that contained Mn4Ta2O9 as the major phase. structure and four compounds MnTaO4 Mn1.4TaO4.2 The Mn3Ta2O8 phase was described by Turnock1 as Mn1.4TaO3.9 and Mn6Ta2O11 with structures undetermined at Mn1.4TaO3.9 stable only above 1160 °C and oxygen partial the time.Two additional Mn–Ta oxides have been reported pressures below ca. 2.5×10-2 atm. at 1200 °C. by Scho�nberg,2 namely the metallic compound Mn3Ta3O with A Guinier–Ha�gg camera with Cu-Ka1 radiation was used the g-carbide type structure and Mn2TaO3 with an alleged for collection of powder diVraction patterns for phase identifi- CoSn (B35 type) structure. The latter of these phases appears cation. The films were measured with a computer-controlled to be quite unique and questionable considering the low microdensitometer. oxidation state of Ta. X-Ray powder diVraction data for structure determination Several of the above Mn2+-containing Mn–Ta oxides were and refinement were collected from a sample contained in a prepared by us for use as precursor materials in the synthesis 0.2 mm diameter spinning capillary on beam line BM16 at the of new Mn–Ta oxynitrides.3 Since the structures of most of ESRF Grenoble with l=0.652 782(3) A° in the 2h range them including ‘Mn6Ta2O11’ and ‘Mn1.4TaO3.9’ were not 1–57°.The detector arm with its nine detectors was scanned known we have investigated their structures and properties. at a continuous rate of 0.5 degrees min-1 and the electronic A study of ‘Mn6Ta2O11’ showed that the correct composition scalers and 2h encoder were read every 200 ms. The data were is Mn11Ta4O21.4 The crystal structure is trigonal (space group) subsequently normalised and rebinned and the counts from with a=5.3776(2) c=34.040(2) A° and can be described as the nine channels combined to yield the equivalent scan with built up from corundum-type Mn4Ta2O9 blocks alternating a step of 0.002°.The direct methods program SIRPOW917 with single MnO layers of octahedra. was used for solving the structure and the GSAS program The compound Mn3Ta2O8 is the oxide described by Turnock package8 for structure refinements. as ‘Mn1.4TaO3.9’. The present study comprises a determination Metal elemental analyses were made in a JEOL 820 SEM of its structure and characterisation of its thermal magnetic (scanning electron microscope) with the EDX (energy and electrical properties. The structure is found to be related dispersive X-ray) analysis system LINK 10000. to the cubic MX2 fluorite type. A vast variety of structure A JEOL 2000 FX microscope operated at 200 kV and with types can be derived from fluorite by ordered removal of the a double tilting goniometer with limitations of ±45° was used X atoms and/or ordering of metal atoms among them the for collecting electron diVraction (ED) patterns.The specimens A2B2X7 (MX1.75) pyrochlore5 and the rare-earth RE2X3 were crushed dispersed in butanol and then transferred to (MX1.5) C-type.6 Mn3Ta2O8 (MX1.6) is a new type within the holey carbon-coated copper grids. class of MX2 fluorites and related structures. Thermal analysis was carried out with a SETARAM Labsys TG-DTA16 instrument. The recordings were made in both Ar atmosphere and air and with heating/cooling rates of Experimental 5–10 °Cmin-1. The magnetic susceptibility was measured with a weak-field Samples of Mn3Ta2O8 were prepared by solid state reaction ac-susceptometer (Lake Shore 7130) in the temperature range in Ar atmosphere using a mixture of appropriate amounts 10–300 K using a magnetic field of 500 A m-1 and a frequency (3:1) of Mn(C2O4) and fine-grained Ta2O5 heated in Ni crucibles in a graphite furnace.The samples were fired at of 500 Hz. J. Mater. Chem. 1998 8(11) 2493–2497 2493 Table 2 Atomic coordinates for Mn3Ta2O8; [tetragonal a= The electrical conductivity was determined with an auto- 11.2782(2) A° c=9.8030(3) A° I41/a Z=4]. mated impedance spectrometer.9 Data were collected at 400–600 °C in the frequency range 10 Hz–1 MHz. The Atom Site x y z U/A° 2 measurements were made in N2 atmosphere on sintered discs 0.5 cm2 in area and 0.1 cm thick with a porosity of ca. 20% Ta 16f 0.42257(7) 0.05661(9) 0.1300(1) 0.00641(2) Mn1 16f 0.3780(5) 0.0505(3) 0.5949(3) 0.0077(1) and furnished with gold electrodes.Mn2 4a 0 1/4 1/8 0.0073(2) Mn3 4b 0 1/4 5/8 0.0071(2) Results O1 16f 0.362(1) 0.198(1) 0.221(1) 0.0043(3) O2 16f 0.460(1) 0.893(1) 0.013(1) 0.0043(2) Compositional analysis O3 16f 0.196(1) 0.312(1) 0.997(1) 0.0056(3) O4 16f 0.310(1) 0.961(1) 0.243(1) 0.0048(2) SEM investigations of polished surfaces of sintered Mn3Ta2O8 compacts revealed an average crystallite size of 10 mm. Twenty EDX point analyses on individual grains yielded a metal composition of 61(1) % Mn and 39(1)% Ta in agreement atoms. The remaining fourth oxygen atom was later located with the starting composition. from diVerence Fourier maps. The structure was refined using 1190 reflections in the 2h X-Ray powder diVraction range 5–57° (d0.65 A° ). The background was fitted by 20 The Guinier–Ha�gg powder pattern of Mn3Ta2O8 was indexed Chebyshev polynomial coeYcients.The half-width of the with a body-centred unit cell using the TREOR90 version of Bragg peaks was 0.014° at 2h=21.4°.MnTa2O6 and Mn4Ta2O9 the indexing program TREOR.10 The cell dimensions a= were included as secondary phases with collective thermal 11.2728(2) c=9.8030(3) A° were obtained using Si as internal parameters lattice parameters and phase fractions refined. A standard and 80 reflections for 2h<88°. The indexed powder total of 74 parameters were refined including 18 positional pattern is given in Table 1 for the first 20 observed lines. It and 8 thermal parameters for Mn3Ta2O8. The obtained atomic agrees with that given by Turnock1 but contains additionrdinates are given in Table 2.Selected interatomic weak reflections and the recorded d values are more accurately distances and bond valence sums11 are given in Table 3 determined. Systematic absences in the powder pattern (hk0 together with expected bond distances calculated from h,k<2n;00l l<4n) indicated the space group I41/a to be Shannon–Prewitt ionic radii:12 O2-(IV)=1.38 A° Ta5+(VI )= possible. The pattern showed furthermore that the sample 0.64 A° Mn2 +(HS,IV )=0.66 A° Mn2 +(HS,VII )=0.90 A° contained small amounts of Mn4Ta2O9 and MnTa2O6 as Mn2+(HS,VIII)=0.96 A° . The fit between observed and calcuimpurities. lated patterns is illustrated in Fig. 1. The corresponding refinement indices are Rwp=0.100 Rp=0.071 DwD=0.90 Structure determination and refinement RF=0.063 and x2=4.8. The refined phase fractions of MnTa2O6 and Mn4Ta2O9 were 8.7(1) and 1.5(1) wt.