摘要:

J. MATER. CHEM., 1994, 4( 12), 1867-1870 Synthesis, Crystal Structure and Properties of [Sr2Cu(C,0,),(H20),]: Precursor of Sr2Cu0, Oxide M. Insausti: M.K.Urtiaga; R. Cortes: J.L. Mesa,a M.I. Arriortuab and T. Rojo*a a Departamento de Quimica Inorganica, Departamento de Mineralogia- Petrologia, lnstitufo de Sinfesis yEstudio de Materiales (ISEM), Universidad del Pais Vasco, Apartado 644, 48080 Bilbao, Spain [Sr2Cu(C,0,),(H20),] (C204*-=oxalate ion) has been synthesized and characterized. The structure has been determined by X-ray diffraction methods. It crystallizes in the triclinic system, space group P T with a =6.349(2)A, b =10.258(2)A, c =15.737(2)A, x =73.21 (l)", =93.66(2)", y =76.44(2)", V= 944.3(4)A3, Z =2. The structure consists of an intricate network of S?' and Cu2+ ions linked by oxalate and H20 groups.A nonacoordinated arrangement around the two strontium ions and a distorted octahedral geometry for the copper(ii) ions are observed. The thermal decomposition of this complex yields a unique and homogeneous phase at comparatively short reaction times and lower temperatures than the ceramic method. Conductivity measurements of the oxide obtained show semiconductor behaviour. As the existence of the copper oxide plane structure seems to play an important role in mechanisms of high-T, superconduc- tivity,' different studies on ternary copper-oxygen systems have been performed. Most of these investigations have been dedicated to phase equilibria and structural details. Among a large number of ternary compounds the oxides present in the MO-CuO binary phase diagram (M =Ca, Sr, Ba) are not only important for a further understanding of crystal chemistry of high-temperature superconductor-related materials but they also have the potential to become new superconductors if subjected to adequate carrier d~ping.~,~ Therefore S~,CUO,~ is an interesting compound because of its possible modification at high pressures to a superconductor with transition tempera- tures of 70 and 100 K.5 These oxides are traditionally prepared by the ceramic method which does not always yield single-phase products of the required stoichiometry.Alternative strategies which decrease the diffusion pathlengths can improve the final products. In this way, metallo-organic precursors hold much promise,6 as this method not only allows mixing of the different metal species on an aiomic scale but also reduces the diffusion distance to ca.10 A. Moreover, suitable precur- sors would result in significant modifications in the micro- structure of the final products. Carboxylates, which have long been known and studied as binding agents, have the disadvantage of incorporating very large amounts of carbon, which later have to be removed.',1° Furthermore, the high stability of some carbonates increases the temperature at which metal oxides are obtained. Accordingly, mixed oxalates may be considered to be among the best possible cuprate precursors. This paper shows the usefulness of the metallo-organic method for the clean and soft synthesis of pure Sr,CuO,.For this purpose, the complex with the formula Sr,Cu(C,O,),( H20), (C2042-=oxalate) has been prepared and structurally characterized. The decomposition of the compound has been extensively studied by TG and DSC. Taking these data into consideration, the complex was heated at different temperatures to obtain the respective oxide. Experimental Synthesis of CSr,Cu(CzO,),( H20)7] [Sr,Cu(C,O,),( H20),] was synthesized by slowly mixing an aqueous solution of 1 mmol (0.354 g) potassium bis(oxa1ate)- copper(rr) dihydrate, prepared as previously," with an aqueous solution of 2 mmol (0.534 g) strontium chloride hexahydrate, contained separately in a diffusion device at room temperature.After 2 days, blue crystals were obtained, which were filtered off and washed with ether. Powder X-ray diffraction data were obtained with a STOE diffractometer, equipped with a germanium monochromator, at temperatures of 293 & 1 K. Physical Measurements Infrared spectra (KBr pellets) were obtained with a Nicolet FTIR 740 spectrophotometer in the range 4000-400 cm-l. TG and DSC measurements were carried out with a Perkin- Elmer System-7 DSC-TGA unit. Crucibles containing 20 mg of sample were heated at 2 'C min-' under dry air. A Bruker ESP 300 spectrometer, operating at X and Q bands, was used to record the EPR powder spectra. The temperature was stabilized by an Oxford Instruments (ITC4) regulator. The magnetic field was measured with a Bruker BKM 200 gaussmeter, and the frequency was determined with an HP5352B-microwave frequency counter.Scanning clectron microscopic (SEM) observations were also carried out to give some indication of the compactness of the oxide using a JEOL JSM-6400. The conductivity was measured using the four- point method', and was calculated by measuring the intensity/ voltage ratio between the points in both directions of the current, in order to minimize the asymmetry effect between the contacts. X-Ray Crystal Structure Determination Preliminary oscillation and Weissenberg photographs for the compound showed triclinic symmetry. A well formed prismatic crystal with dimensions 0.8 mm x 0.4 mm x 0.2 min was mounted on a glass fibre and transferred to an Enraf-Nonius CAD-4 diffractometer with graphite-monochromated Mo-Ka radiation.Final unit-cell dimensions, calculated from a least-squares treatment of 25 accurately centred reflections (8"<8< 12") are given in Table 1, with other crystal data. 5490 unique reflections were collected using o scans, of which 3753 were considered observed [I> 3a(l)]. These were cor- rected for Lorentz and polarization effects. Scattering factors were taken from the International Tables for X-ray Crysta110graphy.l~ The structure was solved using the automatic Patterson interpretation routine in SHELXS-86.14 A Fourier map, calculated with phases based on these ions, revealed the atoms of the complexes. Several cycles of anisotropic refinement were carried out.On application of 1868 Table 1 Data collection and structure refinement of [Sr2Cu(C204)3(H@), 1 compound formula Sr,CuC,H 14019 molecular weight crystal system sp!ce group a14 bl+ c/A a/degrees Pldegrees ))/degrees cell volume/A3 Z 646.78 t riclinic 6.349( 2) 10.258( 2) 15.737( 2) 73.21(1) 93.66( 2) 76.44( 2) 944.3(4) 2 pi Dobslg cm -Dcalck cm -p (Mo-Kx)/cm-F(000) 2.22(4) 2.27 59.6 554 measurements: i (Mo-Ka) 6 rangeldegrees no. of measured reflections 0.71069 5490 1-30 interval h, k, 1 f8,+ 14,+22 refinements: no. of variables 265 selection criterion I>30(I) no. of unique reflections 3753 weighting scheme: W= 1.0/[0~1F~(+p(F~)~]p= R=(CI IFol-IFcl I)/(ClFol) Rw= CWlFol-I~c/)2/~wI~01211'2 0.013325 0.055 0.060 a weighting scheme, the model converged at R=0.055, R, =0.060.The geometric calculations were performed with PARST15 and BONDLA,16 and the molecular illustrations were drawn with SCHAKAL.17 Further details have been deposited with the Cambridge Crystallographic Data Centre. Results and Discussion Crystal Structure The structure can be described as a complex three-dimensional network of Sr2+ and Cu2+ ions coordinated by oxalate and water molecules, indicating the non-existence of discrete mol- ecules. The asymmetric unit is formed by two strontium atoms, two copper atoms situated in special positions (occu- pation factor OS), three oxalate ligands and seven water molecules. A perspective drawing of [Sr,Cu(C,O,), (H20),] as well as the labelling of the atoms is shown in Fig.1. Atomic coordinates and some selected bond distances and angles are given in Tables 2 and 3, respectively. The coordination polyhedra around the Cu" ions can be described as distorted octahedra, with four coplanar bonds, [0(1)-O(2)-O( 1 p-0(2)iv] for Cu(1) and [0(5)-O(6)-O( 5)"'-0( 6)"'] for Cu(2), of nearly equivalent IengJh (ca. 2 A), and two longer tetragonal bonds around 2.5 A (Table 3). The low octahedral distortions around the copper ion, A=O.Ol and A=0.02 for Cu(1) and Cu(2), respectively, were calculated by quantification of the Muetterties and Guggenberger description." The strontium ions are nonacoordinated, and the coordi- nation polyhedra for both ions were analysed using the Cavas and Kepert l9 description, considering the monocapped square antiprism and the tricapped trigonal prism configurations as ideal geometries.The topology for the Sr(1) atom is similar to a monocapped square antiprism, with the atoms 0(3), 0(7), O(11) and oO(10)' [the maximum distance from this plane is 0.203(6)A for O(ll)] and Ow(l), 0(4), O(7)" and J. MATER. CHEM... 1994, VOL. 4 Fig. 1 View of the [S~,CU(C,O,)~(H~O),] molecule with atom numering [i=l+x, y, z; ii=-x, -y, 1-z; iii=l-u, -l-y, 1-z; iv= -9, 1-y, z] (SCHAKAL 88, Ketler, 1988) Table 2 Atomic coordinates (x 104)LSr and Cu x ( 1u5)] and equival- ent isotropic temperature factors (/A2) for [Sr2Cu(C204)3( H20),] atom X Y Z Be, 1346(7) 7780(5) 35040( 3) 1.98(1) 30596(8) -26746( 5) 26373( 4) 2.84( 2) 0 50000 0 6.98( 6) 50000 -50000 50000 3.06( 3) -1166( 13) 3361(7) 447( 4) 6.5(2) 1464( 12) 4469(6) 1217(4) 5.8( 2) -531(15) 2762(7) 1282(5) 4.1(2) 987( 12) 3404( 7) 1725r 5) 3.5(2) -1089(11) 1736( 6) 1750(4) 5.0(2) 1576(8) 2926( 5) 25361 3) 3.5( 1) 3079(8) -3116(4) 44961 3) 2.6( 1) 4346(7) -4854(4) 6155(3) 2.5(1) 2434(9) -2614( 5) 5107(4) 2.3(1) 3152(9) -3629( 5) 6076( 4) 2.2(1) 1310(7) -1390( 4) 5003(3) 2.4(1) 2536(8) -3240( 4) 6714( 3) 3.0( 1) -2997( 7) -2483( 5) 2732(4) 4.1(1) -6222( 6) -558(4) 3102(3) 3.2( 1) -.4308(9) -457(6) 3212(4) 2.6( 1) -2487( 9) -1506( 6) 2934( 4) 2.3( 1) -3688( 7) 390( 6) 3538(4) 5.4(2) -623 (6) -1288(4) 2940( 3) 2.7( 1) 3162(8) 1386(5) 4448( 3) 3.8( 1) 770(8) -4531(5) 30701 4) 4.9( 2) 4788( 8) -4564( 5) 1830( 4) 3.9(1 ) 3351( 17) -760( 10) 1155(6) 9.0(4) 198( 14) -2228 (9) 1114( 6) 7.9(3)1835( 11) -6042( 7) 4864(6) 6.8( 2) 3002( 18) 3435( 10) -470( 7) 10.0(4) ~ Be,=8/3 7t2 CiCj Uijai* aj* aiaj.O(8)" forming parallel planes (Fig. 2). The environment of the Sr(2) atom deviates greatly from the ideal geometries, so it cannot be described as a characteristic polyhedron. The O(12) and O(l0)' atoms act as bridges between the Sr(1) and Sr(2) atoms, forming a qutsiplanar ring, with a maximum deviation of 0.074(5)A for O(10)' J. MATER. CHEM., 1994, VOL. 4 Table 3 Some selected bond distances (/A)and angles (/degrees) with esds in parentheses for [Sr,Cu(C,O,),( H20)7] Sr( 1)-O( 12) Sr( 1)-O( lo)' Sr(1)-0(3) Sr( 1 )-0(4) Sr(1)-O(7) Sr( 1)-O( 11 ) Sr( 1)-Ow( 1) Sr( 1)-O(7)" Sr(1)-O( 8)" CU( 1)-O( 1) Cu( 1)-O(2) CU( 1)-0~(7) 2.650(5) 2.613(4) 2.644( 5) 2.683(5) 2.652(4) 2.551(5) 2.650( 6) 2.759( 5) 2.605(4) 1.958(8) 1.940( 6) 2.487(11) Sr(2)-O(12) Sr(2)-O(10)' Sr( 2)-Ow( 2) Sr(2)-0w(3) Sr(2)-0w(4) Sr(2)-0w(5) Sr( 2)-O( 9)' Sr( 2)-O(5) Sr(2)-O( 6),,' Cu(2)-0(5) CU(2)-O( 6) Cu(2)-0~(6) 2.587(4) 2.607( 5) 2.623 (6) 2.678 (6) 2.629(8j 2.770( 9) 2.555( 5) 2.827( 5) 2.759( 3) 1.933(4) 1.930(5) 2.51 2( 8) O(12)-Sr( 1)-O( lo? 71.4( 1) O(12)-Sr(2)-0( 10)' 723 1) O(I)-C( 1 I-0(3) O(10)-C( 5)-O( 11) O(5)-C( 3 f-O(7) O(1j-Cu( 1)-0(2) 125.0(9) 126.5(6) 126.5(5) 85.3(3) 0(2)-C(2)-0(4) 0(9)-C(6)-0( 12) O(6)-C( 4)-O( 8) 0(5)-C~(2)-0(6) 124.1(8) 125.5(6) 124.1 (5) 86.5(2) symmetry code: i=l+x, y, z; ii=-x, -y, 1-z; iii=l-x, -l-y, 1 -z.0 (12 1 n 0(1( 0 (41 Fig. 2 A ball and stick view of the coordination polyhedron of Sr( 1) in CSr2Cu(C,O'l), (H20171 (see Fig. 1). The ring formed by the atoms Sr( l)-Sr(2)-0( 5)-C(3)-0(7)-Sr( 1)is also quibte planar, with a maximum deviation from plane of 0.144( 5) A for the O(7) atom. The standard notation for bridging carboxylates has been used. According to the description presented by Porai- Koshits," the classification for the oxalates is the following: a-2-a for C(l),C(2) and C(4), s-3-sa for C(3), s-2-a for C(5) and sa-3-a for C( 6). Thermal Treatment The decomposition of the starting material was studied by thermogravimetry in the temperature range 30-850 "C (Fig.3). The data show the occurrence of three consecutive steps: dehydration (40-230 "C), ligand pyrolysis (280-500 "C) and inorganic residue evolution (520-850 "C).The loss of the molecules of water is registered in three endothermic steps with the ratio 2 :2 :3 and is complete at 230 "C. Dehydration supposes a lowering of the coordination number of the alkaline-earth-metal ions, which probably results in a rearrangement of the ligands between the metallic centres. The dehydration enthalpy, calculated from the calorimetric measurements, gives a value of 54 kJ mol-', in good agree- ment with related compounds.' Decomposition of the anhydrous compounds follows immediately after dehydration in the temperature range 280-500 'C, with a total weight loss of 23.2%.In this process 1 1 1 100 300 500 700 TIT Fig. 3 TG(-) and TGD (-.-) curves of [Sr,Cu(C,O,),( H20),] alkaline oxalates and CuO are formed, according to the weight losses and the results appearing for other mixed oxalates.21 In the control of the pyrolysis results, the stability of the metal carbonates may be one of the main factors which induces the formation of mixed oxides. The following reaction schemes are the most reasonable to describe the decompo- sition process. MM'(C,O,)(s)+MCO,(s)+ M'CO,(s) +R(g) (1) MM'(C,O,)(s)+MO(s)+ M'CO,(s)+ R(g) (2) MM'(C,O~)(S)+MO(S)+M'O(s)+R(g) (3) Depending on the stability of the metal Carbonates, reac- tions (l),(2) or (3) can occur.In our case, as alkaline-earth-metal carbonates are stable, the Iigand pyrolysis can be described by reaction (2), which is in good agreement with the theoretical weight losses accompanying the degradation and with X-ray diffraction data. A stable stage is observed between 520 and 790 "C, where strontium carbonate and CuO are formed. At higher temperatures the weight losses continue and the phase Sr,CuO, is formed, as observed bq X-ray diffraction. Nevertheless, SrCO, and CuO phases itre still present. Other experiments performed in a nitrogen atmos- phere yielded similar results. Taking into account this result, different thermal tre, <itments in tubular furnaces were performed in order to obtain the mixed oxide as a unique phase at relatively low temperatures.In this way, the complex [Sr,Cu(C,O,),( H,0),] was fired first at 500°C to remove the organic part then treated at 800°C for 24 h. X-Ray diffraction of the final product was performed and indexation was performed by FULLPROF (pattern-matching analysis)22 in the range 28 =10-70". The diffraction pattern showed the existence of a unique phase of S~,CUO,,~~ which was indexed onothe basis of an >ortho- rhombic cel) Imrnrn, a= 12.704(4) A, b =3.912(2) ,I and c =3.499(2) A. This structure contains planar CuO, squares, which share corners to form single chains, so it has one-dimensional 180"Cu-0-Cu interactions. According to the literat~re,~~calcination at 850 "C with intermittent grinding for 8 days was required to obtain the Sr,Cu03 phase using the ceramic method.Considering the above results, it can be concluded that better results for isolating the mixed oxide have been obtained from the metallo-organic method. In order to detect the presence of carbonates in this final oxide, IR experiments were performed. The Sr,CuO com-pound shows very weak bands at 1445 and 855 cm-l. corre- sponding to impurities of SrCO,, in good agreement nith the elemental analysis (C, 0.4%). The absence of bands at lower frequencies implies that the carbonate ion is not coordinated in the structure, as it is in related corn pound^.^^ EPR experiments on powder samples of the mixed oxide were performed and no signals were observed in the X-band region of the freshly prepared compound, in accord with a high antiferromagnetic ordering.26 Nevertheless, EPR signals appear when the samples are left in air.27 A similar result was observed for MCu02 oxides (M =Ca, Sr, Ba).28 Measurements of electrical resistance from room tempera- ture to 120 K indicate semiconducting behaviour, displaying a large temperature dependence (the resistivity at 125 K is ca.lo6 cm and at room temperature it is lo3 fi cm) (Fig. 4). The activation energy calculated for this range is 0.15 eV. Below 120 K the conductivity becomes so weak that accurate data could not be obtained using our equipment. These data are in accord with those in literat~re.’~ Scanning electron microscopy was also performed for the Sr2Cu03 sample.SEM examinations of portions of the com- pound using backscatter imaging revealed a homogeneous material with no additional phases. SEM photographs of secondary electrons show polycrystalline aggregates formed by small prismatic particles (Fig. 5), the size of which was around 1 pm. Concluding Remarks A new oxalate compound [Sr,Cu(C,O,)(H,O),] has been synthesized and structurally characterized by X-ray diffraction methods. The structure can be described as a complex three- dimensional network of Sr2 + (nonacoordinated) and Cu2+ (hexacoordinated ) ions surrounded by oxalate and water 120 165 210 255 300 T/K Fig. 4 Variation of resistance with temperature for Sr,CuO, -10l.lm Fig. 5 SEM photograph of Sr,CuO, from [Sr2C~(C204)3(H20)7] J.MATER. CHEM., 1994, VOL. 4 molecules, indicating the non-existence of discrete molecules. Thermal decomposition of this compound yielded the Sr2Cu03 mixed oxide at short reaction times and tempera- tures, compared with those obtained from the ceramic method.24 This oxide, which shows the behaviour of a semi- conductor, is composed of homogeneous and small particles, characteristic of metallo-organic precursors. This work was financially supported by the DCiICYT (PB90-05490) and the Accibn Integrada Hispano-Alemana (44B) projects. References 1 J. G. Bednorz and K. A. Muller, Z. Phys., 1986-64, 189. 2 J. Chunlin, C. Chuanmeng, W. Kuihan, L. Sulan, Z. Guiyi, Z. Guofan, Q. Cuefu, B. Weiming, F.Zhanguo and X. Qian, Solid Stute Commun., 1988,65, 859. 