% The synchrotron powder pattern did not exhibit any more respectively corresponding to a total Mn content of 58.3%.Bragg reflections than those observed in Guinier–Ha�gg films and could thus not validate the lower symmetry and larger unit cell implied by the ED study (see below). It was therefore decided to solve the structure using the unit cell obtained from Table 3 Bond distances (A° ) and bond valence sums for Mn3Ta2O8. the X-ray data with a#11.3 and c#9.8 A° . Ta–O3 1.94(2) Mn1–O3 1.98(2) Mn2–O4 4×2.24(1) Observed integrated intensities were extracted from a part O1 1.95(1) O3 2.06(1) O4 4×2.63(1) of the synchrotron data and converted to |F|2 values. A partial O4 2.00(1) O1 2.18(1) mean 2.44 expected 2.34 structure was then derived in space group I41/a by direct O2 2.01(1) O1 2.34(1) methods.The SIRPOW91 program successfully located the O4 2.02(1) O2 2.44(1) Mn3–O2 4×2.00(1) positions of the metal atoms and three out of the four oxygen O2 2.21(1) O4 2.47(1) mean 2.00 expected 2.04 O4 2.57(1) mean 2.02 expected 2.02 mean 2.29 expected 2.28 Table 1 Observed and calculated 2h values for the Guinier–Ha�gg bond valence sums Ta +4.74 O1 -1.86 diVraction pattern of Mn3Ta2O8 up to the twentieth observed line. Mn1 +2.07 O2 -1.93 D2h=2hobs-2hcalc. [l=1.5406 A° cell figure-of-merit M20=98 F20= Mn2 +1.51 O3 -2.12 133 (0.0060 26)]. Mn2 +2.29 O4 -1.86 hkl 2hobs/degrees D2h dobs/A° I/I0 1 0 1 11.955 0.001 7.40 7 2 0 0 15.704 -0.006 5.64 1 1 2 1 19.785 -0.002 4.484 43 1 1 2 21.256 -0.007 4.177 8 2 2 0 22.292 0.005 3.985 7 3 0 1 25.363 -0.001 3.509 14 1 0 3 28.424 0.009 3.137 6 2 3 1 29.964 -0.011 2.980 2 3 1 2 30.977 -0.017 2.885 100 2 1 3 32.629 -0.002 2.742 11 4 1 1 34.017 0.003 2.633 3 4 2 0 35.576 -0.011 2.521 24 3 0 3 36.418 0.010 2.4651 1 0 0 4 36.636 -0.002 2.4509 9 3 3 2 38.501 -0.009 2.3364 1 3 2 3 39.881 0.007 2.2587 3 4 2 2 40.203 0.006 2.2413 2 5 0 1/4 3 1 41.044 -0.002 2.1973 11 4 1 3 43.112 0.007 2.0966 3 Fig.1 Observed (crosses) calculated (solid line) and diVerence 2 2 4 43.308 0.002 2.0875 1 (bottom) X-ray diVraction patterns of Mn3Ta2O8 for 2h=17–29°. 2494 J. Mater. Chem. 1998 8(11) 2493–2497 Electron diVraction group symmetries. The radius of the first order Laue zone indicated furthermore a c axis of ca. 30 A° . The crystal was ED patterns of Mn3Ta2O8 showed reflections that matched then tilted around the b axis.In the [1039] zone [Fig. 1(b)] the I-centred unit cell with a#11.3 and c#9.8 A° that was there are relatively strong superstructure reflections with indiderived from the X-ray data but also sets of weaker reflections ces such as 1 0 1/3 and 2 0 2/3 which can be accounted for that implied both a lower symmetry and a larger unit cell. These by a primitive cell with a tripled c axis. In the [1902] zone superstructure reflections were yielded by all crystallites [Fig. 1(c)] there are weak reflections present with indices such studied. as 1 0 1/2 implying a doubled c axis. The smallest unit cell DiVraction patterns along [001] [1039] and [1902] are shown able to index all reflections is thus found to be a primitive in Fig. 2. The strongest reflections in the <001> zone tetragonal with a¾=a and c¾= 6c#58.8 A° .The sets of super- [Fig. 1(a)] correspond to a fluorite-type subcell. The a axis of structure reflections have diVerent relative intensities. Those the I-centred unit cell is defined by the reciprocal lattice vector that indicate a tripled c axis are the strongest the ones that a*=1/5(2a*f+b*f) as illustrated. The presence of weaker indicate a primitive cell are moderately strong and the ones reflections of the type 100 shows however that the cell is indicating a doubled c axis are the weakest. primitive and thus also that the glide plane perpendicular to Attempts were made to refine the structure both with a the a axis is absent. This suggests P41 or P49 as possible space lower symmetry and/or larger unit cells as implied by the electron diVraction data.Space group symmetries such as I49 P49 and P41 and larger unit cells with c¾=3c and c¾=6c were tried. None of these refinements yielded any significant improvements. This was partly expected since the powder pattern manifests no reflections corresponding to a primitive or larger cell and any information about a superstructure is thus provided only by the main reflections. Refinements with anisotropic thermal parameters were also carried out in order to see if any atoms would show anomalous thermal displacements but none were revealed and these refinements yielded only insignificantly lower R values. Structure description The structure derived for Mn3Ta2O8 is illustrated in Fig. 3. There are four diVerent metal positions one occupied by Ta and three by Mn atoms.The metal atoms are nearly in cubic close packing and the positional shifts from the metal array found in a cubic fluorite structure are small. The metal cubooctahedra around the Mn and Ta atoms are only slightly distorted and the metal–metal distances have a mean value of 3.54 A° and range between 3.23 and 3.81 A° . The structure can be envisaged as built up from four layers of metal–oxygen atom polyhedra that are related to each other by the 41 axis in the c direction. One such layer is shown in Fig. 3(a). The Ta atoms are coordinated by six O atoms forming a distorted octahedron a common coordination polyhedron for Ta5+. The Ta–O mean distance is 2.023 A° which agrees well with an expected value of 2.02 A° . The structure contains pairs of Ta–O octahedra that share an edge and these pairs are in turn connected to each other by cornersharing.The framework formed by the Ta–O octahedra is illustrated in Fig. 3(b). The Mn1 atoms are coordinated by seven O atoms at distances of 1.98(2)–2.57(1) A° . The coordination polyhedra can be idealised as a cube with one corner missing. The mean distance 2.29 A° agrees well with an ionic radius sum of 2.28 A° for Mn2+(HS,VII ) and O2-(IV). The Mn1 atoms are located at positions that are displaced from the Ta atom positions by z#1/2. The structure thus contains strings of Mn1–empty–Ta–empty polyhedra along the c axis related to each other by a 49 symmetry axis. The Mn2 and Mn3 atoms are found on the special positions 4a and 4b respectively both with site symmetries 49 in the channels between the Mn1–Ta strings of polyhedra as seen in Fig.3(b). The Mn2 atoms are 4+4 coordinated by O4 atoms at distances of 2.24(1) and 2.63(1) A° . Each set of O4 atoms forms a tetrahedron and the resulting 8-coordination polyhedron may be described as a distorted cube. The mean Mn2–O distance is 2.44 A° which is longer than the expected value of 2.34 A° . Mn3 is tetrahedrally coordinated by four O2 atoms. The observed Mn3–O2 distance 2.00(1) A° is somewhat shorter than the expected value of 2.04 A° . Bond valence Fig. 2 Electron diVraction patterns for Mn3Ta2O8 along (a) [001] (b) [1039] and (c) [1902]. sums accord with the above bond-length considerations. While J. Mater. Chem. 1998 8(11) 2493–2497 2495 Fig. 4 Molar magnetic susceptity per Mn atom (a) and its inverse (b) versus temperature for Mn3Ta2O8.conductivity determined at 600 °C was s=3.7×10-5 S cm-1 and the relative dielectric constant was calculated from the capacitance to be 30. The temperature dependence of the conductivity was found to be characteristic of a semiconductor with an approximate activation energy value of 1.2 eV. Fig. 3 An illustration of the structure of Mn3Ta2O8 (a) a polyhedral Thermal analysis representation of a (001) section of the structure; (b) a polyhedral A DTA recording for Mn3Ta2O8 heated and then cooled in representation of the Ta atom arrangement. The positions of the Mn1 inert Ar atmosphere at a rate of 10 °Cmin-1 is shown in and Mn2/Mn3 atoms are illustrated by filled and open circles respectively. Fig. 5. The weight change of the sample after the heating– cooling run amounted to less than 0.5%.Small endothermal peaks are observed upon heating at ca. 1340 and 1380 °C they are reasonably satisfactory for the Ta Mn1 and O atoms before Mn3Ta2O8 melts at ca. 1470 °C. These may either too high and too low values are obtained for Mn3 and Mn2 respectively. Magnetic susceptibility The magnetic susceptibility per Mn atom xM of Mn3 Ta2O8 and its inverse xM-1 are shown in Fig. 4 as functions of the temperature T. The susceptibility shows a well-defined maximum at 23 K and a Curie–Weiss law behaviour xM=C/(T-h) above ca. 100 K. The eVective number of Bohr magnetons per Mn atom (meff) was determined from the Curie constant C to be 5.7(1) mB which is close to the expected value of 5.9 mB for Mn2+ in a high-spin state. Electrical conductivity Impedance spectra for Mn3Ta2O8 were measured in a heating –cooling cycle between 400 and 600 °C.The data showed no polarisation at the electrodes and the conductivities were Fig. 5 DTA recordings for Mn3Ta2O8 heated and then cooled in Ar atmosphere. calculated from the low-frequency parts of the spectra. The 2496 J. Mater. Chem. 1998 8(11) 2493–2497 suggest that the phase is of the wolframite (FeWO4) type.13 Its structure composition and formation will be further investigated. Concluding remarks There are a large number of structures related to the fluorite structure which can be derived from it by ordered removal of oxygen atoms or ordering of metal atoms. Mn3Ta2O8 is a new member of this class of compounds. Among the cations of the first-row transition metals the largest ionic radius is found for Mn2+ and the crystal chemistry of oxides containing Mn2+ and Ta5+ ions is expected to be influenced by this fact and by the comparatively large diVerence in size of these cations.The stability of fluorite-related structures increases with the size of the cation and such structures may therefore be expected for higher Mn2+ contents. The metal-to-oxygen ratio decreases Fig. 6 TG curve for the oxidation of Mn3Ta2O8 in air. with increasing Mn2+ content however and the Mn-rich compounds Mn4Ta2O9 (MO1.5) and Mn11Ta4O21 (MO1.4)4 have structures that are related to corundum. The Mn3Ta2O8 originate from phase transitions of Mn3Ta2O8 or be associated compound is accordingly found at an Mn–Ta–O composition with the impurity phases MnTa2O6 and Mn4Ta2O9 present. that satisfies both a relatively high metal-to-oxygen ratio Upon cooling an exothermal reaction starts at approximately MO1.6 and a high Mn content.1440 °C characteristic of a passage through a liquidus curve We have not been able to determine the superstructure and subsequent eutectic or peritectic solidification. In addition implied by the ED studies of Mn3Ta2O8 from the present X- a smaller exothermal transition occurs at ca. 1320 °C indicatray synchrotron data. It is very likely however that the ing a solid-state phase transition. superstructure with c¾=6c is associated with either modu- A TG recording of the oxidation of Mn3Ta2O8 in air upon lations of oxygen atom positions and/or an extended ordering heating at a rate of 5 °Cmin-1 is shown in Fig. 6. The largest of oxygen atoms around Mn2 and Mn3.part of the oxidation occurs in the temperature range Finally it can be remarked that presently available data 300–600 °C and is exothermal. A following smaller increase in indicate a series of uncharacterised phases in the system weight is observed up to ca. 850 °C and two broad endother- Mn2+–Mn3+–Ta5+–O among them the wolframite type phase mal reactions take place above 600 °C. The calculated metal– mentioned above and the phases denoted by Turnock1 as oxygen compositions at the observed plateaus at 600 and MnTaO4 and Mn1.4TaO4.2. Studies of phase formation and 850 °C are MO1.72 and MO1.75 respectively. crystal structures in these systems are presently in progress. A Guinier–Ha�gg film of the sample heated to 1100 °C in We have recently characterised a new cubic fluorite phase the TG run showed that a new phase had formed.Its powder Mn0.6Ta0.4O1.65 by techniques which include XRPD selected- pattern could be indexed by a monoclinic unit cell with a= area ED and high-resolution electron microscopy.14 The ED 4.7574(5) b=5.7296(6) c=5.1133(3) A° b=91.202(9)° and patterns of the phase exhibit prominent diVuse scattering V=139.35 A° 3. The pattern also contained reflections from similar to that reported for other cubic oxygen-deficient fluorite tetragonal Mn3O4 (JCPDS No. 24-734) and three additional compounds. reflections with relative intensities below 6% which could not be attributed to any reported oxide containing Mn and/or Ta. The authors thank Dr. T. Ho� rlin for help with the conductivity The indexed pattern for the new phase is given in Table 4 for measurements.Prof. M. Nygren is thanked for help with the the first 20 observed lines. The unit cell systematic reflection thermal analysis and for support and valuable discussions. absences (h0l l<2n) and the reflection intensities strongly This work has been financially supported by the Swedish Natural Science Foundation. Table 4 Observed and calculated 2h values for the Guinier–Ha�gg diVraction pattern of the Mn–Ta wolframite type phase up to the twentieth observed line. D2h=2hobs-2hcalc [l=1.5406 A° cell figure- References of-merit M20=85 F20=92 (0.0063 35)]. 1 A. C. Turnock J. Am. Ceram. Soc. 1966 49 382. hkl 2hobs/degrees D2h/degrees dobs/A° I/I0 2 N. Scho�nberg Acta Metall. 1955 3 14. 3 J. Grins P.-O. Ka� ll and G. Svensson J. Solid State Chem. 1995 0 1 0 15.464 0.011 5.73 1 117 48.1 0 0 18.640 0.000 4.756 9 4 J. Grins and A. Tyutyunnik J. Solid State Chem. 1998 137 276. 0 1 1 23.305 0.004 3.814 8 5 M. A. Subramanian G. Aravamudan and G. V. Subba Rao Prog. 1 1 0 24.295 -0.006 3.661 50 Solid State Chem. 1983 1 55. -1 1 1 29.770 0.001 2.999 99 6 R. Norrestam Ark. Kemi 1968 29 343. 1 1 1 30.230 -0.008 2.954 100 7 G. Cascarano L. Favia and C. Giacovazzo J. Appl. Crystallogr. 0 2 0 31.183 -0.013 2.866 19 1992 25 310. 0 0 2 35.082 0.003 2.556 30 8 A. C. Larson and R. B. Von Dreele Los Alamos National 0 2 1 35.904 -0.001 2.4992 55 Laboratory Report No. LA-UR-86-748 1987. 2 0 0 37.796 -0.002 2.3783 17 9 T.Ho� rlin Chem. Scr. 1985 25 270. -1 0 2 39.656 0.010 2.2709 4 10 P.-E. Werner L. Eriksson and M. Westdahl J. Appl. Crystallogr. 1 2 1 40.934 0.002 2.2029 8 1985 18 367. -1 1 2 42.797 0.007 2.1113 9 11 I. D. Brown and D. Altermatt Acta Crystallogr. Sect. B 1985 1 1 2 43.494 0.018 2.0790 10 41 244. 2 1 1 45.196 -0.013 2.0046 3 12 R. D. Shannon Acta Crystallogr. Sect A 1965 32 258. 0 2 2 47.638 -0.004 1.9074 10 13 H. Weitzel Z. Kristallogr. 1976 144 238. 2 2 0 49.792 0.001 1.8298 11 14 S. Esmaeilzadeh J. Grins and A.-K. Larsson in preparation. 1 3 0 51.533 0.010 1.7720 35 -2 0 2 51.928 0.006 1.7595 18 Paper 8/04938K -2 2 1 52.828 0.004 1.7316 23 J. Mater. Chem. 1998 8(11) 2493&nda