3 S. Pekker, J. Sasvari, G. Y. Huttirey and L. Wihaly, J. Less-Conznzon Met., 1989, 150, 277. 4 Chr. Teske and H. K. Muller-Buschbaum, 2. Anorg. AIIg. Chein., 1969,371,325. 5 Z. Hiroi, M. Takano, M. Azuma and Y. Takeda, Nature (London), 1993,364,315. 6 T. Rojo, M. Insausti, M. I. Arriortua, E. Hernandez and J. Zubillaga, Thermochim. Acta, 1992, 195.95. 7 M. Insausti, R. CortCs, M. I. Arriortua. T. Rojo and E. H. Bocanegra, Solid State Zonics, 1993,63-65, 351. 8 M. Insausti, J. L. Pizarro, L. Lezama, R. Cortes, E. H. Bocanegra, M. I. Arriortua and T. Rojo, Chein. Muter., 1994.6, 707. 9 H. H. Wang, K. D. Carlson, U. Geiser, R. J. Thorn, H. I. Kao, M. A. Beno, M. R. Monaghan, T.J. Allen, R. B. Proksch, D. L. Stupka, J. M. Williams, B. K. Flanderrneyer and R. R. Poeppel, Znorg. Chem., 1987,26, 1474. 10 H. S. Horowitz, S. J. Mclain, A. W. Sleight, J. D. Druliner, L. Gai, M. J. VanKavclaar, J. L. Wagner, R. D. Biggs and S. J. Poon, Science, 1989,243, 66. 11 D. Y. Jeter and W. E. Hatfield, Inorg. Chim. Acttr, 1972,6, 523. 12 J. Laplume, L’onde Electrique, 1955, 335, 11 3. 13 International Tables for X-ruj, Crystullogruph I, Kynoch Press, Birmingham, 1974, vol. 4. 14 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990,46,467. 15 M. Nardelli, PARST, Comput. Chem., 1983, 7, 95. 16 J. M. Stewart, G. J. Kruger, H. L. Ammon, C. W. Dickinson and S. R. Hall, The XRAY72, System-version of June of 1972, Tech. Rep. TR-192, Computer Science Center, University of Maryland, College Park, MD, 1972.17 E. Keller, SCHAKAL88, A Fortran Program for the Graphic Representation of Molecular and Crystallographic Models, Albert-Ludwigs Universitat, Freiburg, German!, 1988. 18 E. L. Muetterties and L. J. Guggenberger, J. Am. Chern. Soc., 1988, 7, 1383. 19 M. C. Cavas and D. L. Kepert, Prog. Znorg. Chern., 1981,28, 309. 20 M. A. Porai-Koshits, Zh. Strukt. Khim., 1980,21, 146. 21 Y. Saikali, Thermochim. Acta, 1986, 106, 1. 22 J. Rodriguez-Carvajal, FULLPROF, 1990, unpublished. 23 Chr. Teske and H. K. Muller-Buschbaum, Z. Anorg. Allg. Chem., 1970,379,234. 24 M. T. Weller and D. R. Lines, J. Solid State Chem., 1989,82, 21. 25 A. G. Aranda and J. P. Attfield, Angew. Chein., Znt. Ed. Engl., 1993,32,1454. 26 P. Ganguly, K. Sreedhar, A. R. Raju, G. Demazeau and P. Hagenmuller, J. Phys. Condens. Matter, 1989, 1,213. 27 H. Ohta, N. Yamauchi, M. Motokawa, M. Azuma and M. Takano, J. Phys. Soc. Jpn., 1992,61,3370. 28 T. Rojo, M. Insausti, L. Lezama. R. Cortes and M. I. Arriortua, Solid State Commun., submitted. 29 R. C. Lobo, F. J. Berry and C. Greaves, J. Solid Stare Chem., 1990, 88, 513. Paper 4/03738C; Receiced 20th June, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401867

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM.. 1994, 4( 12), 1871-1873 Phase Diagram of the Bi-Sr-Cu-0 System Elena Yu. Vstavskaya," Vladimir A. Cherepanov? Andrei Yu. Zuev,aSimon D. Suttonb and J. Stuart Abell*b a Department of Chemistry, Ural State University, 5 7 Lenin Av, Yekaterinburg, Russia School of Metallurgy and Materials, The University of Birmingham, Birmingham, UK B 75 2TT ~ ~ ~ ~~~ The phases present in the Bi-Sr-Cu-0 system in air, at 865 "C, have been investigated using X-ray diffraction techniques, with particular emphasis on the homogeneity ranges of the ternary oxide phases. Only three such phases were found; a tetragonal form and an orthorhombic form of Bi,Sr,CuO, (both exhibiting homogeneity ranges), and Bi,Sr,Cu,O,. Eight binary oxides were also found. No evidence was found at this temperature for the previously reported phases, Bi,Sr,Cu,O, and Bi,,Sr,,Cu,O,.Since the discovery of superconductivity in the Bi,Sr,Ca, -,CU,O, + 2n system, much scientific effort has been expended in attempting to produce single-phase samples of the higher T, members of the series (higher values of n), with rather less effort being spent on the much lower T,, but nevertheless important base member, Bi2Sr,Cu06 (n=1). In this work we have re-investigated the phase relationships within the Bi-Sr-Cu-0 phase diagram in an attempt to clarify some of the discrepancies in the literature. Bi-Cu-0 System The only phase in this system that has been previously described is B~,CUO,,'-~ which can be fabricated at tempera- tures below its melting point of 840rC.1.5 Two eutectic temperatures were found; 770 'C at 15 mol% CuO and 835 "C at 54 mol% C~0.l.~ Sr-Cu-0 System A number of phases have been reported in this system.Sr,CuO, fprmed in a$ at 930'C has a tetragonal structure (a=3.901 A, c =7.488 A). During annealing at 980 "C a betra- gonalaorthorhombic p$ase transition occurs (a = 12.699 A, b = 3.908 A and c =3.497 A). This transition has been reported to be irreversible.6 Detailed studies of Sr,Cu03 above the phase transition have also been described.' SrCuO, also forms in airs,9 whilst SrCu,02 forms in a reduced oxygen atmosphere." One more phase, with a CuO content of 60-64% has been found, but different authors ascribe different formulae; Sr,Cu,O, (62.5% Cu0),",l2 Sr,Cu,O, (60% CuO)', and Sr4Cu7012 (63.64% CuO).14 Recently, Siegrist et ~11.'~ described it as Sr14Cu,40,, (63.5% oCuO) with an vrthorhom- bic unit cell (a= 11.45 A, b =13.35 A and c= 3.98 A).Sr-Bi-0 System The phase diagram of the Sr-Bi-0 system has been studied by Roth et a1.16 and Guillermo et and a number of phases identified; Bi,SrO,, Bi,Sr,O, and Bi,Sr,06. Ikeda et found pne more pFase BiSr30z (ophorhombic cell: a= 17.147 A. b= 16.758 A and c = 16.998A). Several groups have studied the rhombohedra1 P-pha~e"-'~ and the tetragonal y-phase.21.'2 The P-phase exists between 11and 30 mol% Sr and the ?-phase between 43 and 46 mol% Sr. A detailed study of the Bi,03-Sr0 phase diagram has been made in a previous paper.23 Bi-Sr-Cu-0 System A number of single phases and solid solutions have been reported in the Bi-Sr-Cu-0 sy~tem.'*,~~-~~Th ese include Bi,Sr,CuO, (the 2201 superconductor).24 Bi,,Srl,Cu,O,,'R Bi,Sr,Cu20,,'8 B~,S~,CU,O,'~ and the solid solutions Bi, + 3r2-.CuO, (the so-called Raveau- or R-phase)26 and Bi2+xSr2-~C~l+yOz(.x=0.1-0.6 and y=0-~,/2).~~ Experimental About 230 samples of different composition were prepared by the usual ceramic techniques, using Bi203, SrCO, and CuO all of a purity of greater than 99.5%.Annealing temperatures were in the range 780-865°C and annealing times were 70-250 h, with intermediate re-grinding under alcohol in an agate mortar. Some samples in the Sr-Cu-0 system were fired at 930 'C followed by annealing at 865 "C.All samples were quenched from 865 'C. Phase identification was per-formed by powder X-ray diffraction (XRD) using a DRON-3 diffractometer with Cu-Kr radiation. Data were collccted at 0.04" increments over the range 20" <20 <60", and lattice parameters were calculated using a least-squares method. Results and Discussion Sr-Cu-0 System Three compounds have been found in this system ir air at 865 "C: Sr,Cu03, SrCuO, and Sr,,Cu,,O,, . Compounds with the nominal compositions SrCu,O,, Sr,Cu,O,". and S~,CU,O,~~were not obtained. The sample with the former composition was found to consist of Sr,,Cu,,O,, and CuO, and the last two compositions consisted of varying amounts of Srl,Cu,,O,l and SrCuO,. Sr,Cu,O,, was found to be identical with Sr,,Cu,,O,, within experimental error.Sr-Bi-0 System The following phases have been found in this sysiem in air at 865°C: a 8-phase with a variable composition (0.11dtSr<0.30), Bi,SrO,, a 7-phase with the general formula Bi, +*Sr1 -*O, (0.08<x<0.14), Bi,Sr,O,, Bi2Sr30, and BiSr,O,. Those samples with composition tsr<0.11 either coexist with the liquid phase or are completely molten. The homogeneity range of the P-phase is in good agreement with the literature, that of the ?-phase is broader than previously published. The phase diagram of the Bi,O,-SrO system is described in detail in a previous paper.,, Bi-Sr-Cu-0 System A compound with the nominal composition Bi,Sr !Cu06 (2201), having a tetragonal structure (A) (aM 5.4 A, c c:24 A) has been obtained at a temperature of less than 790°C after firing for 24 h. Increasing the temperature to 865 "C leads to the transformation of A to an orthorhombic structure (B).It was found that phases A and B have considerable homogeneity ranges and that not all compositions of phase A have under- gone this transformation at 865°C. At 790°C the tetragonal phase A has a homogeneity range that includes the sample with the nominal composition Bi,,Sr,,Cu,O, (described earlier as single phase18). This homogeneity range decreases with increasing temperature, as demonstrated by the series Bi,+xSr2-xCu0,. Samples with the following compositions were prepared: x=O, 0.05, 0.1, 0.125, 0.2, 0.3, 0.4, 0.5 and 0.6.After firing at 790°C, analysis by XRD showed all samples to be single phase and tetragonal with the exception of x= 0.4, 0.5 and 0.6, which also contained some excess Bi,SrO,. After firing at 830 "C, the samples with x=0,0.05 and 0.1 had transformed into the orthorhombic phase B. Raising the firing temperature to 865°C showed no change in the position of this boundary. Increasing the Bi component of phase A decreases the melting point and results in a liquid phase at this temperature. In order to confirm the boundaries of the homogeneity ranges of the two phases, lattice parameters were measured and unit-cell volumes calculated for several samples lying on each of lines 1,2 and 3 in Fig. 1. These are presented in Tables 1-3. These results show that neither phase conforms to Vegard's Law, but this is not unusual and many 'non-ideal' solid solutions are known.27 However, outside the single phase regions, where the limiting compositions of the solid solutions remain, the lattice parameters remain constant, and thus define the extent of the phase fields.The homogeneity range of phase A is in good agreement with, but slightly wider than, that obtained by Ikeda et a1." However, Bi,,Sr,,Cu,O,, described by Ikeda et al. as single phase, was found to lie within the B range. Samples in the field, of 2-3 mol% width, between the homogeneity ranges of phases A and B contained both these phases. Because of its relative narrowness it is difficult to conclude whether this is a field of equilibrium coexistence of Bi(%) \ 50 '\ \ \ 60x', 'j70 / 20 BiSr30, Bi2Sr306 Bi2Sr205 Bi2Sr04 Sr(%) Fig.1 Selected area within the Bi-Sr-0-0 phase diagram at 865 "C in air, showing the major phases and phase fields J. MATER. CHEM., 1994, VOL. 4 Table 1 Parameters and unit-cell volumes of phase t3 (compositions on line 1) at 865 "C in air mole fraction of metal component N(i)= n(i)/(nBi +nSr +nCu) 0.360 0.400 0.240 5.418(5) 23.333(7) 24.430( 1) 3088.8 0.375 0.400 0.225 5.418(5) 23.333(7) 24.430( 1) 3088.8 0.400 0.400 0.200 5.422(5) 23.332( 7) 24.436(8) 3091.8 0.410 0.400 0.190 5.424(5) 23.322(6) 24.438( 7) 3091.8 0.425 0.400 0.175 5.427(0) 23.303(2) 24.436(4) 3090.2 0.440 0.400 0.160 5.426(6) 23.341 (7) 24.453(5) 3097.2 0.450 0.400 0.150 5.426(6) 23.341(7) 24.453(5) 3097.2 Table 2 Parameters and unit-cell volumes of phase B (compositions on line 2) at 865 "C in air ~ mole fraction of metal component N(i)= n(i)/(nBi +nSr +nCu) 0.375 0.375 0.250 5.418(5) 23.317(4) 24.416(6) 3084.9 0.380 0.380 0.240 5.418(5) 23.317(4) 24.316( 6) 3084.9 0.390 0.390 0.220 5.420( 1) 23.301(0) 24.330(8) 3085.5 0.400 0.400 0.200 5.422( 5) 23.332( 7) 24.436(8) 3091.8 0.410 0.410 0.180 5.423( 1) 23.316(8) 24.442(0) 3090.7 0.420 0.420 0.160 5.423( 1) 23.316(8) 24.422(0) 3090.7 Table 3 Parameters and unit-cell volumes of phase A (compositions on line 3) at 865 "C in air mole fraction of metal component N(i)=n(i)/(nBi+nSr+nCu) 0.410 0.310 0.280 5.380(8) 24.621(0) 712.8 0.425 0.325 0.250 5.380(8) 24.62 1 (0) 712.8 0.440 0.340 0.220 5.391(7) 24.617(6) 715.6 0.450 0.350 0.200 5.394(4) 24.615(4) 716.3 0.460 0.360 0.180 5.395(5) 24.631(7) 717.1 0.470 0.370 0.160 5.395(5) 24.631(7) 717.1 the two phases, or a metastable region in the vicinity of a transition boundary between them.A similar situation exists near the boundaries of these phase fields and excess CuO. Special attention was paid to the area in the vicinity of the compositions Bi2Sr3Cu20z (C) and Bi,Sr,Cu,O, (D), both described as single phase by Tkeda et a[." Samples with the exact compositions corresponding to the phases C and D, and others varying from them by ra. 1-2 mol% in all direc- tions were prepared. All samples, including those exactly corresponding to phases C and D, were composed of combi-nations of three of the following phases: phase B, phase D, Bi,Sr,O,, SrCuO, and Sr,,Cu,,O,, .Increasing the firing time at 865°C to 150-170 h did not result in the formation of single-phase material. Two series of samples (located on lines 4 and 5 on Fig. 1) were further fired for a total of 200-250 h at 865 "C. XRD showed that the intensities of the reflections from phase B and from the Sr-Cu-0 phases decreased and the refleFtions from B~,Sr,Cu,O, (D),oorthorhombic with II =34.035 A, b = 24.050 A and c =5.389 A, increased. None of the samples were single phase. At all stages of the investigation the sample with J. MATER. CHEM., 1994, VOL. 4 h u)c.-C ?em v u)c 3 0 * I ,,I, ,,,,I , 1 20 25 30 35 40 2Wdegrees Fig.2 XRD pattern from a sample with nominal composition Bi,Sr,Cu,O,. 9.Bi,Sr,CuO, peaks; X, Bi,Sr,Cu,O, peaks nominal composition C contained three phases: B, D and Sr14Cu24041(Fig. 2). Owing to the Sr14Cu24041peaks both being very weak and being situated on the shoulders of peaks of the majority phases, they have not been marked on the figure. Conclusions Equilibrium reactions within this phase diagram are clearly sluggish and can easily prevent the required phase formation unless extreme care is taken during the sintering process. Consequently, the comparison of results from different groups must consider the precise reaction temperatures and times. We have found that at 865°C in air the following binary oxides exist within the Bi-Sr-Cu-0 phase diagram; Sr,CuO,, SrCuO,, Sr14Cu24041,Bi,SrO,, Bil +$r1 -xO, (0.08<x d 0.14), Bi2Sr,0s, Bi,Sr,O, and BiSr,O,.Only two ternary oxides were observed; Bi,Sr,CuO, (existing as both a tetra-gonal and an orthorhombic phase, both with significant homo-geneity ranges) and Bi,Sr,Cu,O,. No evidence was seen for either Bi,Sr,Cu20, (C) or Bi,,Sr,,Cu,O,, which falls within the boundary of the phase B homogeneity range. Over nearly one half of the diagram, in the Bi-rich area, all compositions were liquid at this temperature. The authors are grateful to Dr. Yu. M. Yarmoshenko for useful discussions and help with the structural work.V.A.Ch. is grateful to the Royal Society for the award of a Fellowship. References 1 J-S. Boivin, D. Thomas and G. Iridot, C.R. Seances Acad. Sci., Ser. C, 1973,276, 1105. 2 R. S. Roth, J. R. Dennis and H. F. McMurdie, Phase Diagramsfor Ceramists, American Ceramic Society, Westerville, OH, 1987, Fig. 6392. 3 J. Cassedanne and C. P. Campelo An. Acad. Bras. Cienc., 1966, 3a,33. 4 B. G. Kakhan, V. B. Lazarev and I. S. Shapligin, Russ. J. Inorg. Chem. (Engl. Transl.), 1979, 24, (6): transl. from Z. Neorg. Khim,, 1979,24,1663. 5 E. M. McCarron, M. A. Subramanian, J. C. Calabrese and R. L. Harlow, Muter. Res. Bull., 1988,23, 1355. 6 V. D. Gorobchenko, A. V. Irodova, 0. A. Lavrova md G. V. Laskova, Sverkhprovodimort: Fiz., Xhim., Tekh., 1989,2 (12), 136.7 C. H. Teske and H. Muller-Bushbaum, Z. Anorg. Chem., 1969, 371, 325. 8 N. G. Schmahl and E. Minzl, 2.Phys. Chem., 1965,47,358. 9 C. H. Teske and H. Muller-Bushbaum, 2. Anorg. Chem., 1970, 379, 234. 10 JCPDS card 38-1178. 11 J. Liang, Z. Chen and F. Wu, Solid State Commun., 1990,75, 247. 12 H. Kitagushi, M. Ohno, M. Kaichi, J. Takada, A. Osaka, Y. Miura, Y. Ikeda, M. Takano, Y. Bando, Y. Takeda, R. Kanno and 0.Yamamoto, J. Ceram. Soc. Jpn., Int. Ed., 1988,96,388. 13 J. Hahn, 1. 0. Mason, S-J. Hwu and K. R. Poepelmeier, Chemtronics, 1987,2, 126. 14 C. H. Teske and H. Muller-Bushbaum, 2. Anorg. Chew., 1970, 379, 113. 15 I. Siegrist, L. R. Schneemeyer, S. A. Sunshine, J. V. WasLczak and R. S. Roth, Muter. Res. Bull., 1988,23, 1429.16 R. S. Roth, J. R. Dennis and H. F. McMurdie, Phase Dillgramsfor Ceramists, American Ceramic Society, Westervifle, OH, 1987, Fig. 6428. 17 R. Guillermo, P. Conflant, J-S. Boivin and D. Thomas, Rec. Chim. Miner., 1978, 15, 153. 18 Y. Ikeda, H. Ito, S. Shimomura, Y. Oue, K. Inaba, Z. Hiroi and M. Takano, Phys. C, 1989,159,93. 19 J-C. Boivin and D. Thomas, J. Solid State Chem., 1980,35, 192. 20 J-C. Boivin and D. Thomas, Solid State Ionics, 1981,3-4,457. 21 P. Conflant, M. Drache, J. P. Wignacourt and J. C. Boivin, Muter. Res. Bull., 1991,26, 1219. 22 R. S. Roth, C. J. Rawn, B. P. Burton and F. Beech, J. Res. Natl. Inst. Stand. Technol., 1990,95, 291. 23 E. Yu. Vstavskaya, A. Yu. Zuev, V. A. Cherepanov, S. 11. Sutton and J. S. Abell, J. Phase Equilib. submitted for publication. 24 C. C. Torardi, M. A. Subramanian, J. C. Calabrese, J. Gopalakrishnan, E. M. McCarron, K. J. Morrissey, T. R. Askew, R. B. Flippen, U. Chowdhry and A. W. Sleight, Phys. Reii. B, 1988,38,225. 25 J. A. Saggio, K. Sujata, J. Hahn, S. J. Hwu, K. R. Poeppelmeier and T. 0.Mason, J. Am. Ceram. SOC.,19%9,72,849. 26 D. S. Sinclair, J. T. S. lrvine and A. R. West, Jpn. J. Appl. Phys., 1990,29,2002. 27 A. R. West, Solid State Chemistry and Its Applications, John Wiley and Sons, Chichester, 1984, p. 367. Paper 4/03044C; Received 23rd Mlzy, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401871