ISSN:0959-9428    

DOI:10.1039/a804938k

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Thermal evolution of cobalt hydroxides: a comparative study of their various structural phases Z. P. Xu and H. C. Zeng* Department of Chemical Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260. E-mail: chezhc@nus.edu.sg Received 23rd June 1998, Accepted 11th August 1998 Through an atmosphere-controlled method, a new phase of hydrotalcite-like Co hydroxide with mixed valent states has been synthesised, along with preparations of known a and b phases.Structural and thermal behaviours of all the Co hydroxides have been compared. Three major stages of decomposition are found: (i) 149–164 °C for dehydration of interlayer water, (ii) 185–197 °C and (iii) 219–222 °C for dehydroxylation of hydrotalcite- and brucite-like phases, respectively.Intercalated nitrate anions in hydrotalcite-like phases decompose largely during stage (ii). The oxide Co3O4 starts to form at temperatures as low as 165 °C especially for hydrotalcite-like phases. An intermediate compound, HCoO2, which is formed thermally, decomposes at 258–270 °C. The Co3O4 oxide converts into CoO at 842–858 and 935–948 °C respectively in nitrogen and air, which is much lower than the previously reported range of 1000–1200 °C.Surface areas of calcined samples are found to be proportional to the intercalated anion content. The catalytic activity of the resultant Co3O4 oxides with nitrous oxide is 7.2–8.2 mmol N2O g-1 h-1 at 375 °C, which is comparable to some reported active catalyst systems.conducted in liquid phase under static atmospheric conditions. Introduction Thus, oxidation of divalent transition metal cations by oxygen In recent years, divalent transition metal hydroxides including in static air, if occurring, may not be severe because transport their double hydroxides with trivalent p block (AIII ) or d of oxygen to the liquid phase is limited by the gas–bulk-liquid block metals have received increasing attention owing to their interface.However, if air (and hence oxygen) is constantly unique physico-chemical properties for electrochemical, supplied through bubbling to the liquid phase, the gas–liquid magnetic and catalytic applications.1–8 contact area can be significantly increased. The oxidation of One of the common features for this class of materials is Co2+(3d7) to Co3 +(3d6) has been recently observed in our their layered structure.9,10 A divalent metal cation is located preparation of the LDH compound MgIICoIICoIII-HT using a in the center of the octahedron formed by six hydroxyl groups.dynamic air-flow approach.11 In this connection, without The metal-octahedra then share edges to form two-dimen- involvement of MgII, the possibility of fabricating a CoIICoIII sionally infinite sheets, which is similar to the basic structure hydrotalcite-like phase had been indicated in our previous of brucite [Mg(OH)2].9,10 The brucite-like sheets can stack synthesis of Mg–Co mixed oxide spinels.12 upon each other to build a three-dimensional network accord- Here, we report a systematic investigation on preparation ing to various chemical interactions (mainly hydrogen bond- of Co hydroxide compounds using the dynamic gas-flow of ing) between the sheets.9,10 It has been well known for layered either protective (nitrogen) or oxidative gas (air).In addition double hydroxides (LDH) that when some of the divalent d to formation of a and b phases, the oxidation of Co2+(3d 7) block metal cations are substituted by a trivalent cation, a to Co3+(3d 6) occurs in the dynamic air-flow experiment, positive charge is generated in the brucite-like sheet. To restore leading to generation of a new type of CoIICoIII hydrotalciteoverall charge neutrality of solid, the extra positive charge can like compound. Using the well defined starting compounds (a be balanced by intercalating anion species into inter-brucite- and b phases, along with the newly found CoIICoIII hydrotallike- sheet space, resulting in a hydrotalcite-like phase [HT; cite-like phase), we are in a better position to conduct a named after the mineral compound Mg6Al2(OH)16CO3 comparative investigation on the thermal evolution of all these 4H2O] in most cases.9 Co hydroxides, which has been unclear in the literature owing Nevertheless, the formation and structure of layered mono- to insuYcient materials characterization (such as metal oxitransition metal hydroxides are not studied as explicitly as in dation state and anion content in the interlayer space) and LDH materials.9,10 For example, it has long been known that lack of mechanistic understanding on material formathere are two major types of nickel and cobalt hydroxides (a tion.2,3,6–8,13 The study will also correlate thermal evolution and b phases).1–8 The structure for the latter form (b) has with gaseous chemical constituents and chemical reactivity (an been identified as a brucite-like phase, but the former (a) has important index for catalytic application) of the thermally remained largely unknown regarding its actual formation obtained end product Co3O4 using N2O as a probe molecule.14 mechanism.Among many speculative models proposed for the a phase formation,1–5 two prominent models, ‘hydroxyl Experimental vacancies’ and ‘mixed valent state’ (with mixedM2+ andM3+) appear plausible.2,3 In particular, the ‘hydroxyl vacancies’ Materials preparation model has been experimentally confirmed very recently, which Two series of Co hydroxide samples (N1–N5 and A1–A6) reveals that the nickel and cobalt are strictly in the oxidation with various structural phases (brucite and hydrotalcite-like state of 2+ while the missing OH- groups in brucite-like and their mixtures) were prepared using an atmosphere- sheets are compensated by intercalated anions.8 controlled precipitation method.11 In the sample prepara- We have noted that most of these material syntheses are tion, 20.0 ml of 1.0 M aqueous cobalt nitrate solution [Co(NO3)2·6H2O, >99.0%, Fluka] was added into 100 ml 0.5 M ammoniacal solution in a three-necked round-bottom *Tel: +65 874 2896.Fax: +65 779 1936.J. Mater. Chem., 1998, 8(11), 2499–2506 2499Table 1 Sample nomenclature and preparation conditions further explain the DSC/DTA results, TGA measurements were carried out from 50–950 °C (at two heating rates: Addition Aging Structural 10 °Cmin-1 for 50–900 °C and 2 °Cmin-1 for 900–950 °C) Sample Atmosphere time time phasea with an air flow at 60 ml min-1. The as-prepared precipitates were also heat-treated at 200, N1 N2 15 s 5 min HT(II ) N2 N2 15 s 2 h HT(II ) 300 and 400 °C, respectively, for 2 h with static laboratory air N3 N2 15 s 4 h HT(II )+B(II ) in an electric furnace (Carbolite).Specific surface areas were N4 N2 15 s 6 h B(II ) determined by a multi-point BET method using a Nova-1000 N5 N2 15 s 8 h B(II ) Instrument. Prior to N2 adsorption–desorption measure- A1 Air 4 min 3 min B(II )+HT(II,III ) ment, each sample was degassed with N2 purge for 3 h at a A2 Air 4 min 2 h B(II )+HT(II,III ) temperature lower than its respective heat-treated temperature.A3 Air 4 min 4 h HT(II,III )+B(II ) In catalytic activity tests, 150 mg of 40–60 mesh sample A4 Air 4 min 8 h HT(II,III )+B(II ) calcined at 400 °C (2 h) were used in a tubular quartz reactor A5 Air 4 min 16 h HT(II,III ) (inner diameter=0.4 cm, V=0.088 cm3) in each run.In a A6 Air 4 min 24 h HT(II,III ) typical experiment, N2O gas (1 mol%, balanced with He) was aHT(II )=CoII hydrotalcite-like phase [i.e., a-Co(OH)2]; B(II )=CoII fed at a rate of 30 ml min-1 (F) through the catalyst bed brucite-like phase [i.e., b-Co(OH)2]; HT(II,III )=CoIICoIII hydrotalcite- like phase; order of structural phase appears according to intensity (GHSV=20 500 h-1).The GHSV was also increased to 41 000, of XRD data. 61 500, and 82 000 h-1 to investigate the eVect of feed rate on the catalytic activity. The outlet gases, cooled in a coil, were analyzed by gas chromatography (GC) on a Perkin-Elmer AutoSystem-XL (TCD detector) using a 4 ft Porapak Q flask within a given time (Table 1) under stirring. The atmos- (80/100 mesh) column.The oven temperature of GC was phere for precipitation was controlled by bubbling the solution maintained at 120 °C, and the flow rate of He carrier gas was with either purified nitrogen (Soxal, O2 <2 vpm) or purified 40 ml min-1. The catalytic activity for N2O decomposition air (Soxal, O2=21±1%, H2O<2 vpm, and hydrocarbons was evaluated in terms of conversion percentage, X=(Pi,N2O- <5 vpm) at a rate of 40 ml min-1 at room temperature.It Pf,N2O)/(Pi,N2O+0.5Pi,N2OPf,N2O), where Pi,N2O and Pf,N2O are should be noted that there are three preparative parameters inlet and outlet partial pressures of nitrous oxide.15,16 (atmosphere, addition time, and aging time) varied in these experiments (Table 1).The final pH values of the filtrate were in the range 8.5–8.2 depending on the addition/aging time in Results and discussion the experiment. Preparation of Co hydroxides with designed phases Materials characterisation In this work, three preparative parameters (atmosphere, addition time, and aging time) were varied. As indicated in Crystallographic information on the precipitates was Table 1, two diVerent hydrotalcite-like phases and one pure investigated by X-ray powder diVraction (XRD).The XRD brucite-like phase or their mixtures can be obtained using an patterns with diVraction intensity versus 2h were recorded in appropriate combination of these parameters. Fig. 1 shows a Shimadzu XRD-6000 X-ray diVractometer with Cu-Ka some representative XRD patterns which indicate that the radiation (l=1.5418 A° ) from 8–40° at a scanning speed of synthesis atmosphere gives a greater influence on crystallo- 3° min-1.Chemical bonding of cobalt–oxygen, hydroxyl and graphic structure of the resulting precipitates. some anions (mainly nitrate) was studied by FTIR (Shimadzu In a nitrogen atmosphere, an a phase precipitate can be FTIR-8101) using the potassium bromide (KBr) pellet techobtained upon fast addition and short aging (N1, Table 1).nique. The spectra were measured with a resolution of 2 cm-1 EA investigation reveals that there is no trivalent cobalt but and 100 scans were accumulated. Elemental analysis (EA) for there is anion intercalation in this phase (Table 2). On the nitrogen and carbon contents in the as-precipitated samples was performed using a Perkin Elmer 2400 CHN elemental analyzer.Cobalt content in the precipitate samples was determined by thermogravimetric analysis (TGA, Shimadzu TGA- 50) based on the end products Co3O4 at 600 and CoO at 950 °C. The trivalent cobalt content in some important precipitates (N1, N5, A1 and A6) was determined by a redox titration method in which 10.00 mg of solid sample were dissolved in 20.0 ml 1.0 M HCl solution upon gentle heating.The produced Cl2 was gradually purged with N2 (60 ml min-1) and passed through a 50.0 ml 0.01 M KI solution mixed with starch indicator. The resultant blue mixture was then titrated against a Na2S2O3 solution (0.0100 M) until disappearance of the blue coloration.DiVerential scanning calorimetry (DSC, Netzsch DSC200), diVerential thermal analysis (DTA, Shimadzu DTA-50), and thermogravimetric analysis (TGA, Shimadzu TGA-50) studies were carried out with various gas backgrounds in order to understand the thermal evolution of the prepared cobalt hydroxides. Samples in DSC measurements were heated from 40–400 °C at a rate of 10 °Cmin-1 under nitrogen with a gas flow rate of 15 ml min-1.To diVerentiate thermal processes at various heating stages, FTIR spectra were recorded for samples that were heated to a specified temperature in DSC measurements. In DTA measurements, the heating/cooling rate was Fig. 1 Representative XRD patterns for as-prepared Co hydroxides: kept the same as in DSC, but with air gas-flow at 60 ml min-1 N1, N5, A1, and A6, noting that major diVraction peaks of the samples are located in the reported 2h range. and with nitrogen gas-flow at 100 ml min-1, respectively.To 2500 J. Mater. Chem., 1998, 8(11), 2499–2506Table 2 Elemental analysis and TGA results for some representative samples Sample [Co3+]/[Co]a [NO3-+2CO32-]/[Co]b Structural phasec L1 (%)d L2 (%) L3 (%) N1 0 0.19 HT(II ) 8.9 11.8 4.6 N5 0 0.02 B(II ) 0.8 9.1 4.6 A1 0.04 0.13 B(II )+HT(II,III ) 3.6 13.4 4.0 A6 0.28 0.32 HT(II,III ) 8.0 16.5 4.7 Sample Co (%) NO3- (%) CO32- (%) H2O (%) Chemical formula N1 55.2 6.6 2.1 10.9 CoII(OH)1.81(NO3)0.11(CO3)0.04·0.65H2O N5 63.1 0 0 0.5 CoII(OH)2·0.03H2O A6 52.0 14.0 1.5 5.1 CoII0.72CoIII0.28(OH)1.96(NO3)0.26(CO3)0.03·0.3H2O aMole ratio of trivalent cobalt to total cobalt in the precipitate samples.bMole ratio of total charges of anion species to total cobalt in the precipitate samples; CO32- results from ambient CO2 dissolution. cThe phase notation is the same as in Table 1. dL1, L2 and L3 are weight loss percentages from 40–160 °C, 160–230 °C, and 230–500 °C. basis of XRD/EA/TGA studies, the precipitate (N1) of the hydrotalcite-like phase has an inter-brucite-like-sheet distance of 8.09 A° , and a chemical formula CoII(OH)1.81(NO3)0.11 (CO3)0.04 0.65H2O. In contrast, a sample similarly prepared in nitrogen but with a longer aging time is a pure brucite-like compound (N5, Fig. 1). Again, cobalt is strictly divalent and there are virtually no anions or