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(12), 1875-1881 Wet Chemical Syntheses of Ultrafine Multicomponent Ceramic Powders through Gel to Crystallite Conversion P. Padmini and T. R. Narayanan Kutty Materials Research Centre, Indian Institute of Science, Bangalore -560 012, India Coarse B"+O,,,-xH,O (10 <x <120) gels, free of anionic contaminants react with A(OH), solutions under refluxing conditions at 70-100°C giving rise to nanoparticles of multicomponent oxides (A=Ba, Sr, Ca, Mg or Pb; B=Zr, Ti, Sn, Fe, Al or Cr). These include ABO, perovskites and their solid solutions, polytitanates, hexaferrites and related phases, aluminates with spinel or tridymite structure and chromates. The nanosized crystallites are often in metastable phases, such as cubic BaTiO, at room temperature or superparamagnetic hexaferrites.Through the same route, luminescent phosphors of aluminates doped with rare-earth metals could be prepared. The present results indicate the general features of the gel-crystallite (G-C) conversion involving the instability of the metal hydroxide gel brought about by the disruption of the ionic pressure in the gel as a result of the faster diffusion of A2+ ions through the solvent cavities within the gel frame work. This is accompanied by the splitting of the bridging groups like B-(OH)-B or B-0-6, leading to the breakdown of the gel into crystallites. G-C conversion has advantages as a method of synthesis of ceramics in terms of operational cost and procedural simplicity. Wet chemical synthesis (Table l)i-3of ultrafine ceramic pow- ders continues to be a subject of intense research activity as the products exhibit advantages over powder derived from conventional ceramic routes.The main advantages of these processes are the increased homogeneity and high surface area of the resulting powders which lead to relatively high reactivity and hence low sintering temperatures. Sol-gel pro- cessing is the most widely employed route and involves a colloidal sol that is converted into a gel through ageing. The gel is subsequently calcined, giving rise to crystalline prod- UC~.~-~The powder characteristics, such as particle size, par- ticle shape, crystallinity, phase content, surface area and purity, are mostly dependent on the conditions of ~alcination.~ As an alternative method, the gel can be converted directly to crystallites, even in the presence of solvent, because of the instability of the gels caused by the influx of alien ion^.^,^ The perovskite titanates and their solid solutions have been pre- pared by this method.$ This technique differs from the sol-gel process in that high temperature calcination is not necessary for the formation of multicomponent powders.G-C conver-Table 1 Different chemical processes for the preparation of multicom-ponent oxide systems I. synthesis from complex precursors (thermal decomposition): (a) oxalate route (h)citrate route (c) catecholate route (d) acetate route (t')complex cyanide route 11. (a) co-precipitation (b)freeze-drying 111.evaporative decomposition: (a) spray pyrolysis (b) liquid mix process IV. sol-gel processing: (a) mixed alkoxide route (b) carboxy-alkoxide route (c) hydroxide-alkoxide route V. hydrothermal synthesis VI. gas-phase reactions (plasma or laser technique) VII. self-propagating combustion sion can take place even with coarser gels so that the raw materials need not include expensive organometallics or alkox- ides. G-C conversion is not restricted to titania gel but can be extended to other metal hydroxide gels and ceramic powders of multicomponent oxides, e.g. ferrites, aluminates, zirconates, polytitanates. The general reaction involved in this technique is the breakdown of the gel network because of the change in ionic pressure brought about by the chemical influx of aliovalent ions.It is generally thought that the reactive gels need be prepared from alkoxides or organometallics, but the present results show that coarser gels prepared from any soluble compound can be equally reactive provided there are no interfering anionic contaminants and that the gel is not aged. In comparison to all the known wet chemical techniques, this method has advantages in operational cost and procedural simplicity. Experimental Gels of the hydrated metal hydroxides B"+(OH),*xII,O or B"+O,,,.xH,O (where B=Ti4+, Zr4', Sn4+, A13+. Fe3+, Cr3+ etc.) were prepared by the addition of ammonium hydroxide at 30-40 "C to the corresponding chloride, nitrate or sulfate solutions until the pH was 6-8.The gels were washed free of the anions and ammonium ions. The presence of anionic contaminants such as SO,,-, NO3-or C1-impedes the reaction. No special care was taken to control the particle size of the gel. The extent of water content in the gel was determined quantitatively through weight loss measurements by thermo- gravimetric analysis. The gel was suspended in A(OH), solu- tion, where A =Ba2+, Sr2+, Pb2+, Ca2+ etc., in a flask fitted with a water-cooled reflux condenser. Air in the vessel was displaced by nitrogen and entry of carbon dioxide prevented by an alkali guard tube. The reaction was carried out at 70-100°C for 0.5-4 h. The solid phase remaining in the reaction vessel was filtered off, washed free of A(OH)2and air dried.Unlike the gels, the solid phase formed was crystalline, as identified by electron or X-ray diffraction. The solid phase is not crystalline to X-rays when the concentration of A(OH), is <0.02 mol I-'. The kinetics and mechanism involved in the reaction will be dealt with in a later paper. Phase identification of the powders was carried out by X-ray powder diffraction using a Philips PW 1050/70 J. MATER. CHEM., 1994, VOL. 4 diffractometer with a step scan facility. Particle size and shape were evaluated by the intercept method of micrographs from the transmission elecp-on microscope (TEM) (JEOL 200 CX, 200 kV) having a 2 A resolution. The chemical compositions of the products and any contaminants were determined by wet chemical analysis using atomic absorption spectroscopy (AAS). Mossbauer spectra were recorded at constant acceler- ation in conjunction with a Nuclear Data Instruments ND60 multichannel analyser using a 57C0 source in a rhodium matrix.The experimentally observed Mossbauer spectra were curve-fitted by a least-squares method by computer, assuming Lorentzian lineshapes. The photoluminesence (emission and excitation) spectra were recorded at room temperature using a Hitachi 650-10s fluorescence spectrophotometer equipped with a 150 W xenon lamp and a Hamamatsu R928F photo-multiplier. Thermal analyses were performed on a simul-taneous TG-DTA instrument from Polymer Laboratory, STA 1.500.Results During the initial stages of the reaction a considerable decrease in gel volume was brought about by the disintergration of the coarse gel by the influx of A'+ ions. This decrease in volume was observed only when there was chemical interaction between the A2+ and B"+O,!,*xH,O gel. Otherwise the gel remained as a sticky mass with little volume change. As the reaction proceded, the gel lost its appearance and was con- verted into a flowing powdery mass. The amount of solvent entrapped in the gel was ca. 95% by weight, as it is for most metal hydroxide gels. The relatively open network of gels filled with water may form enhanced diffusion paths for A2+ ions to penetrate the reaction sites. The gel stability could be altered by the use of an organic solvent such as ethanol or acetone. Powders prepared in ethanol exhibited a more uniform particle size distribution. The multicomponent oxide phases prepared during the current investigations are listed in Table 2.These systems were chosen because of their wide-ranging applications. Titanates, Zirconates and Stannates The preparation of the perovskite phases has been reported.8 The perovskite phases prepared by G-C conversion are frequently metastable; e.g. BaTiO, has cubic symmetry at Table 2 Phases formed at 100 "C through G-C crystallite conversion titanates BaTiO,, SrTiO,, PbTiO,, (Ba,Sr)TiO,, (Ba,Pb)TiO,, (Ba,Ca)TiO,, Ba(Zr,Ti)O,, Ba(Sn,Ti)O, (1.05 :1.0 in all cases), BaNd,Ti,O,, (1.05:2.0 :5.0), Nd,Ti,O,, (4:9) zirconates BaZrO,, SrZrO,, (Ba,Sr)ZrO,, Ba(Ti,Zr)O,, Sr(Ti,Zr)O, (1.05 :1.0 in all cases) Ba polytitanates BaTi,O,, +TiO, (1 :6) BaTi,O,, +BaTi,O, (1 :4.5) BaTi,O, (1 :3) Ba,Ti,,O,, (1:2) BaTi,O, (1 :1.5) BaTiO, +BaTi,O, (1:4) [in presence of Mg2+] ferrites BaFe,,O,,, SrFe,,O,, (1.05:6.0 in both cases), BaCo,Fe,,O,, (1.05:2.0: KO), Ba,Ni,Fe,80,, (2.05 :2.0: 14), Ba,MnzFe3,06, (4.05 :2.0 :18) aluminates SrAl,O,, BaA1,0,, CaAl,O, (1.05 : 1.0 in all cases), BaAll2Ol9, SrA1,,0,, (1.05:6.0 in both cases), BaMgAl,,O1, (1.05: 1.0:5.0), BaMg,Al,,O,, (1.05 :2.0 :8.0), Sr,,,Mg,Al,,O,, (5.5:6.0:22.5) chromates BaCr,O,, SrCr,O, (1.05 :1.0 in both cases) The molar ratios in the reaction mixtures are in brackets.room temperature, [Fig.l(a)]. This is true also for titanate solid solutions, even when strongly structure-distorting ions such as Pb2+ or Ca2+ are present. Chemical analyses were carried out to confirm the purity of the resultant phases. The chemical composition of the annealed products have 58 wt.% Ba and 20 wt.% Ti for BaTiO, and 49 wt.% Ba and 32 wt.% Zr for BaZrO, powders, roughly corresponding to 1:1 mole ratios in both cases. Chemical tests of the solutions were performed at different stages of the reaction after ultracentri- fugation in order to detect the B component. These tests were negative. As the temperature of the reaction was 6 100cC, the possibility of dissolution of the B"+On,2*xH20 gel in A(OH)2can be excluded. Moreover, if the dissolution-crystal- lisation process prevailed, definite morphological features of the crystallites would be expected, which is not observed for these products formed at such low temperatures and 1 atm pressure.Hence a G-C transformation may be envisaged where the structural rearrangement caused by A2+ ions within the gel is followed by dehydration. As dehydration is slower at lower temperatures, there is a partial retention of OH- and H20 in the structure. The presence of such network- modifying impurities enhances structural disorder so that the cubic phase remains metastable." The particle size decreases with increasing Ba(OH), concentration or A :B ratio. The extent of aggregation depends upon the initial Ba(OH)2concentration. Crystallites formed from TiO, xH20 and 0.4 mol 1-' Ba(OH), are nearly spherical and are 5-15 nm in diameter.The degree of aggregation is greater for BaTiO, formed from higher A :B ratios and Ba(OH), concentrations.* The particle size ranges from 0.5 to 5 nm. Only H,O was evolved during heating. Heating the sample to nearly 1200"C for 2 h in air, and subsequent cooling to room temperature, yields tetragonal BaTiO,. TG of this sample shows ca. 4-5% weight loss at 200-300 "C. The weight remains nearly constant up to 1200°C. The same trend is observed in the case of zirconates, ferrites and aluminates. Semiconducting BaTi0, ceramics prepared by this route were found to give excellent PTCR (positive temperature coefficient of resistance) characteristics, possibly because of the absence of alkali, which is commonly retained in aged gel products.* This clearly shows that the products obtained by G-C conversion are phase-pure. For A :B <0.67, Ba polytitanates were formed, as shown in Table 2.The actual phase stabilized depends upon the A: B molar ratio and not the effective concentration of A(OH),. It can be seen from Table 2 that the A:B ratio originally maintained in the reaction mixture does not coincide with that of the corresponding solid phases recovered because minor amounts of A(OH), remain unreacted and are finally washed out. Polytitanates such as BaTi,O, and Ba,Ti,O,, are of considerable interest for microwave dielectric appli- cations, particularly as microwave resonators." Fig.1 (b) shows the X-ray diffraction pattern of BaTi,O, formed by this route. This polytitanate has a relative permittivity of ca. 40 at 10 GHz.12Ternary oxides, such as BaNd2Ti5014, have also been synthesized by the present method, with a reported relative permittivity of ca. 80 at 10 GHz.', Impurities play an important role in the stabilisation of the phases. For example, when Mg2+ is present as an impurity, a mixture of BaTiO, and BaTi,O, is obtained with Ba :Ti z 1:1 in the starting composition. When Ba:Ti=1:4 and 5-10 mol% of Mg2+ is present, the only phase formed is BaTi,O,. However, incorporation of Mg2+ into the titania framework was found to be impossible so that ilmenite, MgTiO,, which has 6 :6 coordination, is not formed by this route.Ferrites Reactions were carried out with iron(r1r) hydroxide gels and A(OH), for various compositions indicated by the phase J. MATER. CHEM., 1994, VOL. 4 28/degrees Fig. 1 X-Ray diffraction pattern of (a) BaTiO, exhibiting cubic symmetry (metastable) at room temperature and (b)BaTi,O, diagram of the BaO-MeO-Fe,O, systems14 (Fig. 2) where Me=Mn, Fe, Zn, Co, Ni or Mg. The compositions of the various ferrites are given by the points M, W, X, Y, Z and U in the phase diagram. These hexagonal ferrites with end- member compositions of BaFe12019 or SrFel,Olg are well kno~n.'~~'~The Fe3+ ions occupy five non-equivalent lattice sites in interstices between the oxygen ions. The compositions of the other ferrites are listed in Table 2.The as-prepared powders are found to be X-ray amorphous but crystalline by electron diffraction. The particle size calculated by the linear intercept method from the micrograph [Fig. 3(a)] is of the order of 3-10 nm and the corresponding electron diffractog- 50 50 BaO Me0 Fig. 2 Phase diagram of BaO-MeO-Fe,O, system where Me stands for divalent metals such as Mn, Fe, Zn, Co, Ni or Mg and the points M, W, X, Y, Z and U corresponds to the compositions; M, BaFelzO19; W, BaMe,Fe,,O,,; X, Ba,Me,Fe,,O,,; Y, Ba,Me,Fe,,O,,; Z, Ba,Me,Fe,,O,,; and U, Ba,Me,Fe,,O,, ram [Fig. 3(b)] shows a ring pattern that is characteristic of a mostly polycrystalline material with ultrafine particle size. Ultrafine particles of ferromagnetic and ferrimagnetic mate- rials often exhibit superparamagnetic behaviour, as shown by the Mossbauer spectra of Ba ferrite, Fig. 4(a).The superpara- magnetic state refers to single-domain particles exhibiting a uniform spontaneous magnetisation.'6-'8 The isomer shift, which gives an idea of the chemical bonding, was calculated to be 0.32. Superparamagnetic behaviour was found to persist for the powders even after they had been heat treated at 400-500 "C for 96 h. X-Ray crystallinity was achieved only when the powders were heat treated at temperatures >950 "C: their Mossbauer spectra exhibit multiplets characteristic of Ba ferrite [Fig. 4(b)]. For each hyperfine component, five extra lines are seen, which correspond to five different crystal sites occupied by Fe3+ ions.Thus the microscopic and Mossbauer studies clearly point to the fact that the G-C conversion route gives rise to nanosized ceramic pobders in a metastable state. Aluminates Various aluminate phases formed through the G-C conver-sion route are listed in Table 2. Generally they fall into three structural types: (1)hexa-aluminates," AAl12019; (2) AA1,04, where A =Ba2+, Sr2+ etc., which are related to the distorted 'stuffed' tridymite structure;20$21 (3)hexa-aluminate derivatives with differences in the stacking sequence of the spinel blocks. Phases with distinguishable X-ray patterns can be prepared by varying the A:Al ratio. Unlike the titanate or zirconate systems, the Mg2+ ions can be incorporated in the aluminium- oxygen network because of the similar co-ordination of MgO, and AIO, tetrahedra.AAl,O, phases are formed only at higher concentrations of A(OH),. The distorted tridymite lattice retains the A2+ ions within the open channel which is along the pseudo-hexad axis. Compounds of general formula H15nm Fig.3 TEM micrographs of (a) as prepared powders of Ba-ferrite with particle size of the order of 3-10nm and (h) the selected area diffraction pattern of (u) A,A1203+n (n< 1) have been reported by hydrothermal prep- arations.22 However, under hydrothermal conditions, the pre- dominant stable phase has n = 1, i.e. AA1,0,. This phase does not show any tendency to take up Mg2+ ions, possibly because of the large size of the open channels, whereas the incorporation of Mg2+ ions in hexa-aluminate derivatives, e.g.BaMgAlloO1,, does take place. The changes in the X-ray diffraction pattern between the end-member SrA1,,0,,, and BaMgAlloO,, are shown in the Fig. 5. The crystallite size of the aluminates lie in the range 10-25 nm. Boric oxide is usually added in minor concentrations (< 2 wt.%) for attaining better crystallinity through particle size enhancement for al~rninates.~~Hence small amounts of B203 were introduced in the reaction mixture during the synthesis. By this method, improved crystallinity is observed for the aluminates (particle size 30-1 10 nm). The most significant feature of the aluminates prepared by G-C conversion is their ability to retain rare- earth-metal ions which act as luminescent hosts for indepen- dent ionic luminescent centres.This is typically illustrated with Eu2+ in SrA1,,0,, and by multiple dopants, e.g. Ce3++ Mn2+ +Tb3+ in BaAl12019 after reduction in an N,-H, atmosphere at 950 "C. The single-band luminescence J. MATER. CHEM., 1994. VOL. 4 (b1 I I1 1 I I1 I1 I I1 I1 I Ill 1 108642024 6810 velocity/m m-' Fig. 4 Mossbauer spectra of Ba-ferrite (a) as prepared powder and (b)heat treated at 950 "C spectrum of Eu2+ in SrAll,Ol9 is shown in Fig. 6. The excitation band is maximum around 290 nm and the emission band around 398 nm. The (4f)6(5d)' excited state of Eu2+ shows considerable relaxation effects in hexa-aluminates, as indicated by the large Stokes shift. In the case of other aluminates, AAln03 + n, Eu2+ luminescence is dominated by the green band and the 398 nm band.22 Ce3+ acts as a good sensitizer in most aluminates.This is illustrated with BaAl120,, co-doped with Ce3+, Mn2+ and Tb3+ in Fig. 7. This material has multiple excitation bands corresponding to the 5d-4f state of Ce3+ with various ionic pairs such as Ce3+-Ce3', Ce3+-Mn2+(Tb3+).The Ce3+-sensitized lumin- esence (i,,,= 270 nm) gives rise to a strong green band around 520nm. This emission arises from Mn2+ in the tetrahedral site. Along with the Mn2+ emission band, those of Tb3+ can also be observed. However, when the ibex,is shifted to longer wavelength (310 or 340 nm), the emission band due to Tb3+ alone is observed.These results show that efficient luminescent phosphor materials can be prepared by the G-C conversion route. Chromates The chromates listed in Table 2 were also synthesized through the G-C conversion route. They were found to be crystalline, with orthorhombic symmetry.20 The particle size of the chro- mates ranged from 20 to 150 nm. Discussion The present results show that G-C conversion is a general technique for the preparation of multicomponent oxides as nanosized particles, provided the gel is reactive. Results from various systems indicate that the first phase formed, is in general, metastable because of the retention of the OH-in the structure. Metastability is observed even in the case of the ferrites wherein the as-prepared powders exhibit superpara- magnetic behaviour.The basic reactions involved in hydro- thermal and G-C conversion are the same, except that J. MATER. CHEM.. 1994, VOL. 4 Cu-Ka I-$ I I I I I I 1 I 1 I 24.2 38.4 52.6 66.8 28Ydegrees Fig. 5 X-Ray diffraction pattern of (a)SrA1,,0,, and (b)BaMgAl,,O,, 100 N+100 i ii 80 80;;i 3 h .