water intercalated in this compound.The inter-brucite-like-sheet distance determined by XRD is 4.66 A° , which is identical to the literature data for the b phase.17 From N1 to N5 (Table 1) the sequential evolution from CoII(OH)1.81(NO3)0.11(CO3)0.04 0.65H2O to CoII(OH)2 can be conducted by changing the aging time. A twophase material is seen in sample N3 in which both CoII(OH)1.81(NO3)0.11 (CO3)0.04 0.65H2O and CoII(OH)2 are present.The above CoII hydrotalcite-like to CoII brucite-like transformation occurs under a protective atmosphere of nitrogen. However this transformation can be steered in the reverse direction when nitrogen is replaced by air. A predominant brucite-like phase can be prepared within 4 min (A1, Table 1 and 2, Fig. 1). Surprisingly, this newly formed brucite-like phase can be transformed into a pure hydrotalcite-like phase Fig. 2 DSC curves for the sample series prepared under nitrogen upon prolonged aging in air. As demonstrated in the A1–A6 atmosphere: N1–N5. series, bi-phasic mixtures are observed in A1–A4 while hydrotalcite- like compounds are found in A5 and A6. Related to this phase evolution, the Co3+ to total cobalt ratio is increased evolution revealed by XRD, a smooth transition from the from 0.04 to 0.28 from the sample A1 to A6 (Table 2) and hydrotalcite to brucite-like phase can be seen from these about 30% of initial Co2+ has been oxidized to Co3+ in A5 DSC curves.and A6. In response to the increase in positive charge, a DSC scans for air-prepared samples A1–A6 are shown in similar amount of negative anionic species is found in the Fig. 3. As can be seen, thermal events corresponding to interlayer space (i.e., 0.28 vs. 0.32 determined for A6; Table 2). depletion of the interlayer water and dehydroxylation of The chemical formula for both A5 and A6 is thus hydrotalcite-like structure take place at ca. 152–164 °C (first CoII0.72CoIII0.28(OH)1.96(NO3)0.26(CO3)0.03·0.3H2O according peak) and 185–197 °C (second peak) respectively, in all to XRD/EA studies and the inter-brucite-like sheet distance samples.In good agreement with the XRD results, which for this compound is 7.98 A° . show a predominant brucite-like phase and a small hydrotal- In view of the above results, Co hydroxides can apparently cite-like phase in A1 and A2 (Table 1), the DSC investigation be tailor-made with a desired structure and chemical composiin Fig. 3 also indicates the co-existence of the two phases. tion, including electronic configuration for the cations, using Compared to hydrotalcite-like phase, the brucite-like phase is the current approach. thermally more stable. The third endothermic eVect (217–219 °C) due to dehydroxylation of the brucite-like struc- Thermal decomposition of Co hydroxides ture is observed in A1 and A2.The endothermic eVect observed at ca. 258 °C in A5 will be explained below. Fig. 2 shows results of DSC investigations on the nitrogenprecipitated samples N1–N5. Low-temperature endothermic It is noted that thermal behaviours of N1 and A6 considerably diVer (Fig. 2 and 3), although they both have hydro- humps at ca. 130 °C in N1–N3 can be assigned to depletion of surface adsorbed water, while bands at 149–158 °C can be talcite-like structures (Fig. 1). In view of the absence of trivalent cations, the CoII hydrotalcite-like structure in N1 attributed to the release of interlayer water molecules.18–20 Collapse of the hydrotalcite-like structure is observed at ca. (a phase; CoII(OH)1.81(NO3)0.11(CO3)0.04·0.65H2O) can be described well by the ‘hydroxyl group deficiencies’ model.2,8 190–195 °C for N1–N4 (only a small shoulder in N4, i.e., a trace phase of HT).Finally large endothermic bands at By contrast, the CoIICoIII hydrotalcite-like phase in A6 (CoII0.72CoIII0.28(OH)1.96(NO3)0.26(CO3)0.03·0.3H2O) can be 219–222 °C in N4 and N5 can be assigned unambiguously to decomposition of the brucite-like structure, which normally ascribed to formation of common hydrotalcite-like phase that contains both divalent and trivalent cations due to the partial occurs at a higher temperature.18–20 Similar to the structural J.Mater. Chem., 1998, 8(11), 2499–2506 2501839 cm-1: n3 and n2 modes of NO3- with D3h symmetry) to a tilted or vertical position, which is indicated by a newly emerging peak at 1010 cm-1 (n1 mode of NO3-) of C2v symmetry.21,22,26 Although the final calcined products are all Co3O4, thermal evolution paths may diVer considering the substantial diVerences of starting materials in chemical composition, oxidation state, and molecular structure (Table 2).To diVerentiate the decomposition paths, the formation process of Co3O4 was further investigated by DTA using nitrogen and air.Fig. 5 shows DTA results for N1–N5; decomposition processes in nitrogen are quite similar to those in DSC (Fig. 2) in which nitrogen was also used. However, the thermal behaviour changes considerably in air-DTA experiments. For example, exothermic peaks at 172 °C (N5) and 189 °C (N4) can be attributed to Co2+ oxidation to Co3+ in the brucite-like phase.The peaks are still exothermic even considering the decomposition (which is endothermic): 27 3 Co(OH)2+1/2 O2=Co3O4+3 H2O DHro=-25 kJ mol-1 Co3O4 (1) For the A1–A6 series shown in Fig. 6, the endothermic peaks Fig. 3 DSC curves for the sample series prepared under air atmosphere: A1–A6. at 211 °C (A1) and 206 °C (A2) observed in nitrogen atmosphere can be assigned to the decomposition of a mixed brucitelike phase.Nevertheless, this endothermic eVect is cancelled oxidation of Co2+. The speculated ‘mixed valent state’ model3 out by the exothermic oxidation of the brucite-like phase for can be thus confirmed. With respect to the formation mechanthe same samples measured in air (A1 and A2, Fig. 6). ism, the increase in peak intensity and sharpness in A6 (second Following the DSC /XRD /FTIR /DTA results, weight peak, Fig. 3) reflects an increase in interaction between cations losses before 500 °C in TGA can be broadly classified into and intercalated anions due to presence of trivalent cobalt. three major types (L1, L2 and L3; Table 2). The first, between More anionic species are found in the CoIICoIII hydrotalcite- 40 and 160 °C can be ascribed to dehydration of samples like phase than in the CoII HT-like phase (0.32 in A6 vs. 0.19 (surface and/or interlayer water). This assignment is supported in N1; Table 2). It is thus believed that stronger electrostatic by the fact that a negligible loss is observed for the essentially attraction would lead to a more intense endothermic eVect brucite-like phase N5 while larger weight losses are seen for observed during the dehydroxylation. all hydrotalcite-like samples (N1>A6>A1&N5; Table 2).The second stage between 160 and 230 °C is attributed to Thermal evolution to Co3O4 oxides dehydroxylation of Co hydroxides, as all samples exhibit the Owing to structural and chemical diVerences, diVerent thermal largest losses over