-5.z 60 E 60 C0, Q,c c C .-C .-.-P .-9 c c. aQ3 40 5 40 2020 0 200 300 400 500 6000 200 300 400 500 Unm NVnm Fig. 7 Excitation and emission spectra (300 K) of BaAl,,O,, doped Fig.6 (u) Excitation and (b)emission spectra (300 K) of SrAlI2Ol9 with Ce3+, Mn2-and Tb3+.(u) i.,,=520 ntn (Mn”); (b) &,= 398 nm and i.,.=290 nm 270 nm; and (c) Ewe, =3 10nmdoped uith Eu”; iem= m~nocrystallites~"~~are produced in the former route whereas the latter method gives rise to polycrystallites.Metal hydroxide gels are in general polymeric chains, forming an entangled network in which the solvent is entrapped. The stability of the gel is dependent on three factors:26( 1) polymer-polymer affinity; (2) rubber elasticity; and (3) the ionic pressure. The sum of these three components is the osmotic pressure which determines whether the gel will take up fluid or expel it. If any one of the factors is altered, the gel becomes unstable and the gel network collapses irreversibly. In the present context, it is the ionic imbalance that contributes to the collapse of the gel network. The polymer network tends to ionise, giving rise to H+ or OH- depending upon the nature of the chemical species in the network.Whether H+ or OH-is dominant will depend on the pH at which the gel stabilises. For B"+O,,,.xH,O gels, ionisation leads to OH- ions, which are able to move around the entrapped solvent within the gel and contribute a pressure similar to that of the randomly moving gas molecules. Electrically they are neutralised by the positive charge retained by the polymer network. The rapid influx of aliovalent ions such as A2+ into the gel cavities containing entrapped solvent can upset the charge balance. Hence the system has to rearrange itself to maintain neutrality. Since there is a continu- ous influx of ions, the interactive stability between the network and the solvent breaks down.The imbalance in the electrical charge has to be countered by an appropriate amount of H+ or OH-. Under such a continuous influx of A2+ ions it becomes difficult to maintain the charge balance and hence the gel collapses. The collapse of a gel can be brought about at constant temperature and constant solvent composition by changing the pH of the solution or adding a salt. Both of these factors can alter the effective ionisation of the polymer network. The influx of A2+ can couple with the ionic charges on the framework. The presence of anionic contaminants, OH J. MATER. CHEM.. 1994, VOL. 4 such as SO4,-, NO,-or C1-, can impede this reaction. These anions may try to couple with the H+ as well as A*+ and maintain neutrality. If this happens, the gel persists.Hence the presence of such ions impedes G-C conversion. Divalent ions in low concentration are more effective in bringing about gel collapse than monovalent ions. Splitting of bridging groups such as B-(0H)-B and B-0-B, accompanied by chemical rearrangements are possible as the gel shrinks. This causes the splitting of the coarser gel into finer particles so that the reaction rate is increased by the influx of A:+ ions. This results in polyhedra that are free of solvent, giving rise to a stable multicomponent oxide lattice. The process is illustrated in the case of TiO, .xH,O gel which is converted directly into ATiO, crystallites by the influx of A2+ ions (Fig. 8).It is inevitable that OH-ions are retained randomly in the place of 0,-sites. Bridging hydroxy groups escape the crystallites only after heat treatment. The basic mechanism involved in the formation of the multicomponent oxides may be the de- olation of the bridging groups such as B-(OH)-B and B-0-B (because of the increase in pH within the gel) followed by oxolation, leading to the formation of Bn+O,,,, which are charge compensated by the A2+ ions to form a stable multicomponent oxide lattice. The presence of hydrophilic solvent such as an alcohol or acetone can bring about a faster reaction because of their affinity for H,O, so that the water molecules from the gel do not get back to the gel network. The stabilit? of the gel is solvent dependent.However, this is not the prime factor in the present case, since the presence of such solvents without A(OH), does not lead to the disintegration of the gel network. Conclusions A disordered framework can give rise to an ordered crystalline lattice because of the decrease in the free energy of the system. Fig.8 Reaction mechanism involved in the formation of ATiO,, where A=Ba2+ or Sr2+; 0,H,O; 0,OH-; 0,Ti4+:0,A2+ J. MATER. CHEM., 1994, VOL. 4 However, it tends to result in metastable phases. Results from various systems show that G-C conversion can be easily adapted for the preparation of multicomponent oxide systems. References 1 P. P. Phule and S. H. Risbud, J. Muter. Sci., 1990,25, 1169. 2 A. D. Hilton and R.Frost, in Electronic Ceramic Materials, ed. J. Nowotny, Trans Tech Publications, Switzerland, 1992, p.145. 3 J. Twu and P. K. Gallagher in Properties and Applications of Perovskite-type Oxides, ed. L. G. Tejuca and J. L. G. Fierro, Marcel Dekker, New York, 1993, p.1. 4 L. L. Hench and D. R. Ulrich, Ultrastructure Processing of Ceramics, Glasses and Composites, Wiley-Interscience, New York, 1984, p.43. 5 C. W. Turner, Am. Ceram. Bull., 1991,70, 1487. 6 C. J. Brinker and G. W. Scherer, J. Non Cryst. Solids, 1985,70,31. 7 G. Tomandl, H. Rosch and A. Stiegelschmitt, in Better Ceramics through Chemistry, ed. C. J. Brinker, D. R. Clark and D. R. Ulrich, Materials Research Society, Pittsburg, 1988, vol. 3, p.281. 8 T. R. N. Kutty and P. Padmini, Muter.Res. Bull., 1992,27, 945. 9 M. I. Diaz-Guemes, T. G. Carreno, C. J. Serna and J. M. Palacios, J. Muter. Sci., 1989,24, 1011. 10 R. Vivekanandan and T. R. N. Kutty, Powder Tech., 1989,57,181. 11 H. M. O’Bryan and J. Thomson JR., J. Am. Ceram. SOC., 1974, 57. 522. 12 D. J. Masse, R. A. Purcel, D. W. Readey, E. A. Maguire and C. P. Hartwig, Proc. IEEE., 1971,59, 1628. 13 K. Wakino, K. Minai and H. Tamura, J. Am. Cerurn. SOC.,1984, 67, 278. 14 H. J. Vink, Philips Tech. Rev., 1962/63,24, 364. 15 P. B. Braun, Philips Res. Rep., 1957, 12,491. 16 B. D. Cullity, Introduction to Magnetic Materials. Addison-Wesley, London, 1972, p.410. 17 C. I?. Bean and J. D. Livingston, J. Appl. Phys., 1959,30, 120s. 18 Y. Ishikawa, J. Appl. Phys., 1964,35, 1054. 19 A. L. Lindop, C. Mathews and D. W. Goodwin, Acta Crystallogr., Sect. B, 1975, 31,2940. 20 F. P. Glasser and L. S. Dent Glasser, J. Am. Ceram. Yoc., 1963, 46, 377. 21 H. D. Megaw, Crystal Structures, A Working Approuch, W. B. Saunders, London, 1973, p.312. 22 T. R. N. Kutty, R. Jagannathan and R. P. Rao, Muter. Res. Bull., 1990,25, 1355. 23 B. Smets, J. Rutten, G. Hoeks and J. Venlijsdonk, J. Ektrochem. Soc., 1989, 136, 2119. 24 T. R. N. Kutty, R. Vivekanandan and S. Philip, J. Rlater. Sci., 1991,25,3649. 25 T. R. N. Kutty, R. Vivekanandan and P. Murugaraj. Muter. Chem. Phys., 1988, 19, 533. 26 T. Tanaka, Sci. Am., 1981,244, 110. Paper 4/01268B; Receizjed 2nd March, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401875

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 12), 1883-1887 Accommodation of the Misfit Strain Energy in the BaO(100)/Mg0(100) Heteroepitaxial Ceramic Interface using Computer Simulation Techniques Dean C. Sayle,*+“Stephen C. Parker“ and John H. Hardingb a School of Chemistry, University of Bath, Avon, UK 6Al 7AY AEA Technology, 6424.4, Harwell Laboratory, Didcot, Oxfordshire, UK OX1 I ORA Static atomistic simulation techniques have been employed to investigate the accommodation of the misfit strain energy in the BaO(l00)/Mg0(100) interface. The materials return to their natural (bulk) lattice parameters a few planes from the interface, while maintaining expanded or contracted lattice parameters at the interface to ensure charge matching of counter ions. BaO also forms three-dimensional islands when grown on MgO(l00), in accordance with molecular beam epitaxy results.This behaviour is attributed to the instability of a monatomic BaO layer on MgO compared with a BaO bilayer. The formation and structures of thin film oxide interfaces have received much attention recently, particularly in the field of superconductivity. An understanding of the interfacial structure at the atomic level is critical to the development of thin films. For example, the modification of the thin film crystal structure as a result of the interface is important for the many applications of thin-film interfaces, which depend on the materials maintaining long range structural coherence. The formation of a heteroepitaxial interface necessitates matching two incommensurate lattices.This can be achieved by compression or expansion of either or both of the compo- nent materials in order to accommodate the misfit. The resulting strain introduced into the interface will destabilise the interface. In this paper, we employ atomistic simulation techniques to investigate the accommodation of the misfit strain energy induced in forming a heteroepitaxial ceramic interface. The relaxation of ions can destroy the long-range coherence in the overlaying thin film, resulting in a deterior- ation of the material properties. As an example, we investigate the BaO/MgO interface. This was chosen for its simplicity and because robust potentials (including a consistent oxygen potential for the component materials) are available.Furthermore, this interface has appli- cations in the field of thin-film superconductors, where the BaO plane can terminate the YBa,Cu,O,~, thin film at the YBa,Cu,O,.S interface., Cotter et have investigated the BaO/MgO interface4 and, more recently, McKee et al.’ have investigated the stability and the growth of optical-quality perovskites on MgO with either BaO or TiO, terminating the BaTiO, at the interface plane.5 First, we investigate a bulk BaO( 100)/Mg0( 100)interface. In particular, we investigate the behaviour of the interface when it meets a surface and thus allows interface edge effects to be included for the first time in our simulations. Second, we investigate a monolayer of BaO( 100)on a bulk MgO( 100) substrate.Finally we examine the formation of a ‘BaO mono- layer’ on an MgO substrate. Simulation Code and Potential Model The simulations in this work were performed on a CRAY X-MP4j8 at the Atlas Division of the Rutherford Appleton Laboratory, using the energy minimisation codes MIDAS, t Present address: Royal Institution of Great Britain, 21 Albermarle Street, London, UK W1X 4BS. and CHAOS.’ The MIDAS program considers the crystal as a stack of planes periodic in two dimensions (Fig. 1). The stack is divided into two regions: region 1, where the ions are allowed to relax explicitly; and region 2, where the ions are held fixed relative to each other. Region 2 can, howe\er, relax as a whole, enabling the crystal to either ‘expand’ or ‘contract’. Region 2 is included to ensure that the potential for the ions at the bottom of region 1 are calculated correctly.The top of region 1 is the free surface, unless two such blocks are placed together, enabling the cohesive energy of the perfect crystal to be calculated. A heteroepitaxial interface is created if two dissimilar materials are placed together. The CHAOS code was employed to study the sequential addition of isolated BaO units to the MgO substrate. The approach is to divide the crystal ‘surrounding’ the BaO units into two regions (Fig. 2): an inner region 1, where all the ions are allowed to relax explicitly; and an outer region 7, which extends to infinity and is treated using a quasi-continuum approximation.We use an ionic model for the solid with full ionic charges. The long-range Coulomb interaction is treated by the standard Ewald summation. The short-range potentials betm een the -+-+-+-7block 2 Jregion I1-*-.-*--.-a # -surface iace region I region I -+-+-+-J .block 1 -+-+-+-region11 -+-+-+-+-+-+-+ -+-+-+-+-+-+-+ (b 1 Fig. 1 Schematic representation of the crystal regions for interface and surface calculations employed in the MIDAS code defect region IIb -00 Fig. 2 Representation of the two-region strategy used in the CHAOS code ions' were obtained by fitting to the bulk crystal properties. The shell model' was used to describe the electronic polaris- ability of the component ions.The potential parameters are given in Tables 1 and 2. Epitaxial Constraints The incommensurat? relationship betwten the lattice param- eters of BaO (2.77 A) and MgO (2.10 A) leads to a prohibi- tively large, two-dimensional interfacial unit cell. In a previous paper," we employed a near-coincidence site lattice theory to construct BaO/MgO interfaces of low misfit (less than 3%) and showed them to be more stable than interfaces of higher misfit. The misfit strain energy was alleviated by explicit relaxation of the interfacial ions, mainly perpendicular to the interfacial plane. Relaxation of these ions parallel to the plane of the interface was, however, restricted as the calculations were performed at constant surface or interfacial area.The relaxation behaviour is inevitably influenced by the constant surface area boundary conditions imposed on the simulation, which are necessary to ensure periodicity in neighbouring unit cells. The obvious problem in simulating interfaces without this constraint is that the incommensurate lattices could lead to an infinitely large unit cell if the standard configuration for the calculation is used. However, we can perform this type of calculation by creating a surface perpendicular to the interface. Cotter et aL4 have experimentally examined BaO( 100)layers on an MgO( 100) substrate conforming to the $a,( MgO) = a,( BaO) configuration or, more correctly, using coincidence- =site-lattice notation," CiaOIMgO1/2(0=45").We employ this interfacial configuration as a basis for our studies. This configuration is the smallest interfacial primitive unit cell with Table 1 Short-range parameters describing the short range potential energy terms between the component ions.The analytical function is of the form: V(r)=Aexp(-p/r) -CrP6. species AIeV PlA C/eV k6 Ba2'-02- 905.7 0.3976 0.0 Mg2+-02- 1428.5 0.2945 0.0 02--02- 22764.3 0.149 27.879 Table 2 Shell-model parameters employed to describe the polaris- ability of the component ions. The analytical function is of the form: LX= Y2/k. species Y/eV k/eV k2 Ba2+ 9.203 443.46 Mg2+ rigid ion O2--3.0 51.836 J. MATER. CHEM., 1994, VOL. 4 a lattice misfit of less than 10% and requires the BaO overlayers to be rotated 45" with respect to the MgO substrate.Three distinct configurations arise (Fig. 3); first, with the barium and oxygen of the BaO lattice constrained to sit above the magnesium sublattice of the MgO crystal surface; second, with the barium and oxygen constrained to sit above the oxygen sublattice; third, with the BaO sited above the inter- stitial sites of the MgO lattice. The most stable configuration is the first. We therefore use this interfacial configuration as a starting point for the calculations. Our method is to construct this interface and then create a surface perpendicular to the interfacial plane (Fig. 4).The relaxation perpendicular v-u 0 magnesium 0 oxygen @ barium Fig. 3 Representation of three configurations of BaO overlayers on an MgO( 100) substrate.The BaO is constrained to adopt positions above the oxygen sublattice of the MgO, above the magnesium sublattice and also at interstitial positions. interface -unit cell constant area MgO --. I I I I I I I 1 Fig. 4 (a) Original MIDAS interface construction constrained to constant interfacial area. Relaxation unconstrained only in the [1001 direction. Relaxation in [OOl] and [OlO] directions restricted to maintaining constant interfacial area. (b)Modified interface construc- tion enabling unconstrained relaxation in the [OlO] direction and hence allowing for an interfacial area change. Relaxation in [OOl] and [1003 directions restricted to maintain constant area. J.MATER. CHEM., 1994, VOL. 4 to the surface ([OlO] direction) or in the plane of the interface is now not restricted by the constant surface area constraint and therefore relaxation in the [OlO] direction can also be associated with a change in the interfacial area. However, in order to ensure periodicity in neighbouring unit cells, the constant surface area boundary condition must still be imposed. This results in undesirable interface-interface inter- actions. It is therefore essential to have the [100) direction of the unit cell as large as possible to minimise these interactions. In these calculations, the accommodation of misfit strain is partitioned between the BaO and MgO, on the basis of their respective bulk modulii, so as to introduce as little strain energy into the system as possible.Only 1.7% of the total 7% mismatch is therefore assigned to be accommodated by the MgO and the remaining 5.3% to the BaO lattice (MgO is expanded and BaO compressed). Results Bulk BaO( lOO)/MgO( 100) Interface Plate 1 shows the behaviour of the ions at the interface region after full relaxation. It is a side view of the interface; a thin slice is shown for clarity. A contraction of the BaO lattice away from the interface is clearly evident. The driving force for this relaxation is to relieve the strain in the system with the MgO and BaO crystals adopting their natural lattice parameters away from the interfacial plane. The BaO returns to its natural (bulk) lattice parameter five planes away from the interface region, while maintaining an artificially expanded lattice parameter at the interface.It is therefore apparent that the local interfacial strain is more than compensated for by Plate 1 Behaviour of the ions at the interface region after full relax- ation for a system with 10 BaO and 10 MgO planes explicitly relaxed in the [1001 direction. Oxygen is coloured red, barium white and magnesium purple. -view C Fig. 5 Construction of the interface allowing for relaxations in the plane of the interface [OlO] direction, for monolayer coverage. Views A, B and c' are indicated on the figure. the favourable cation-anion interactions across the interface. The contraction follows a smooth progression from the +5.3% mismatch at the interface.The MgO exhibits similar behaviour with the MgO planes away from the interface region adopting their natural lattice parameter while MgO planes at the interface region remain artificially compressed. However, MgO was compressed by only 1.7%; the behaviour is therefore less obvious from the figure. These findings are consistent with the results of Mckee et nL5who write 'the BaTiO, lattice parameter relaxes to its strain free bulk value within 10 unit cells from the interface.' Relaxation of the magnesium ions towards the BaO anions and away from the BaO cations (in the [loo] dirxtion) is clearly seen (in accordance with the results of Cotter et ~1.~). It is reassuring to see this behaviour as the relaKation is A c Plate 2 Three projections of the BaO monolayer on MgO after relaxation.Oxygen is coloured red, barium white and mignesium purple. Projections A, B and C are indicated in Fig. 5. constrained by the constant surface area boundary condition perpendicular to the interfacial plane ([ 1001 direction). A more subtle point to note is that the surface relaxation of the BaO and MgO lattices remains consistent with experimental observations of the ions at the surfaces, i.e. the oxygens relax out of the surface and the cations relax towards the bulk of the crystal.12.13 BaO( 100) Monolayer on MgO( 100) Substrate The calculations above have shown the relaxation of a bulk BaOiMgO interface when the constant interfacial area bound- ary conditions have been partially removed.We now investi- gate the relaxed structure of a BaO monolayer on an MgO substrate, again with the boundary conditions partially lifted. Fig. 5 shows the construction of the BaO monolayer on the MgO substrate. The BaO monolayer is not constrained by J. MATER. CHEM.. 1994, VOL. 4 further BaO layers and is therefore free to relax in both the [OlO] and [loo] directions. Of course, as with the bulk interface, any relaxation in the [OlO] direction might also be associated with an interfacial area change. The constant-area condition must again be imposed and will result in the interface being repeated in the [1001 direction. Plate 2 shows three projections of the structure of the monolayer interface after relaxation.Substantial reconstruc- tion occurs at the interface region. Furthermore, the BaO monolayer has resulted in a perturbation of the MgO substrate as far as 7 interatomic spacings from the interface. View A shows the BaO overlayer to be attached or 'pinned' to the top of the MgO substrate. In view B, magnesium and oxygen ions have migrated from the MgO crystal to the top of the BaO monolayer. View C suggests a substantial modification of the BaO cubic structure after relaxation to an almost hexagonal symmetry. These relaxations enable the BaO to Plate 3 Representations of the relaxed configurations of from one to eight BaO units [(a)-(h)]placed on the MgO( 100)surface. (i) illustrates the unrelaxed starting configuration of eight BaO units placed on the MgO(100) substrate. J.MATER. CHEM., 1994, VOL. 4 relieve the high strains induced at the interface. The BaO achieves a higher surface area compared with its natural (bulk) value by changing to hexagonal symmetry, i.e. the barium and oxygen ions are then three coordinated instead of four coordinated within the monatomic BaO plane. The removal of the constant-area constraint was developed using the BaO( IOO)/MgO(100)system because both BaO and MgO are relatively simple structures. It can also be used to model more complex interface systems. The application of this methodology to such systems must therefore wait for increases in computer size and speed, as these types of calculation are very computationally demanding.BaO Monolayer Formation on MgO( 100) from BaO Subunits In this section, we adopt a different approach by allowing the BaO monolayer to form from sequential additions of BaO units on the surface. The CHAOS code is employed for these calculations. This code can investigate isolated BaO clusters and hence there are no interactions between neighbouring BaO clusters. Furthermore, the calculations are not subject to constant surface area boundary conditions. Plate 3[(u-(h)] shows the relaxed structures of from one to eight BaO units added to the MgO substrate. Plate 3(i) shows the unrelaxed starting configuration of eight BaO units placed on the MgO( 100) substrate. The initial structures before relaxation were constructed by placi_ng BaO units flat on the substrate adopting the a,( BaO)=,/2a,( MgOj configuration.For one BaO unit on the MgO substrate [Plate 3(aj], the barium ion is seen to move away from the surface to reduce the unfavourable Ba-Mg interaction. The magnesium directly below the oxygen of the BaO unit has migrated out of the surface towards the oxygen, resulting in an increased energy contribution to the system because of the close proximity of the oppositely charged ions. For two BaO units on the surface [Plate 3(b)] the a,(BaO) =$a,(MgO) configuration is still apparent after relaxation (the BaO remains directly above the magnesium sublattice of the MgO, see Fig. 3). The magnesium ions directly below the oxygens of the BaO units are again seen to move out of the MgO surface thereby decreasing the Mg-0 distance. The barium ions are seen to move away from the magnesium ions, above which they are constrained to sit, because of unfavourable Ba-Mg interaction. For three BaO layers the BaO units appear to have moved from the MgO surface. For four BaO units, the BaO has split into two sets of two BaO units; however, the units do not adopt the configuration observed for two BaO units. This may be because the interaction between the two units pushes the structure towards an alternative local energy configuration.With five or more BaO units, cluster formation of BaO is observed on the MgO substrate. For eight BaO units a three- dimensional island structure is observed. In previous work,14 we have found that two layers of BaO on MgO are more stable than a monatomic layer.We tentatively suggest there- fore that the formation of a ‘bilayer’ of BaO is facilitated by three-dimensional clustering of the originally monatomically flat BaO. Fig, 6 shows the calculated energies of adding BaO layers to the MgO substrate per BaO unit. The calculated energy of one BaO unit is lower than the energy of a BaO cluster containing two or more BaO units. We would therefore expect single BaO units to be observed rather than clusters at low concentrations of BaO on the MgO surface. At higher BaO loadings (i.e. monolayer coverage) the BaO does not form a coherent monatomically flat epitaxial film but instead BaO clusters form. Recent molecular beam epitaxy results’ suggest that BaO grown on an MgO(100) surface does not grow monatomically flat epitaxial films but forms three-dimensional 5: 1887 -30.4 -.-c -30.6 %m 9 -30.8 QU~2 -31.0 5? t a, -31.2 -31.4 12345678 BaO units added/cluster size Fig.6 Calculated energies of from one to eight BaO units nlaced on the MgO( 100) surface after relaxation islands, in accordance with our results. The molecular beam epitaxy results are for a monatomic loading on the MgO substrate, equivalent to 50% monolayer coverage based on the MgO surface. Conclusion The methodology devised for the partial removal of the constant-area boundary condition has enabled the strain energy imposed on the interface to accommodate the misfit to be dissipated by interfacial area changes as a direct consequence of interfacial relaxation.For the bulk BaO( 100)/Mg0(100) interface, the BaO and MgO crystals relax to their strain-free bulk values within 5 planes of the interface. The planes at the interface remain artificially lengthened (BaO) or compressed (MgO) to accom- modate the mismatch. We attribute this relaxational behaviour to the excellent charge matching across the interfacial plane, more than compensating for the strain energy induc.ed into the system to accommodate the misfit between the two materials. Finally, we have identified that Ba0 forms three-dimen- sional islands when grown on MgO( loo),in accordance with molecular beam epitaxy results. This behaviour is attributed to the instability of a monatomic BaO layer on MgO com- pared with a BaO bilayer.We thank the AEA Harwell and the SERC for financial support and Biosym Technologies for the provision of the Insight software. References 1 R. Simon, Phys. Today, 1991,44,64. 2 D. Labalestier, Phys. Today, 1991,44, 74. 3 D. C. Sayle, S. C. Parker and J. H. Harding, Mol. Simul., 1994, 12, 127. 4 M. Cotter, S. Campbell, R. G. Egdell and W. C. Mackrodt, Su$. Sci., 1988,197,208. 5 R. A. Mckee, F. J. Walker, E. D. Specht, G. E. Jellison, L. A. Boatner and J. H. Harding. Phys. Rez>.Lett., 1994,72,2741. 6 P. W. Tasker, Harwell Report, AERE-R9130,1978. 7 D. M. Duffy and P. W. Tasker, Harwell Report, AERE-R11059, 1983. 8 G. V. Lewis and C. R. A. Catlow, J. Phys. C, 1985,18,1149. 9 B. G. Dick and A. W. Overhauser, Phys. Rev., 1958,112,”O. 10 T. X. T. Sayle, C. R. A. Catlow, D. C. Sayle, S. C. Parker and J. H. Harding, Philos. Mag. A, 1993,68, 565. 11 A. P. Sutton and R. W. Balluffi, Acta Metall., 1987,35,2177. 12 K. H. Reider, Surf. Sci., 1982,118, 57. 13 V. E. Heinrich, Rep. Prog. Phys., 1985,48, 1481. 14 D. C. Sayle, Ph.D. Thesis, University of Bath, 1992. Paper 4/03828B; Received 23rd June, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401883

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4( 121, 1889-1892 Two New Diphosphates with SrV,(P,O,), Structure: Mercury and Lead Phases S. Boudin, A. Grandin, A. Leclaire,” M. M. Bore1 and B. Raveau Laboratoire CRISMAT, ISMRA et Universite de Caen Bd.du Marechal Juin, 14050-Caen Cedex, France Two new diphosphates of the AV2(P207)2series involving a trans configuration of the bidentate P207groups have been synthesized for A =Hg or Pb. Their structure, determined from a single-crystal X-ray diffraction study is similar to that of the Sr-phase. They crystallize in the Pi space group, 2 = 1,u =4.848(1) A, b =6.892(1)A, c =8.077(2) A, CI =92.65(1)’,b =93.26(1)”,7 =106.23(1)”, for the Hg phase and a =4.804(1) A, b =7.1 13(1) A, c =7.898(2) A, x =89.78(1)”, ,!I=92.62(1)”, ;1=106.10(2)“, for the Pb phase.The crystal structures, solved by the heavy-atom method, were refined. The final agreement factors are R=0.027 and 0.033 for the Hg and Pb phases, respectively. A comparison with the other isotypic members of this series A =Sr, Cd and Na,MoP,O, (0.25 <x <0.50) is presented. The great similarity between the lead and strontium phases is shown, demonstrating the absence of a lone-pair effect of Pb” in this structure. The particular behaviour of the Hg phase is emphasized, due to the ability of this element to adopt the dumbbell (or 2+4) coordination; the strong covalent character of Hg” induces a significant distortion of the PO4 tetrahedra, rarely observed to date in transition-metal phosphates. The studies performed on V”’ diphosphates with the general pounds are isotypic of the mixed valent molybdenum diphos- formula AV,( P207),allow two different structural types to phate Na,MoP,O, (0.25dxd0.50, x =0.30? x =0~0~~).The be distinguished according to the nature of the divalent recent synthesis of the cadmium diphosphate CdV2( P,0,)26 cation A.For a large A cation, like barium, the [V2P401,], confirms the great flexibility of this [V2P2014]m host lattice. framework consists of VO, octahedra linked to two bidentate In fact, the stability of this second structural form seems to P207 groups in a cis configuration [Fig. l(a)] so that be governed by the ability of the ‘interpolated’ cations (sodium, BaV,( P207),’ exhibits tunnels where barium cations are strontium, calcium or cadmium) to adopt a more or less located.This kind of framework is very stable. It was observed distorted octahedral coordination. In this respect, the study for the first time for BaTi,(P207)22 and has also been seen of the behaviour of Hg” and Pb” in these phosphates is of for BaMo,( P207),., interest, since the first species is known for its ‘2+4‘ coordi-When the size of the bivalent cation is smaller, the nation, whereas the second is intermediate in size between [V2P201,], framework is also built up from V06 octahedra strontium and barium and often exhibits a lone-pair effect. and diphosphate groups, forming tunnels. However, in this We report here the synthesis and single-crystal study of two case the two P207 groups linked to one V06 octahedron are new isotypic diphosphates, HgV,( P207),and PbV,( P207)2 in a trans configuration [Fig. l(b)].This was shown by Hwu with a trans configuration of the P207 groups. and Willis, for SrV,( P207),. These authors could also prepare the isotypic phosphate CaV,( P,0,)2.4 In fact these two com- Experimental The crystal growth of the title compounds was performed in two steps. An intermediate mixture, with the compo-sition ‘V2P4015’ and ‘Pb2V1,2P4014’ for the Hg and the Pb phases, respectively, was first prepared, starting from the appropriate molar ratios of V205, H(NH4),P04 and Pb(CH3CO2)-2.3H2O. The ‘V2P4015’ mixture was he.ited at 653 K, and the ‘PbV1.2P4014’ mixture was heated at 773 K for 2 h in order to eliminate CO,, NH,, H,O and CH,.An appropriate amount of HgO and V was added to the ‘V2P4015’ mixture in a second step in order to obtain the ‘Hg,V,P4OI7’ composition. In the same way, 0.8 mol of V was added per ‘PbV,,,P,O,~ formula unit to reach the composition PbV2P4OI4. These mixtures were placed in an alumina crucible and heated in an evacuated silica ampoule for 24 h at 1073 and 1373 K, respectively. They were cooled to 1073 K at a rate of 1K h-’ and to 873 K at a rate of 0.5 K h-’, respectively, and finally quenched to room tem- perature. Under these conditions yellow-green crystals of the mercury phosphate and green crystals of the lead phase were extracted. Their composition, HgV2( P,0,)2 and PbV2( P207),, was confirmed by microprobe analysis. The lead diphosphate PbV,( P207),could be synthesized as a pure phase in the form of polycristalline samples by a similar method.It was heated at 1223 K for 24 h and quenched Fig. 1 VP,O1, unit with two bidentate PzO, groups in (a) cis to room temperature. The X-ray powder diffractogram configuration, (b) trans configuration (Table 1) was recorded on a Philips diffractometer with J. MATER. CHEM., 1994, VOL. 4 Table 1 Intereticular distances in PbV,( P,O,), Table 2 Positional parameters and their estimated standard deviations h k 1 dob,/A d,,,,/A I h k 1 dobslA dcalc/A I 00 1 7.8588 7.8890 33 220 2.2192 2.2176 8 ~~ ~ ~ 0 10 0 1 1 1 0 0 1 T o 1 oi 1 Ti I T I 1 1 i 1 12 12 i 1 2 0 021 1 0 2 6.8229 5.1276 4.6071 4.4381 4.0552 3.9295 3.8095 3.1802 3.0016 2.9918 3.2020 3.1295 3.0641 6.8334 5.1418 4.6108 4.435 1 4.0637 3.93 15 3.8038 3.2031 3.1743 3.1248 3.0685 3.0057 2.9925 16 5 30 12 13 8 33 30 24 34 8 100 41 201 2'21 113 2i2 122 113 20-2 123 132 032 202 22 2 014 2.1863 2.1556 2.1201 2.0907 2.0457 2.04 13 2.0357 2.0061 1.9818 1.9640 1.9516 1.9015 1.8878 2.1857 2.1566 2.1224 2.0932 2.0469 2.0440 2.0319 2.0074 1.9831 1.9647 1.9513 1.9019 1.8903 7 5 2 6 9 12 12 5 10 5 2 4 2 Hg V(1) V(2) 0 0 0.5 0.38 1 7( 1) 0.9714(1') 0.1755(4) -0.1218(4) -0.3752(4) 0.1 322(4) 0.7310(4) 0.4739( 4) 0.2071(4) 0 0 0.5 0.75873(8) 0.35984(8) 0.18 12(3) 0.3041( 3) 0.4425(3) 0.7447( 3) 0.5318(3) -0.1107(3) -0.1916( 3) 0.5 0 0.5 0.80628(8) 0.74191(8) -0.1913( 3) -0.1 584( 3) -0.0624( 3) 0.5893(3) 0.6837( 3) 0.6283(3) 0.8472( 3) 1.5 15( 2) 0.395( 6) 0.399( 6) 0.463 (7 0.425( 7 0.77(2) 0.82(2) 0.78(2) 0.77(2) 0.86(2) 0.77( 2) 0.71(2) 10 2 2.9358 2.9309 7 130 1.8481 1.8486 6 1 T 2 00 3 2.8884 2.6272 2.8925 2.6297 26 13 231 123 1.8418 1.8154 1.8401 1.8171 9 3 atom X 4' 7 B,,lA2 0 2 -2 0 2 2 1 12 12 2 12 2 12 0 21 o 121 12 1 2 00 2.5929 2.5691 2.5288 2.5127 2.4568 2.4404 2.3939 2.3470 2.3119 2.3051 2.5944 2.5709 2.5309 2.5 158 2.4583 2.4403 2.3950 2.3520 2.31 11 2.3054 8 11 5 4 5 11 4 3 13 14 131 if4 212 133 113 033 040 132 114 124 1.7894 1.7758 1.7675 1.7472 1.7358 1.7133 1.7080 1.6904 1.6789 1.6604 1.7895 1.7768 1.7679 1.7488 1.7366 1.7139 1.7083 1.6909 1.6790 1.6620 3 4 6 3 3 4 4 5 2 2 0 0 0.5 0.38 17(4) 0.9728( 4) -0.088( 1) 0.125( 1) 0.178( 1) -0.376( 1) 0 0 0.5 0.7706(2) 0.3842( 2) 0.2023( 7) 0.3514( 7) -0.0956(7) -0.1764(7) 0.5 0 0.5 0.7978(2) 0.7490(2) -0.2047(7) -0.1474(7) -0.0709( 7) 0.5892( 7) 1.503( 7) 0.47( 2) 0.50(2) 0.49( 3) 0.48(3) 0.89(8) 0.92(8) 0.98( 9) 0.88(8) 0.703( 1) 0.4457( 7) 0.7066( 7) 0.97(8)0 3 0 2.2765 2.2778 2 132 1.6096 1.6098 3 0.474(1) 0.7565( 7) 0.6163(6) 0.79(8)2 i 1 2.2633 2.2636 3 240 1.6012 1.6015 3 10 3 2.2401 2.24 5 22'2 1.5867 1.5871 1 0.197( 1) 0.5567( 6) 0.8519(6) 0.69(8) 1 i 3 2.2258 2.2246 8 042 1.5727 1.5729 4 Anisotropically refined atoms are given in the form of the isotropic equivalent displacement parameter defined as: Cu-Ka radiation and indexed in a triclinic cell, in agreement Be, =4/3 [Plla**+P22b*2+p33c*2+Plza* b* cos ;I*with the parameters obtained from the single-crystal study.Attempts to synthesize pure HgV,( P,O,), were unsuccessful +PI3a*b* cos P* +P2,b*c* cos a*]. using the previous experimental procedure; only the vanadium phosphate V( PO,), and metallic Hg were obtained as major products. The [V,P,O,,], framework of these two diphosphates is very similar to that previously described for SrV,( P,O,),,, Results and Discussion Na,MoP,O, (0.25 <x<0.50, x =0.30,5a x =0.5OSb) and CdV2(P20,),.6 A projection of the structure onto the (100) Two crystals of HgV,(P,O,), ad PbV2(P,0,), with dimen- plane (Fig. 2) shows that the latter can be described by the sions 0.116 mm x 0.077 mm x 0.051 mm and 0.026 mm x 0.026 mm x 0.026 mm, respectively, were selected for the struc- tural determination.The cell parameters were determined at 294 K from a least-squares refinement based upon 25 reflec-tions with 18"<0<25". The intensities were measured on a CAD-4 Enra!