this temperature range.The final stage evolution behaviours and thermal stability of the Co hydrox- between 230 and 500 °C can be ascribed to a continued ides are expected upon heat-treatment. Fig. 4(a) shows FTIR decomposition process for large-sized particles (mass transport spectra for the stage-calcined N1 sample (see Experimental limiting) and a decomposition of HCoIIIO2. section). As can be seen, fingerprint IR absorptions at 580 According to TGA all samples show a small weight loss and 660 cm-1 for the Co3O4 spinel phase are fully developed between 270 and 274 °C within the total L3.However, this after heat-treatment at 165 °C.12 Upon the lowering of intensit- loss becomes much more pronounced in samples A4 and A5 ies for hydroxyl group and intercalated nitrate ion bands at with a total loss L3 of 7.3% and 7.1%, respectively, compared 3430 cm-1 and 1384 and 839 cm-1,21–23 the metal-oxygen to L3=4.0–4.7% in Table 2.The weight losses at 270–274 °C vibrational absorption modes at 564–580 and 660 cm-1 correspond to the endothermic peaks at 262–270 °C found in increase markedly.9,24 Fig. 4(b) shows in FTIR spectra the the DTA scans of A4 and A5 (Fig. 6; and similarly observed metal–OH vibration of the A1 sample at ca. 494 cm-1,9,24 in the DSC scan of A5, Fig. 3), which can be assigned to after heat-treatment at 165 °C. It is thus confirmed that sample decomposition of cobalt oxide hydroxide [HCoIIIO2]. This A1 is thermally more stable than N1. Furthermore, the trivalent Co compound also has a hexagonal-layered structure, hydroxyl group (3630 cm-1) in the brucite-like phase is still layers of which are bonded to each other by hydrogen bonding very pronounced at this temperature and it is only significantly and decomposes in the same temperature range under vacuum reduced at around 200 °C when dehydroxylation commences.or oxygen atmosphere: 28 It should be noted that, regardless of types of initial precipitates, the final decomposed products of N1 and A1 at higher 12 HCoO2=4 Co3O4+6 H2O+O2 (2) temperatures are exclusively in the spinel form (IR bands at 564 and 660 cm-1 for Co3O4).12 The oxidative formation of It has been established thermogravimetrically that in the temperature range 120–190 °C trivalent Co hydroxide first Co3O4 phase is completed with precipitation of nitrate anion, in the absence of ambient oxygen.In this connection, a small forms HCoO2 before being converting to Co3O4 at 240–300 °C.13,28 Furthermore HCoO2 can also be oxidatively peak at 1270 cm-1 for N1 heated to 190 °C can be assigned to the asymmetric vibration of monodentate nitrate ion [asym- prepared in an ambient atmosphere via oxidation of b- Co(OH)2.28 Since the TGA measurements were conducted metric vibration mode: nas(ONO2)].20,25 Nitrate ion evolution during heating can be seen clearly in A6 [Fig. 4(c)] that with air, divalent cobalt can be oxidised to the trivalent HCoO2 on heating. contains the largest amount of anions (Table 2). In addition to the formation of monodentate species [1310 cm-1 for the The pronounced HCoO2-decomposition for the A4 and A5 samples in TGA/DTA/DSC can be attributed to the presence nas(ONO2) and 1470 cm-1 for the symmetric vibration mode ns(ONO2)] 20,25 at 165 °C, the nitrate anion also undergoes a of trivalent cobalt plus the on-site formed HCoO2.However, HCoO2 decomposition is not observable in DTA/DSC symmetry-lowering from a flat-laying configuration (1384 and 2502 J. Mater. Chem., 1998, 8(11), 2499–2506Fig. 4 FTIR spectra for three as-prepared samples: (a) N1, (b) A1, and (c) A6 after heating from room temperature (unmarked) to the temperatures (marked) of some major thermal events in the DSC scans (10 °Cmin-1 in nitrogen). Fig. 6 DTA curves for A1–A6 sample series measured under nitrogen Fig. 5 DTA curves for N1–N5 sample series measured under nitrogen and air atmospheres.and air atmospheres. measurements for A6 even though it contains trivalent cobalt. the same for all samples including the anion-free N5. This This diVerence can be related to distribution homogeneity of view is further supported by FTIR studies in Fig. 4(a)–(c) trivalent cobalt in the precipitates. Compared to the A5 which show a significant reduction in the nitrate ion absorption sample, the second endothermic peak of the DSC-scan for A6 at 1384 cm-1 over this temperature range.(Fig. 3 and similarly in DTA scans of Fig. 6) is much sharper. This is also observed for other well aged samples (>24 h, not Transformation between Co3O4 and CoO listed in Table 1). As mentioned earlier, the sharpness of DSC peak indicates a well defined phase. The above observations It is known that Co3O4 is a thermodynamically stable form under an oxygen containing atmosphere.The stability of the lead to our belief that a more homogeneous distribution of trivalent cobalt cations among the divalent ions leads to a prepared Co3O4 was examined under inert gas (nitrogen) or an oxygen-containing atmosphere (air). DTA investigation on dimunition of the HCoO2 phase.Based on the TGA data, it can be concluded that the anionic decomposition and restoration of Co3O4 during heating-cooling cycles is shown in Fig. 7 and 8. The endothermic peaks at species decompose almost simultaneously with dehydroxylation of the Co hydrotalcites (N1, A1 and A6; Table 2) in ca. 846–858 °C for samples N1–N5 heated under nitrogen (Fig. 7) can be described by the forward reaction of the stage two, since the losses (L3) at stage three are essentially J.Mater. Chem., 1998, 8(11), 2499–2506 2503Fig. 9 TGA curves for some selected Co hydroxides during Co3O4= 3 CoO+1/2 O2 phase transformation. Fig. 7 DTA heating-cooling cycles for N1–N5 sample series measured under nitrogen and air atmospheres; arrows indicate the heating– cooling directions.The observed similar DTA-heating–cooling cycles for both sets of samples indicate that all Co hydroxides should have similar chemical constituents over the temperature range studied. This point has been further confirmed with the TGA investigation shown in Fig. 9. In good agreement with the DTA results, there is no further weight loss at temperature >500 °C until the Co3O4 phase decomposes.Conversion of Co3O4 to CoO occurs at >910 °C (note: a slow heating rate of 2 °Cmin-1 is used). The detected weight loss is in the range of 6.23–6.46%, very close to the theoretical value of 6.62% according to eqn. (3). On the basis of DTA/TGA results, the Co3O4 to CoO transformation in these hydroxide-derived spinel phases occurs at lower temperatures compared with the reported data of 1000–1200 °C under air.20,21 As revealed in XRD patterns, the crystallinity of the low-temperature formed Co3O4 is low, which may ease oxygen transport during phase conversion and result in the observed low transformation temperature.Catalytic evaluation of the thermally formed Co3O4 Fig. 10 shows specific surface area data for some representative samples calcined at various