-Nonius diffractometer, using Mo-Ka radiation (i=0.71069 A) in the range -9 <h <9; -13 <k <13;0 <E <16, and, -9<h<9; -14<k<14; 0<1<15, for the Hg and Pb phases, respectively. Three reflections were periodically meas- ured in order to confirm that there were no significant vari- ations. A total of 4456 reflections for the Hg phase (4475 for the Pb phase) were collected. The structure determinations were carried out with 2725 reflections having I >341) for the Hg phase and 1205 for the Pb phase.The data were corrected for Lorentz and polarization effects. No absorption were per- formed and secondary extinction corrections were calculate? for the Hg ehase only. Th? crystal data are a =4.848( 1) A, h =6.892( 1) A, c =8.07'g2) A, a =92.65( l)", p =93.26( l)G,y 7 106.23( l)', Vo=258( 1) A3 for the Hg phase and a =4.804( 1) A, b =7.113( 1) A, c =7.898( 2) A, cc =89.78( 1)", p =92.62( l)G, y = 106.10(2)", V =259.0( 9) A3 for the Pb phase, triclinic Pi space group. The structures were solved by the heavy- atom method using the SDP chain program., The refinement of the atomic coordinates and the anisotropic thermal factors, listed in Table 2, led to R=0.027, R,=0.029 and R=0.033 and R,=0.036 for the Hg and Pb phases, respectively. Fig.2 Projection of the structure AM,(P,O,), slightly inclined to a J. MATER. CHEM., 1994, VOL. 4 stacking along b of [VP,O1l], mixed layers of V06 octahedra A second important feature of this study deals with the and PO, tetrahedra with [VO,], layers of VO, octahedra. distortion of the PO, tetrahedra in the Hg phase. Each P(2)This framework forms tunnels where the mercury and lead tetrahedron of this phas?, exhibits one abnormally long ions are located. P(2)-0(4) bond, of 1.58 A, in addition to that corresponding The examination of the interatomic distances (Table 3) to the bridging oxygen of the P207 group. This is easily shows the VO, octahedra of the two compounds are not very explained by the fact that O(4) is also linked to mercury, distorted. The amplitude of V( 1)-0 distances is slightly more forming a rather strongly covalent Hg-0 bond.importaqt for the Pb phase, wit! distances ranging from 1.94 It is worth comparing these structural distortions with to 2.07 A against 1.95 to 2.02 A for the Hg phase. On th,e those of the other isotypic phosphates of strontium, cadmium other hand, the V(2)-0 distances range from 1.93 to 2.10 A and sodium (Table4). Except for the Hg phase, all the for the Hg phase and are $ghtly more spread than those of compounds exhibit the same distortion of the PO4 tetra-the Pb phase (1.95-2.07 A). The most striking difference hedron, i.e. Lhree normal P-0 bonds ranging fromc 1.480(2) between the two structures is the coordination of mercury to 1.548(6)A, and a longer one of 1.572(2)-1.616(2) A charac-compared to lead.Lead exhibits a distorted octahe(ra1 coordi- teristic of the bridging oxygen of the diphosphate group. Thus, nation, Pb-0 distances ranging from 2.50 to 2.81 A (Table 3), mercury appears as one of the rare elements that is able to whereas mercury is characterized by its classical '2+4' coordi-distort the phosphate matrix. Considering the four vmadium nation, i,e. a dumbbell coordination with two oxygens located diphosphates AV2(P207),, one observes from the evolution at 2.09A, yith four additional oxygens much further away of the A-0 distances that the amplitude of the distortion of (2.79-2.93 A), forming a very flattened HgO, octahedron. the AO, octahedron is very similar for cadmium, strontium Table 3 Distances (/A)and angles (/degrees) in polyhedra HgV,( P207)2 structure PbV,( P,0,)3 structure V(1) 0(1) O(1') O(2) O(2') O(3) O(3') 0(1) O(1') O(2) O(2') O(3) O(3') O(I) 2.024(2) 4.028(3) 2.794(3) 2.902(3) 2.839(3) 2.780(3) 2.072(5) 4.144(8) 2.815(7) 2.907(8) 2.816(8) 2.864(8) O(1') 180.0(0) 2.024(2) 2.902(3) 2.794(3) 2.780(3) 2.839(3) 180.0(0) 2.072(5) 2.907(8) 2.815(8) 2.864(8) 2.816(8) O(2) 87.83(8) 92.17(8) 2.004(2) 4.009(3) 2.691(3) 2.897(3) 88.2(2) 91.9(2) 1.973(5) 3.945(8) 2.748(7) 2.788(8) O(2') 92.17(8) 87.83(8) lSO.O(O) 2.004(2) 2.897(3) 2.691( 3) 91.9( 2) 88.2(2) lSO.O(O) 1.973(5) 2.788( 8) 2.748(7) O(3) 91.22(8) 88.78(8) 85.77(8) 94.23(8) 1.949(2) 3.897(3) 89.0(2) 91.0(2) 89.2(2) 90.8(2) 1.942(6) 3.884(8) O(3') 88.78(8) 91.22(8) 94.23(8) 85.77(8) lSO.O(O) 1.949(2) 91.0(2) 89.0(2) 90.8(2) 89.2(2) 180.0(0) 1.942(6) V(2) O(4) O(4") O(5) O(5") O(6) O(6") V(2) O(4) O(4") O(5) O(5") O(6) O(6") O(4) 2.101(2) 4.202(3) 2.838(3) 2.869(3) 3.018(3) 2.745(3) O(4) 1.969(6) 3.937(8) 2.774(8) 2.766(8) 2.900(7) 2.818(8) O(4i) 180.0(0) 2.101(2) 2.869(3) 2.838(3) 2.745(3) 3.018(3) O(4") 180.0(0) 1.969(6) 2.766(8) 2.774(8) 2.818(8) 2.900(7) O(5) 89.38(9) 90.62(9) 1.931(2) 3.861(3) 2.751(3) 2.775(3) O(5) 90.2(2) 89.9(2) 1.949(5) 3.897(8) 2.794(7) 2.896(8) O(5") 90.62(9) 89.38(9) 180.0(0) 1.931(2) 2.775(3) 2.751(3) O(5") 89.9(2) 90.2(2) lSO.O(O) 1.949(5) 2.896(8) 2.794(7) O(6) 95.43(8) 84.57(8) 89.50(9) 90.50(9) 1.977(2) 3.954(3) O(6) 91.7(2) 88.3(2) 87.9(2) 92.1(2) 2.073(5) 4.147(8) O(6") 84.57(8) 95.43(8) 90.50(9) 89.50(9) lXO.O(O) 1.977(2) O(6") 88.3(2) 91.7(2) 92.1(2) 87.9(2) 180.0(0) 2.073(5) O(li") 1.521(2) 2.572(3) 2.474(3) 2.544(3) O(1'l') 1.527(6) 2.554(8) 2.486(8) 2 529(7) O(3lv) 117.1(1) 1.494(2) 2.546(3) 2.412(3) O(3"') 114.9(3) 1.502(6) 2.563(8) 2441(7) O(6) 108.0(1) 114.3( 1) 1.536(2) 2.528(3) O(6) 107.9(3) 114.3(3) 1.548(6) 2 542(7) O(7) 108.3( 1) 101.6(1) 106.6( 1) 1.616(2) O(7) 107.9( 3) 103.7( 3) 107.7( 3) 1.600(5) O(2') 1.480(2) 2.500(3) 2.491(3) 2.492(3) WV) 1.489(6) 2.463(8) 2.492(8) 2.509( 7) O(4") 109.3( 1) 1.584(2) 2.532(3) 2.495(3) 0(4") 108.7(3) 1.543(6) 2.527(8) 2.193(7) O(6) 113.7( 1) 110.6( 1) 1.495(2) 2.496(3) O(6).112.6(3) 111.9(3) 1.507(6) 2.512(8) 0(7"') 109.4(1) 104.5(1) 108.9(1) 1.572(2) O(7") 109.3(3) 105.6(3) 108.5(3) 1.587(5) Hg-0(4), 2.093(2)A; Hg-O(4"'"), 2.093(2) A; Hg-O( 1'1, Pb-0(4], 2.506(5) A; Pb-0(4"I1:), 2.506(5) A; Pb-O( 1'1,2.796(2) A; Hg-O(J"'), 2.796(2) A; Hg-0(6"), 2.939(2) A, 2.605(5) A; Pb-O(~"'), 2.605(5) A; Pb-O(6"), 2.817(5) A,Hg-O(6'"), 2.939(2) A.Pb-0(6'"), 2.817(5) A. Symmetry codes: -7i -x -y ii 1-x 1-y 1 -z ... 111 x l+Y lsz iv l+x 1+y l+z v l+x 4' 1+z vi l+x y l+z vii x Y z+l ... Vlll -x -Y 1-z ix x-1 y-1 Z J. MATER. CHEM., 1994, VOL. 4 Table 4 Comparison of the interatomic distances in the diphosphates AV,( PZO7),and Na,MoP,O, compounds CdV,(P,O,), HgV,(P,O7)2 SrV,(P,O,), PbV2.(P207)2 Na,MoP,O," NaMo,P,O,,b Sloping characters: Mo " Ref. 5(a). Ref.5(b). d (A-O)/A 2.307( 1)-2.586( 1) 2.093(2)-2.93( 2) 2.48 1(2)-2.761(2) 2.506(5)-2.817( 5)2.331 (6)-3.205(6)" 2.335(7)-3.20( l)b -0 distances. lq v -O)/A 2.005( 1) 1.998(2) 2.000(2) 1.996(5) 2.046 (2)" 2.043 (3)b d [V( 1)-O]/A 1.914(1)-2.148( 1) 1.949( 2)-2.024( 2) 1.944(2)-2.069( 2) 1.942(6)-2.072( 5) 1.972(2)-2.022(2)' 1.973(3)-2.014(3)b ti[V(2)--O]/A 1.932(1)-2.070( 1) 1.9i1(2)-2.101(2) 1.952(2)-2.092( 2) 1.949(S)-2.073( 5) 2.060 (2)-2.115 (2)" 2.0.~7(3)-2.111(3)b and lead in spite of the different sizes of these cations. Note the striking similarity between lead and strontium whose SrO, and PbO$ octahedra exhibit distances of 2.48-2.76 and 2.50-2.81 A, respectively. This shows that there is no lone-pair effect of Pb" in PbV,(P,O,),.As pointed out above, mercury exhibits a different behaviour owing to its tendency to adopt the dumbbell coordination. The size of the VO, octahedron, charactFristic by the aveJage V-0 distances, ranging from 2.006 A for Cd to 1.996 A for Pb is not signifi- cantly affected by the size of the A cation. On the other hand, the distortion of the VO, octahedra is closely related to the nature of the A cations. The Sr and Pb phosphates that are characterized by similar A -0 distances exhibit very similar VO, octahedra; the latter are not stron8ly distorted, with V-0 distances ranging from 1.94 to 2.09 A (Table 4). On the other hand, cadmium, probably because of its smaller size, induces a larger distortion of the VO, octahedra, especially for the V(l) octahFdra for which V-0 distances ranging from 1.91 to 2.14A are seen.In the same way, the V(2) octahedra of the Hg phase are slightly more distorted than those of the Sr and the Pb phases; nevertheless the V(l) octahedra are less distorted compared to Sr and Pb phases because of the strong anisotropic coordination of the mercury. It is difficult to compare the molybdenum phosphate Na,MoP,O, (0.25 <x <0.50, x =0.30,5" x =0.505') with the vanadium diphosphates because of the existence of the mixed valency of the molybdenum MO"'-MO'~.Nevertheless it is worth pointing out that the NaO, octahedra exhibit a much greater distortion than the other A cations with Na-0 distances ranging from 2.33 to 3.20 A.This is due to the fact that sodium does not sit on the symmetry centre as it does in the vanadium diphosphates. Note that the M 00,octahedra are almost regular. Two new members of the series of vanadium diphosphates AV,(P,O,), with a trans configuration of the PzO, groups have been synthesized. The large size range of the A cations and their various electronic configurations show the high flexibility of this structure. Among these compounds, the mercury phase is remarkable for the ability of mercury to induce a distortion of the PO4 tetrahedra that has rarely been observed to date. These results encourage us to study the behaviour of mercury in other phosphates of transition elements. References 1 L. Benhamada, A. Grandin, M. M. Borel, 4. Leclaire and B. Raveau, Acta Crystallogr., Secl. C, 1991,47, 2437. 2 S. Wang and S. J. Hwu, J. Solid State Chem., 1991.90, 31. 3 A. Leclaire, J. Chardon, M. M. Borel, A. Grand~nand B. Raveau, 2.Anorg. Allg. Chem., 1992,617, 127. 4 S. J. Hwu and E. Willis, J. Solid State Chem., 1991, 93, 69. 5 (a) A. Leclaire, M. M. Borel, A. Grandin and B. Raveau, 2. Kristallogr., 1988, 184, 247; (b) K. L. Lii and J. J. Chen J. Solid State Chem., 1989,78, 178. 6 S. Boudin, A. Grandin, M. M. Borel, A. Leclaire and B. Raveau,Acta Crystallogr., Sect. C, 1994,50, 840. 7 B. A. Frenz and Associates, Inc., 1982, SDP Structure Determination Package College Station, Texas, I JSA. Paper 4104146A; Received 7th July, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401889

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 12), 1893-1895 Crystal Structure of U,Pt,Sn: A New Derivative of the Tetragonal U,Si,-type Structure P. Gravereau: F. Mirambet,“ 6. Chevalier,*aF. Weill,” L. Fournes? D. Laffargue??”F. Boureeb and J. Etourneaua a Laboratoire de Chimie du Solide du CNRS, 351 Cours de la Liberafion, 33405 Talence Cedex, France ” Labomtoire Leon Bri//ouin, (CEA-CNRS), Centre d’Efudes de Saclay, 9I 79I Gif-sur-Yvetfe, France The crystal structure of the new ternary stannide U,Pt,Sn has been investigated by both X-ray powder diffraction and electron diffraction. It crystallizes in a tetragonal unit cell with a =768.1(1) pm and c =739.1(1)pm. This crystal structure is a new superstructure of the tetragonal U,Si,-type which appears on account of the existence of short Pt-Pt distances.The crystal structure of U,Pt,Sn is described taking into consideration those of UPt and UPt,Sn. Recent investigations have been devoted to the new family of ternary compounds U2M,X containing 3d, 4d or 5d transition- metal element as M and indium or tin as the X The magnetic properties of these compounds are strongly influenced by the nature of M. For instance in the series U2M,Sn with M =Ru, Rh or Pd, U,Ru,Sn is a Pauli paramag- net whereas U2Rh2Sn and U2Pd,Sn order antiferromag- netically at TN=25( 1) and 42( 1)K, re~pectively.~,~These properties are correlated likely to the number of d electrons of the M component which partly governs the strength of the 5f( U)-rzd(M) hybridization. We have previously shown that the stannides U2M,Sn with M =Fe, Co, Ni, Ru, Rh and Pd crystallize in the tetragonal ordered version of the U,Si,-type structure (P4/rnbnz space group).A compound containing platinum U,Pt,Sn has already been reported with the ideal U,Si,-type [a=767.8(5) pm and c=369.9(2) prn]., Recently, we have claimed that its X-ray diffraction pattern could not be indexed in the P4/mbm space group.4 In order to solve this discrepancy, we have investigated the crystal structure of U,Pt,Sn again by both X-ray powder diffraction and electron diffraction. In the present work we show that U,Pt,Sn exhibits a superstructure corresponding to a new deformation of the U,Si,-type structure. Experimenta1 U,Pt,Sn was prepared by direct melting of the constituents using an induction levitation furnace under a purified argon atmosphere, followed by an annealing treatment under vacuum at 800°C for 2 weeks. Microprobe examination was used to check both the homogeneity and the composition of the sample.This analysis is based on the measurements of U Ma1, Pt La, and Sn La, X-ray radiations which are compared to those of UPt,Sn used as reference. This procedure indicates a good homogeneity of the sample and the experimental atomic percentages [U, 40.6( 2)%; Pt, 39.7( 2)”/0 and Sn, 19.7(2)’?/0]are in agreement with the atomic composition of U2Pt,Sn (u,40%; Pt, 40%; Sn, 20%). The lattice parameters have been obtained by a least-squares refinement method with the help of Guinier X-ray powder data (Cu-Ka,) using Si( 5N) as an internal standard.The crystal structure of U,Pt,Sn has been refined by the Rietveld profile method.6 The data were collected on a Philips PW 1050 diffractometer using Bragg-Brentano geometry with Cu-Ka radiation and a take-off angle of 6”. The pattern was scanned in steps of 0.02 (28) from 20” to 120” with a constant counting time of 40 s. The electron diffraction investigation was carried out on a JEOL 2000 FX microscope, operating at 200 kV, equipped with a double tilt specimen stage. For this experiment, the melted or annealed sample was crushed in methanol and a drop of the suspension was deposited on a holey carbon support film. Results and Discussion The X-ray powder pattern of U,Pt,Sn shows the presence of reflections which could not be indexed on the basis of the tetragonal U3Si2 unit cell as for U,Fe,Sn (Fig.l).’ A precise examination of these reflections reveals the presence of a tetragonal unit cell having a c parameter twice as large as that determined for other U,M,Sn stannides. At room tem- perature, the unit-cell parameters of U,Pt,Sn, deduced from X-ray powder pattern, are: a=768.1(1) pm and c= 739.1(1) Pm. Electron Diffraction Study Fig. 2 shows some selected-area electron diffraction patterns obtained on the U,Pt,Sn sample; the zone axis is [OOl], [OiO] and [Olj], respectively, for Fig. 2(a),(h) and (c). These patterns are indexed on the basis of the tetragonal U,Si, unit cell. However, some reflections, indicated by arrows in Fig.2(b) and (c),remain unexplained by this indexation, which demonstrates the existence of a superstructure. To account for these additional reflections, the c parameter of 1J,Pt,Sn must be doubled in comparison to the c parameter of U3Si,. ei : ’ ’ . : : ’ -ctcect+.. : : : : : : 20 30 40 50 60 70 a 2adegrees Fig. 1 Rietveld refinement of the X-ray powder pattern of U,Pt,Sn (+indicates the reflections which cannot be indexed in the U,Si, unit cell) (a1 Fig. 2 Electfon diffraction patterns of U2Pt2Sn along [OOl] (a),[OiO] (h) and [012] (c) zone axis (arrows indicate the superstructure; (hkl) indices are given in the ideal U3Si2unit cell) Also, this study reveals no systematic extinction for (hkl) reflections, which is consistent with a P Bravais lattice.Nevertheless Fig. 2(b) shows that for (h01) reflections an additional condition must be considered: h +I # 2n. This obser- vation indicates the presence of an n-type glide plane. The corresponding extinctions along the a*,b* or c* axes [Fig. 2(u) and (c)] vanish on account of the double diffraction phenomenon. Structure Determination A tiny single crystal was isolated from a crushed annealed U,Pt,Sn sample. A study by a Buerger precession and a Weissenberg camera confirmed the tetragonal symmetry. Systematic extinctions were observed for (hOl) with h +1# 2n leading to three possible space groups: P4,/mnm7 Pan2 and P4,nm. Unfortunately, the poor quality of the single crystal did not allow it to be used for crystal-structure determination. A structural model deriving from the U,Fe,Sn type’ can be found in a unit cell with a doubled c parameter and P4,/mnrn symmetry.This hypothesis has been refined by the Rietveld profile method on X-ray powder data (Fig. 1). As currently obtained for Rietveld studies of intermetallic compounds, both an extra-lorentzian contribution to the pseudo-Voigt profile function is observed (presence of mechanical constraints induced by the melting procedure used for the synthesis of the compounds)’ and some slightly negative values for the isotropic thermal parameters B of U and Pt atoms (micro- absorption induced by surface roughness). Refinements with free or fixed B parameters showed no significant variation of the atomic coordinates obtained (<1 esd).They are summar- ized in Table 1 and were obtained with the reliability factors R,, =0.100 and R,=0.058. The relevant interatomic distances are given in Table 2. The projection of the structure of U,Pt,Sn onto the xy plane is shown in Fig. 3. The Sn and Pt atoms are located, respectively, inside [U,] and [u,] distorded prisms. Important features distinguish this structure from the ideal U3Si, type: (i) U atoms form zig-zag chains running along the c axis; (ii) each Pt atom is coordinated by six U atoms, forming a distorded [u,] trigonal prism; moreover Pt atoms J. MATER. CHEM., 1994, VOL. 4 Table 1 Atomic parameters of U2Pt2Sn (esds calculated according to ref. 15) atom site x 1’ B/A2 U(1) U(2)Pt Sn 4f 4g8j 4d 0.3407(4) 0.1860(4) 0.1281(4) 0 0.3407(4) 0.8140(4) 0.1281(4) 1I2 0 0 0.221618) 114 0.2 0.2 0.3 0.4 Table 2 Selected interatomic distances (pm) in tetragonal U2Pt2Sn 346.1(4) 404.1 (4) 382.5(4) 382.5(4) 421.6(4) 421.6(4) 370.7( 4) 370.7(4) 283.1(6) 288.3(6) 302.7( 6) 295.0(6) 342.9(2) 335.7(2) Sn-4U( 1) 342.9(2) 283.1(6) 4U(2) 335.7(2) 302.7( 6) 4Pt 302.8(4) 288.3(6) 295.0(6) 278.3(8) 327.6(8) 302.8(4) U 00 0 0.5 Pt o k0.22 0 f0.28 Sn @ a.25 Fig.3 Crystal structure of U2Pt2Sn(projection onto the ry plane) are located near one triangular face of this prism; (iii) along the c axis, the Pt atoms form short (327.6 pm) and long (411.5 pm) Pt-Pt distances through the basal face of the [U,] prism, thus forcing the U atoms to move perpendicularly to the Pt-Pt bond; (iv) Sn and Pt atoms are not located on the same atomic plane perpendicular to the c axis.All these structural characteristics seem to be due to the presence of the Pt atom in the [u,] trigonal prism. U,Pt,Sn can be considered as a ternary ordered substitution of the Zr3Al, phase.’ The three different zirconium sites observed in Zr,Al, are occupied by uranium and tin atoms in U,Pt,Sn, whereas the platinum atoms are located at the aluminium site. Some deformation of the [U,] prism has been observed in the structure of the binary compound UPt which crystallizes in the monoclinic PdBi phase deriving from the orthorhombic CrB phase.’ In this case, the displacement of the platinum atom inside the [u,] prism is responsible for the doubling of the original u and c parameters of the orthorhombic CrB phase.The origin of this behaviour has been explained by J. MATER. CHEM., 1994, VOL. 4 veIocity/mm s-' 44-20 24 6 Fig. 4 *I9Sn Mossbauer spectrum of U,Pt,Sn at room temperature Parthe by considering the size of the [RE,] prisms found in the structure of many equiatomic binary compounds REX (RE=rare-earth metal and X =Si, Ge, Ga, transition-metal element) built by linking the trigonal prisms." For example, the compounds REPt show compressed [RE,] prisms with the ratio h/l< 1 (h=height of prism and I= average edge length of the triangular prism base). This compression leads to a deformation of the prism as in UPt or U2Pt,Sn.Short Pt-Pt distances also exist in the ternary stannide UPt,Sn which adopts the hexagonal ZrPt,Al type structure." In this case, the Pt atoms form pairs having 285pm as interatomic distance. Each uranium atom has seven uranium nearest neighbours in U2Pt2Sn: five in the xy plane and two along the c axis (Fig. 3 and Table 2). As a result, the U sublattice can be considered as a three-dimensional framework where one U-U distance (346.1 pm) is short. Note that in other ternary stannides known in the uranium-platinum-tin system, the U-U distances are much longer: 468 and 455.5 pm for UPtSn and UPt2Sn.11-'2 Another interesting comparison concerns the U-Pt and U-Sn distances observed in the ternary stannides U,Pt,Sn, UPtSn and UPt,Sn.The U-Sn distances in U,Pt,Sn (4 x 342.9 pm) are comparable to those determined in UPt,Sn (6 x 346.2 pm) but larger than that observed for UPtSn (6 x 330.9 pm). On the contrary, the U-Pt distances are shorter in U,Pt,Sn (2 x 283.1 pm) than in the other Pt-based ternary stannides, suggesting that 5f( U)-5d( Pt) hybridization may be important. l19Sn Mossbauer Spectroscopy At room temperature, the lI9Sn Mossbauer spectrum of U,Pt,Sn exhibits one quadrupole doublet because the Sn site possesses tetragonal point symmetry (Fig. 4). The Mossbauer parameters are S=2.07(2) mm s-', A=0.34(2) mm s-l and r=0.85(2) mm s-'. Note that U,Pt,Sn exhibits a smaller quadrupole splitting than those observed for the other U,M,Sn ternary stannides with M =Fe, Co, Ni, Ru, Rh, Pd.4 This can be explained by the amplitude of the deformation of the [U,] prism surrounding Sn atoms in U,Pt,Sn (Fig.3). Conclusion The U,Pt,Sn stannide crystallizes in a new ordered version of the tetragonal U3Si,-type structure. The occurrence of this crystal structure can be related to the shifting of the Pt atoms inside the trigonal [U,] prims, forming one of the building units of the crystallographic arrangement. This observation appears to be a feature for the compounds based on platinum since our preliminary study performed on U,Pt,In reveals that it is isostructural to U,Pt,Sn. Note: The crystal structure of U,Pt,Sn was reported by us at '24iemes Journees des Actinides' (15-19 April 1994, Obergurgl, Austria).13 At this conference a similar superstruc- ture was noted by other authors for both U2Pt,Sn and U2Ir,Sn.l4 References 1 F.Mirambet, P. Gravereau, B. Chevalier, L. Trut and J. Etourneau, J. Alloys Compounds, 1993,191, L1. 2 M. N. Peron, Y. Kergadallan, J. Rebizant, D. Meyer, J. M. Winand, S. Zwirner, L. Havela, H. Nakotte, J. C. Spirlet, G. M. Kalvius, E. Colineau, J. L. Oddou, C. Jeandey and J. P. Sanchez, J. Alloys Compounds, 1993,201, 203. 3 Z. Zolnierek and A. Zaleski, Proc. 232me JournCes des Actinides, Schwarzwald (Germany), 1993, Abstract 06.6. 4 F. Mirambet, B. Chevalier, L. Fournes, P. Gravereau and J. Etourneau, J. Alloys Compounds, 1994,203,29. 5 F. Mirambet, Thesis, University of Bordeaux I, no.1050, 1993. 6 J. Rodriguez-Carvajal, Collected Abstracts of Powder Diffraction Meeting, Toulouse, France, 1990, 127. 7 P. Gravereau, H. Guengard, F. Mirambet, L. Trut, J. (irannec, B. Chevalier, A. Tressaud and J. Etourneau, Muter. Sci. forum, to be published. 8 C. G. Wilson and F. J. Spooner, Acta Crystallogr., 1960,13,358. 9 A. Dommann and F. Hulliger, Solid State Comnun., 1988, 65, 1093. 10 E. Parthe, in Structure and Bonding in Crystals, ed. M. 0.Keeffe and A. Navrotsky, Academic Press, New York, 1981, vol. 11, p. 256. 11 Z. Zolnierek, J. Mugn. Magn. Muter., 1988,76-77,231. 12 K. H. J. Buschow, D. B. de Mooij, T. T. M. Palstra, G. J. Nieuwenhuys and J. A. Mydosh, Philips J. Res., 1985.40,313. 13 P. Gravereau, F. Mirambet, B. Chevalier, F. Weill, L. Fournks, D. Laffargue and J. Etourneau, 24iemes Journees des Actinides, Obergurgl, Austria, 1994, Abstract PB12. 14 L. C. J. Pereira, J. M. Winand, F. Wastin, J. Rebizant and J. C. Spirlet, 24iemes JournCes des Actinides, Obergurgl, Austria, 1994, Abstract PB9. 15 J. F. Berar and P. Lelann, J. Appl. Crystallogr., 1991, 24, 1. Paper 4/04299T; Received 14th July, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401893

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(12), 1897-1901 Synthesis and Catalytic Properties of Magnesia Fine Powders prepared by Microwave Cold Plasma Heating Kazuo Sugiyama,* Yasushi Nakano, Hirofumi Souri, Eijiro Konuma and Tsuneo Matsuda Faculty of Engineering, Saitama University, 255 Shimo-okubo, Ura wa 338, Japan A fine powder of magnesia was prepared using microwave cold plasma as the heat source, and its crystal structure and catalytic properties were examined. Magnesium hydroxide was used as the raw material, and magnesia samples were prepared with variable parameters of microwave output and treatment time. After the start of heating, the temperature of the plasma immediately rose to 1100 "C.Magnesia fine powders were formed by heating the hydroxide for 1-2 min. The samples obtained had a large specific surface area (e.g.350 m2 g-'>). The morphology of the samples was observed by scanning electron microscopy, and they were seen to be net-like with micropores of a few hundred nanometres in diameter as opposed to the flat morphology obtained in general electric furnace heating. Solid basicity measurements revealed that the magnesia powders had strong surface basic sites of 27.0 6 H -<33.0. When the catalytic properties were examined using the condensation of benzaldehyde and the oxidative coupling of methane, which are typical base-catalytic reactions, the magnesia prepared by plasma heating exhibited high activity in both reactions. Commonly used processes for manufacturing powdered mate- rials include the preparation of fine particles by mechanical grinding of coarse particles,' the nuclear growth of ions or molecules in the liquid pha~e~,~ and the condensation of metal va~our.~ Recently, thermal plasma processes for manufacturing fine powders of carbides and nitrides have been st~died.',~ In these processes, solid, liquid and gaseous materials are injected into a thermal plasma torch to produce fine powders.In the plasma spraying method,' for example, a metal salt is sprayed and vaporized in a plasma at several thousand degrees centigrade, and then quenched to form dense particles. It is interesting to investigate the properties of powder materials produced by a cold plasma, because a cold plasma (also called a glow discharge plasma) is several hundred degrees centigrade lower than a thermal plasma.We have previously reported' that almost all types of metal oxide fine powder could be produced quickly by microwave cold plasma heating and that the powders thus produced had a bulky structure and surfaces free of hydroxy groups. Bulky metal oxides with large surface areas are said to have a large number of surface sites that are active in catalytic reactions. For example, properties such as the capability of extracting hydrogen from methane or the H2-D2 exchange capability of magnesia, which has vacant surface sites and oxygen ions, are kno~n.~~'~ Magnesia prepared by microwave cold plasma heating is expected to have active surfaces. In this study, a magnesia catalyst was prepared using microwave cold plasma heating and its application to catalytic reactions was examined.A magnesia fine powder with a large specific surface area, which is difficult to produce by conven- tional methods, was successfully prepared. The sample mor- phologies were observed by scanning electron microscopy and the solid basicity of the powders was determined by the indicator method. We then evaluated catalytic properties on the basis of the condensation of benzaldehyde" and the oxidative coupling of methane,'2"3 typical basic catalytic reactions. Experimental Microwave Plasma Heating The microwave plasma heater used for sample preparation is shown in Fig. 1. Descriptions of each unit of the heater were reported in detail previously.* This apparatus consists of a microwave generator, an impedance adjustment unit, an appl- icator and an evacuation unit.The output power of the microwave could be altered within the range 0-1000 W. To prepare a sample, a quartz reactor (capacity 200 ml), in which was placed the raw material powder, was placed in the applicator. The internal pressure of the reactor was reduced by the evacuation unit and a predetermined level of microwave power was applied. Very rapidly the inside of the reactor attained the plasma state. Microwave irradiation was stopped after a predetermined period of time to complete the heating process. Samples and Physical Measurements Magnesium hydroxide (Konoshima Chemicals) was used as the starting material.The crystal structures of the raw material and the samples prepared were determined with an X-ray powder diffractometer (XRD; Rigakudenki RAD-C). The morphology of each sample was observed with a scanning electron microscope (SEM; Hitachi S-5000) at an acceleration voltage of 1 kV in order to avoid damaging the surface morphology of the samples. The specific surface area was determined by the BET method using nitrogen gas adsorption. The plasma temperature was measured using an optical fibre thermometer (Accufiber M-lo), which had a small black-body sensor at the end of a sapphire rod.14 The basicity of samples was determined by the indicator Dower vacuum vacuum Pump liquid N,trap plunger Fig. 1 Schematic diagram of microwave cold plasma heating apparatus method,15 in which the Hammett indicator was used to colour 0.1 g of the sample in 30 ml of dehydrated benzene.This operation was carried out in a dry nitrogen gas atmosphere. The hydroxy group modes of magnesia were observed with an FTIR spectrometer (JEOL JIR-100). The desorption behaviour of carbon dioxide gas from the samples was studied by temperature-programmed desorption. In this method, carbon dioxide gas adsorbed on prepared samples that had previously been evacuated at 100"C for 1h. Then the desorbed gas was analysed with a quadrupole mass spectrometer (Varian MAG-5152). The amount of base in each sample was determined by the pulse adsorption method. A known volume of carbon dioxide was repeatedly adsorbed until the sample was saturated with the gas.The amount of adsorbed gas was estimated by TCD gas chromatography. Catalytic Reactions For plasma heating, the catalyst preparation conditions were 200 W output power and 1.5 min heating time, while for evacuation heating, it was 600°C for 2 h, typical calicination conditions16 for generating maximum amounts of basic sites and the highest base strength on the magnesia. The condensation of 7 ml of benzaldehyde was carried out at 180°C on 0.07 g of catalyst in a batch reactor. The benzaldehyde (Wako Pure Chemical Industries) was purified by distillation under reduced pressure. The reactants and products were analysed using an FID gas chromatograph equipped with an OV1 capillary column.A fixed-bed flow system reactor was used for the oxidative coupling of methane supplied at a rate of 36mmol h-l on 1.Og of catalyst at 750°C. The methane (purity 99.95%, Takachiho Chemicals) was decarbonated by passing it through a soda lime column before it was used in the reaction. The molar ratio of methane to oxygen was 5 :1. Helium was used as the carrier at a flow rate of 30 ml rnin-l. Reactants and products were analysed with a TCD gas chromatograph. An activated carbon column was used for analysis of oxygen and carbon monoxide, while a Porapak-Q column was used for other products. Results and Discussion Preparation of Magnesia A stable orange plasma was produced in the reactor by irradiating 200 W of microwave power into the reactor filled with 10 g of magnesium hydroxide.Similar plasma states were produced with 300 and 400 W microwaves. Fig. 2 shows the XRD patterns of the raw material and the samples prepared by plasma heating. Magnesia was produced within 1.5 min when a microwaves output power of 200 W was used. Although the results are not shown here, magnesia could be prepared in 1min when the hydroxide was heated with microwaves at output powers of 300 or 400 W, i.e. a very short period of time compared with a conventional evacuation heating in an electrical furnace at 600 "C, which requires more than 2 h for magnesia preparation. The products obtained by 1.5 rnin of heating at 200 W had broader XRD peaks than the raw material, implying that the product is made up of smaller particles than the raw material.The specific surface area of the magnesia prepared by plasma heating was 382 m2 g-', ca. ten times greater than that of the raw material (37 m2 g-'). This specific surface area is also larger than that obtained by evacuation heating, which does not generally exceed 250 m2 g-' l7even when the preparation is carried out with great care. The surface area of magnesia prepared by evacuation heating in an electric furnace at 600 "C for 2 h was 205 m2 g-'. J. MATER. CHEM., 1994, VOL. 4 llA1,,J0. 