temperatures for 2 h.As can be seen, brucite-like compounds, N5 and A1 (A1 contains a small amount of hydrotalcite-like phase), show higher specific surface area at low temperature (200 °C). However, they show Fig. 8 DTA heating–cooling cycles for A1–A6 sample series measured under nitrogen and air atmospheres; arrows indicate the heating– cooling directions.following chemical equilibrium: Co3O4=3 CoO+1/2 O2 (3) However, this decomposition occurs at a much higher temperature (941–946 °C) for the same series under air. As it is reversible, the above reaction shifts to the left when the temperature is lowered and there is suYcient oxygen in the gaseous phase [PO2=2.1×104 Pa using air at this experimental setting (total pressure=1 atm)].As indicated by the large exothermic peaks at 804–828 °C (Fig. 7), the reverse reaction of eqn. (3) is exothermic due to oxidation of divalent cobalt.29 A similar observation is seen for the sample series A1–A6 in Fig. 8 over the same heating–cooling range. Since there is no oxygen, the reverse transition is not observed in the DTA Fig. 10 Specific surface area versus calcination temperature for some representative Co hydroxide samples.experiments using nitrogen (Fig. 7 and 8). 2504 J. Mater. Chem., 1998, 8(11), 2499–2506Table 3 Kinetic data of nitrous oxide decomposition from some representative catalysts Structural Ea/ Specific surface Sample phasea kJ mol-1 ln A areab/m2 g-1 N5 B(II) 94 19.5 29 A1 B(II)+HT( II,III) 80 16.7 40 N1 HT(II) 83 17.4 42 A6 HT(II,III) 79 16.8 52 aThe phase notation is the same as in Table 1.bUpon calcination at 400 °C. i.e. nitrous oxide decomposition is largely carried out on the Co3O4 surface phase, rather than CoO. Decomposition of nitrous oxide on a metal oxide surface involves an electron transferring from a low-oxidation state metal cation to an adsorbed nitrous oxide molecule.33,34 For many metal oxide catalysts, charge transfer from a low-valence metal cation to the adsorbed N2O is often considered as a fast surface reaction.Based on this mechanism, a typical Langmuir–Hinshelwood rate equation can be derived.15 In particular, the following simplified equation can be obtained Fig. 11 Conversion versus temperature curves for some representative considering the adsorption step as a controlling step and a Co hydroxides calcined at 400 °C.negligible inhibiting role of O2 in the decomposition reaction:15,35,36 -dPN2O/dt=kPN2O (4) significantly reduced specific surface areas at elevated temperatures. On the other hand, the surface areas of hydrotalcite- After integration, eqn. (4) becomes: like samples (N1 and A6) are systematically higher than those ln{ln(Pi,N2O /Pf,N2O)}=ln A-ln(F/V )-Ea/RT (5) of N5 and A1 at 300 and 400 °C.In particular the surface areas of these high-temperature calcined samples are pro- where V and F have been defined in Experimental section, portional to the anion content in the precursor compounds, and A is Arrhenius pre-exponential factor, and Ea is the i.e., SA6>SN1>SA1>SN5 (see Table 2 for the anion content).apparent activation energy of decomposition.35 Eqn. (5) has Fig. 11 shows the chemical reactivity of the above thermally been employed in the current work to provide a correlation formed samples using nitrous oxide as a probe gas at 400 °C. between the preparative method and catalytic activity of These conversion versus temperature curves were obtained at Co3O4.Since the ln{ln(Pi,N2O/Pf,N2O)} versus 1/T plots are all a GHSV of 20 500 h-1. Although they are prepared from fitted very well to straight lines, decomposition kinetics of various starting Co hydroxides, the 400 °C-formed oxides nitrous oxide on these Co3O4 oxides can be described as first occur strictly as Co3O4 crystallographic phases,30 as revealed order with partial pressure of nitrous oxide with apparent by XRD before and after nitrous oxide decomposition.Owing activation energies of 79–94 kJ mol-1 as listed in Table 3, to the same Co3O4 phase, the observed catalytic activity varies noting that these values are quite similar. only slightly. Although surface area is usually critical in determining catalytic activity, a variation in specific surface Conclusions area by a factor of ca.two has been shown to have little eVect (N5 versus A6, Table 3). Therefore, the subtle diVerences In summary, the synthetic chemistry and thermal evolution of among these curves can be attributed to a combined eVect of one new and two known Co hydroxides have been investigated total variation in specific surface area, number of reaction systematically with a wide range of characterisation and sites per specific surface area, and activity of a reaction site.analytical methods (XRD/EA/DSC/FTIR/DTA/TGA/GC). Overall all Co3O4 oxides derived from these monohydroxides Three major decomposition stages of Co hydroxides have been are highly active, when compared with the literature conversion identified: (i) 149–164 °C for dehydration of interlayer water, data.For example, the activity of the present catalytic oxides (ii) 185–197 °C and (iii) 219–222 °C for dehydroxylation of at 375 °C is in the range 7.2–8.2 mmol N2O g-1 h-1 (GHSV= hydrotalcite and brucite-like phases, respectively. Intercalated 82 000 h-1, N2O=1 mol% balanced with He), compared with nitrate anions decompose largely at the stage (ii).During the 1.6–2.3 mmol N2O g-1 h-1 (GHSV=30 000 h-1, N2O= above decomposition, Co3O4 oxide starts to form at tempera- 0.1 mol% balanced with He) for an active hydrotalcite-like tures as low as 165 °C especially for hydrotalcite-like phases. compound (CoIIMgIIAlIII-HT) derived catalyst operated at The intermediate compound HCoO2, formed during the ther- 350–400 °C.31,32 mal evolution, decomposes at 258–270 °C.It is found that As the decomposition reaction occurs on the surface, the Co3O4 oxide formed freshly from thermal decomposition of actual crystallographic phase of surface Co3O4 should be the hydroxides coverts to CoO at 842–858 °C and 935–948 °C further addressed. Using XPS, it is known that the transform- under nitrogen and air, respectively.Surface areas of calcined ation in eqn. (3) occurs at ca. 347 °C on the surface of Co3O4 samples are found to be proportional to their intercalated in an oxygen atmosphere of 1×10-3 Pa, which is ca. 186 °C anion content. The resultant Co3O4 materials are catalytically lower than for thermodynamic calculation for the bulk phase active and the activity is comparable to some reported active Co3O4 to CoO transition under the same PO2.29 Since the catalyst systems.For example, the activity of Co3O4 oxide oxygen partial pressure generated by the current nitrous operated at 375 °C is in the range 7.2–8.2 mmol N2O g-1 h-1 oxide decomposition is much higher than 1×10-3 Pa (GHSV=82 000 h-1, N2O=1 mol% balanced with He). 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ISSN:0959-9428    

DOI:10.1039/a804767g

出版商:RSC    

年代:1998

数据来源: RSC