20 30 40 50 60 70 80 219ldegrees Fig.2 X-Ray diffraction patterns for Mg(OH), (Cb)and MgO (0) prepared by microwave cold plasma heating. (a) Raw material; and after heating for (b)0.5 min, (c) 1min, (d) 1.5min.Fig. 3 shows the specific surface areas of magnesia samples obtained by plasma heating under various heating time and microwave output power conditions. When the microwave output power was 200 W, the specific surface area increased rapidly after 1rnin of heating and attained the maximum value, 382 m2 g-', in 1.5 min. When output powers of 300 and 400 W were used, maximum values of the specific surface areas were observed between 1and 2 rnin of heating, but these maximum values were lower than that obtained at 200 W. Thus, using this heating method, magnesia with maximum specific surface area can be prepared by controlling the microwave output power and heating time. Fig. 4 shows the change in plasma temperature with time from the start of heating.With microwaves of 200 W, the plasma reached a temperature of 1050 "C 2 rnin after the start of heating. The plasma temperature was 1180 "C for 300 W microwaves. When using microwaves at 400 W output power, the plasma temperature measurement was stopped after 1 min -400r pb, 0123456 tlmin Fig. 3 Relationship between specific surface area and heating time under different output power conditions: (0)200 W, (A)300 W, (0)400 W. Sample: MgO; 10 g. J. MATER. CHEM., 1994, VOL. 4 9c 0:0 246810 tlmin Fig. 4 Temperature vs. time plots for plasma heating under different output power conditions: (0)200 W, (A) 300 W, sample: MgO; 10 g. because it reached 1200 "C, the upper limit of the temperature sensor used.The plasma temperature could be adjusted by controlling the internal pressure of the reactor and the output power of electromagnetic waves.18 In the microwave cold plasma heat- ing process, the initial pressure in the reactor after inserting the samples was 130 Pa. When the thermal decomposition of the sample began, the reactor pressure rose to ca. lo3 Pa due to an increase in gas desorbed from the sample. The rate of increase in temperature after the start of heating and the maximum attainable temperature depended on both the microwave output power and the increase in pressure in the reactor. The internal pressure of the reactor attained its peak with the progress of sample decomposition by heating, which in turn raised the temperature to its maximum level.Samples obtained around the maximum temperature had larger specific surface areas. When heating was continued after the maximum temperature, the specific surface areas gradually reduced. The specific surface area of magnesia obtained by microwave cold plasma heating is therefore strongly influenced by the plasma temperature and heating time. Morphological Observations Fig. 5 and 6 show SEM micrographs of the surfaces of magnesia prepared by evacuation heating in an electric furnace and by microwave cold plasma heating. The magnesia pow- ders were prepared by heating the hydroxide for 2 h at 600°C in an electric furnace, or by plasma heating for 1.5min at 200 W.The magnesia prepared in the electric furnace had the familiar flat morph~logy,'~ while the magnesia prepared by plasma heating had a net-like morphology with a very large number of micropores, 100-300 nm in diameter. The higher magnification photographs of the magnesia prepared by plasma heating showed that the areas surrounding the micro- pores are also porous. Magnesium hydroxide has a hexagonal structure in which layers of OH and Mg alternate vertically along the c axis.20 Decomposition due to dehydration on heating occurs on the planes that include the OH layers which are between Mg ions. When magnesium hydroxide is decomposed by heating it in an electric furnace for several hours, nuclei of magnesia grow within the layers while the hexagonal structure is maintained.21,22 In microwave plasma heating, however, these layers of the magnesium hydroxide structure undergo two heating actions,8 i.e.dielectric heating from the inside of the samples induced by the microwaves and heating on the surfaces of samples by the plasma gas molecules, within only 1 or 2min. This rapid dehydration prevents the magnesium hydroxide from retaining its layer structure. 0.6pm Fig. 5 SEM photographs of MgO prepared by heating under vacuum in an electric furnace: (u) x 10000, (h) x 50000 Solid Basicity Magnesia, a typical basic oxide, has been extensively studied in terms of its solid basicity by means of the indicator method and the gas adsorption method et~.~~In general, the highest base strength of magnesia16 is 18.4<H-<22.3, nhich is obtained when, for example, magnesium hydroxide is decom- posed by heating it in air at 550-600T.In this study, magnesia prepared in an electric furnace adsorbed 4-ni troanil- ine (pK, = +18.4) and diphenylamine (pK, = +22.3) and developed the basic colour in both cases, but it did not adsorb aniline (pK, = +27.0) and did not colour. On the other hand, magnesia prepared by plasma heating at 200 W for 1.5 min purple-red, the basic colour of aniline, but it was not coloured with triphenylmethane (pK, = +33.0). A similar basicity was observed for magnesia prepared by heating the hydroxide for >1min with 300 or 400 W microwaves. There are therefore strong basic sites of 27.0 <H-< 33.0 on the surface of magnesia prepared by plasma heating.The desorption of carbon dioxide and the amounts of carbon dioxide adsorbed were measured for magnesia samples pre- pared by microwave cold plasma heating and by evacuation heating in an electric furnace. Fig. 7 shows the TPD profiles of carbon dioxide. There were differences in the desorption peaks at higher temperatures: the magnesia prepared in an electric furnace had peaks at 200 and 3OO0C, while that prepared by plasma heating had additional peaks at 380 and 470 "C.This supports the assertion that the magnesia prepared by plasma heating has stronger basic sites than the magnesia prepared by conventional evacuation heating. (a 1 0.6pm Fig. 6 SEM photographs of MgO prepared by plasma heating: (a) x 10000,(b) x 50000 J 100 200 300 400 500 60 desorption TI0C Fig.7 TPD profiles of COz adsorbed on magnesia: (0)MgO pre- pared by plasma heating, (a)MgO prepared by heating under vacuum in an electric furnace The amount of carbon dioxide adsorption was 0.31 mmol g-' for magnesia prepared in an electric furnace and 0.65 mmol g-' for magnesia prepared by microwave cold plasma heating (Table 1). Therefore that the magnesia pre- pared by plasma heating has a larger number of basic sites. Fig. 8 shows the IR absorption spectra. The samples pre- pared by evacuation heating in an electric furnace had a peak24 attributable to carbonyl groups (1300-1450 cm-') and a weak broad peak of hydroxy groups (near 1650cm-'), while samples prepared by plasma heating did not exhibit these peaks and showed superior IR transmittance.J. MATER. CHEM., 1994, VOL. 4 Table 1 Effect of preparation conditions on base amounts of MgO preparation method conditions base amount: mmol g-l evacuation heating 600 'C, 2 h 0.31 plasma heating 200 W, 1.5 min 0.65 4000 3000 2000 1600 1200 wave nu mber/cm-' Fig. 8 1R spectra of (a)Mg(OH)zraw material, (b)MgO prepared by heating under vacuum in an electric furnace, (c) MgO prepared by plasma heating Decarbonation and dehydration by microwave cold plasma heating results in the solid basicity of magnesia. Catalytic Properties Benzaldehyde condensation was carried out in the presence of magnesia powder catalysts prepared by microwave cold plasma heating and the observed reaction activity is shown in Fig.9. This reaction has an induction period which lasts until the catalytic surface is saturated with intermediate benzyl alcoholate." The products of the reaction consisted mainly of benzyl benzoate and a small amount of benzyl alcohol. The magnesia prepared by plasma heating had a higher activity by weight in terms of conversion to benzyl benzoate than the magnesia prepared by evacuation heating in an electric furnace. The kinetics of benzaldehyde condensation were first order" with respect to benzaldehyde after the induction period. The rate constant per surface area of the magnesia catalyst pre- pared by evacuation heating was 1.1 x lop5min-' m-2, while 2o r I A 0fl 0 1 2 3 4 reaction time/h Fig.9 Condensation activity of benzaldehyde on MgO catalysts. Reaction temperature 180"C; 7 mI benzaldehyde. 0.07 g catalyst. (0)Prepared by plasma heating, (e)prepared by heating under vacuum. J. MATER. CHEM., 1994, VOL. 4 h 40r 01 23 45 reaction timeh Fig. 10 Methane conversion and C, selectivity in the oxidative coup- ling reaction of methane over MgO catalysts. Reaction temperature 750 'C; 1.0 g catalyst. (0)Conversion, (A) C2 selectivity of MgO prepared by plasma heating, (0)conversion, (A)C, selectivity of MgO prepared by heating under vacuum. that of the magnesia prepared by plasma heating was 2.6 x min-' m-'. The catalytic oxidative coupling of methane was examined on the magnesia samples.The products of this reaction were carbon monoxide, carbon dioxide, ethane and ethene. As shown in Fig. 10, the magnesia catalyst prepared by plasma heating had a higher activity and C, selectivity than the sample prepared by evacuation heating. These results agree with those reported in ref. 25 in that the catalyst with a strong basicity yielded the a activity to C, products. The magnesia catalyst prepared by plasma heating had strong basic sites, which would be effective for activities of both the benzaldehyde condensation and the oxidative coup- ling of methane. Conclusions (1) Magnesia fine powders with large specific surface areas could be prepared by microwave heating the hydroxide for a few minutes.(2) The magnesia powders had a net-like surface mor-phology with micropores of a few hundred nanometres in diameter, and also had strong basic sites of 27.0 6H-< 33.0. (3) The high catalytic activity of magnesia prepared by this method was observed both in the condensation of benz- aldehyde and the oxidative coupling of methane. References 1 Q.-Q.Zhao and G. Jimbo, Adv. Powder Technol., 1991,2,91. 2 A. G. Walton, J. Phys. Chem., 1964,67,1920. 3 J. J. F. Scholten, A. M. Beers and A. M. Kiel, J. Catal., 1975, 36, 23. 4 K. S. Mazdiyasni, C. T. Lynch and J. S. Smith, J. Am. Ceram. SOC., 1965,48, 372. 5 P. Kong, T. T. Huang and E. Pfender, IEEE Trans. Pltrsma Sci., 1986, 14, 357. 6 K. Watari, K. Ishizaki and T.Fuyuki, J. Mater. Sci. Lett., 1989, 8, 641. 7 T. Ono, M. Kagawa and Y. Shono, J. Mater. Sci., 1985, 20, 2483. 8 K. Sugiyama, Y. Nakano, H. Aoki, Y. Takeuchi and T. Matsuda, J. Muter. Chem., 1994,4, 1497. 9 D. J. Driscoll, W. Martir, J-X. Wang and J. H. Lunsford, J. Am. Chem. SOC.,1985,107,58. 10 M. Boudart, A. Delbouille, E. G. Derouane, V. Indovina and A. B. Walters, J. Am. Chem. SOC.,1972,94,6622. 11 K. Tanabe and K. Saito, J. Catal., 1974,35,247. 12 H. Jrnai, T. Tagawa and N. Kamide, J. Catal., 1987,106, 394. 13 X. D. Peng and P. C. Stair, J. Catal., 1991,128,264. 14 R. R. Dils, J. Appl. Phys., 1983,54, 1198. 15 0.Johnson, J. Phys. Chem., 1955,59,827. 16 K. Tanabe, Solid Acids and Buses, Academic Press, New York, 1970, p. 50. 17 H. Hattori, N. Yoshii and K. Tanabe, Proc. 5th. Int. Congr. CataI., 1973, 10,233. 18 A. von Engel, Electric Plasmas: Their Nature and Uses,Taylor and Francis, London, 1983, p. 162. 19 S. Coluccia and A. J. Tench, J. Chem. SOC., Faraday Trans 1,1979, 75, 1769. 20 L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithaca, 3rd edn., 1960,p. 553. 21 F. Freund, Ber. Deutsch. Keram. Ges., 1970,47, 739. 22 R. S. Gordon and W. D. Kingery, J. Am. Ceram. SOC., 1966, 49, 654. 23 K. Tanabe, CATALYSIS-Science and Technology, ed. J. R. Anderson and M. Boudart, Springer-Verlag, Berlin, 1981, p. 241. 24 J. H. Taylor and C. H. Amberg, Can. J. Chem., 1961,39,535. 25 V. R. Choudhary and V. H. Rane, J. Catal., 1991,130,411. Paper 4/02254H; Received 15th April, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401897

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 12), 1903-1906 Microwave-Hydrothermal Processing for Synthesis of Layered and Network Phosphates Sridhar Komarneni,*+ Qing Hua Li and Rustum Roy Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA Novel microwave-hydrothermal processing was used to increase the kinetics of the synthesis of technologically important layered and network phosphates by one to two orders of magnitude. The new powder processing technique, which is 'environmentally benign', appears to favour the formation of layered phases. A new 13.2 A [d(OOl)]layered Ti phosphate has been synthesized by this technique. This phase shows high caesium exchange selectivity and it may be useful for separation of radioactive Cs from acidic nuclear wastes and in decontamination of the environment after accidental releases of Cs.The utilization of microwave plasmas in the processing of materials such as diamond films and of microwaves in sinter- ing of ceramics is now widespread. We have been developing a new direction for the use of microwaves in materials processing as we recently reported on the microwave enhance- ment of the kinetics of hydrothermal reactions by one to two orders of magnitude in the synthesis of ceramic The phrase, 'microwave-hydrothermal processing' was coined by us1 for the use of microwave field during the hydrothermal reaction. Hydrothermal processing is well established in science and technology where reactions are carried out under closed-system conditions at elevated temperatures and pressures, usually of water, typically below 1000"C and 1000 MPa.3-6 The main classical advantages of the hydrother- mal process were that oxide (and other materials) could be made to react several hundred degrees below what could be achieved by any other method.In addition low-temperature polymorphs (e.g. quartz) or hydroxylated (e.g. clays) or hydrated (e.g. zeolites) phases could be synthesized only by this process.6 Recently a new advantage has been added. Since hydrothermal processes are closed-cycle they can be environ- mentally benign. The addition of a microwave field during a hydrothermal reaction was first utilized for the dissolution reactions in the analysis of inorganic materials such as rocks, soils and sediment^.',^ However, the combination of micro-waves with hydrothermal processing for the synthesis of fine powders was first demonstrated in our This use of microwaves under hydrothermal conditions generally only increased the kinetics of a reaction, and did not produce new products. In only one case has it resulted in an apparently new layered phase of alumina plus H20.This prompted us to investigate the role of microwaves in the hydrothermal synthesis of layered crystalline Zr and Ti phosphates, which are insoluble acid salts formed by polybasic acids such as phosphoric and several hydrolysable polyvalent cations such as Zr and Ti9 These crystalline, layered phases are an excellent group of exchangers which are not only acid resistant but also radiation resistant and hence could possibly be used in nuclear waste separation and immobilizati~n.~*~~ The three-dimensionally linked network structures of Zr, Ti and Sn phosphates have been shown to be candidates for useful ionic conductors, radioactive waste hosts and low thermal expansion materials.' '-13 The synthesis of these lay- ered and network phases, however, is a slow process requiring one to several Thus the objectives of the present study were (a) to investigate the catalytic role of microwaves on the kinetis of the hydrothermal synthesis of crystalline, Zr -t Also with the Department of Agronomy.and Ti layered and network phosphates and (b)to determine whether any new layered phases could be synthesized using the microwave-hydrothermal (MH) reactions.Experimental The various layered crystalline Zr and Ti phosphates were synthesized following the general procedures previously described14 except that MH conditions were used here instead of conventional hydrothermal (CH) conditions. The roles of pH, temperature and time on the formation of Zr and Ti phosphates were determined by using the respective gels prepared by three methods as described in Tables E and 2. The gels were treated under MH conditions using a commer- cially available microwave digestion system (MDS-2000,CEM Corporation) which operates at 2.45 GHz. The system is controlled by pressure; a maximum pressure of 200 psi-/- which -f 1 psi x6.895 x lo3 Pa.Table 1 MH synthesis of layered Zr phosphates MH conditionsb sample no. methods" treatment pH before time/h reaction' phase fc irmation by XRDd 1 I 2 0.2 x-ZrP 2 I 0.5 0.2 a-%rP 3-7 8 I I 2 2 0.3; 0.48; 0.53; 0.57; 0.75 0.59 a-%rP 3-ZrP; trace Y-ZrP 9-12 13, 14 I I 2 2 0.84; 2.62; 3.40; 3.99 3.8; 4.0 y-%rP y-2rP; trace x-ZrP 15, 16 17-19 20 I1 I11 I11 2; 6 2 2 1.89 0; 0.48; 0.92 2.43 y-ZrP x-%rP Y-ZrP a Method I: Zr oxychloride (25 ml, 1 mol 1-') was titrated ciropwise into a Na,HPO, solution (250 ml, 2 mol kg-') while stirring and the pH was adjusted with conc. HCl. Method 11: Zr oxychloride (50 ml, 1 mol 1-I) was added dropwise to boiling NaH,PO, (l00m1, 6mol 1-') while stirring and the pH was adjusted with conc.HCI. Method 111: Zr oxychloride (5 ml, 1mol 1-') was mixed with H,PO, (10 ml, 6mol 1-') and the pH was adjusted with LiOH in some cases. bPressure is 200psi in all cases which is equivalent to 194°C. 'The pH after reaction was measured in several cases but did not change significantly. a-ZrP [Zr( HPO,),.H,O]; y-ZrP = [Zr( HPO4),.2H20]. All the samples were washed four times with 1 mol 1-' HCl to prepare H+ forms then three times with deionized water prior to XRD. J. MATER. CHEM., 1994, VOL. 4 Table 2 MH synthesis of network and layered Ti phosphates MH conditions' ~ sample no. methods' pressure/psi pH before reaction' phase formation by XRDd 1-4 I 200 0.21; 0.65; 0.83; 2.57 NTP 5 I 65 1.71 NTP 6 I 30 2.48 amorphous 7, 8 9 I1 I1 200 69 0.27; 1.60 1.6 NTP NTP 10 I1 30 1.6 NTP; amorphous 11 IT1 200 0 a-TiP 12 I11 200 0.97 yTiP 13-15 I11 200 1.64; 3.04; 4.23 new TIP ~~ a Same as in Table 1 except titanium oxychloride was used.'Treatment time was 2 h in all cases. 200 psi = 194"C; 69 psi = 150 C; 65 psi = 148°C; 30psi=121 "C. 'The pH after reaction was measured inoa few cases but did not cFange significantly. dNTP [NaTi,(P04),]; a-TiP [Ti(HP04)2.H,0 (7.6 A phase)]; y-TiP [Ti(HP0,),.2H20 (11.6 A phase)]; New TIP [13.2 A phase]. All the layered phases lvere washed four times with IMHCl to prepare Hfforms then three times with deionized water prior to XRD. corresponds to the steam pressure of pure water at 194°C of the d(001) peaks.All the experimental pLirameters were that can be reached with this system. Results and Discussion a-'ZrP [Zr (HPO,),.H,O] crystallized at a lower pH than y-ZrP [Zr( HP04),.2H,0] in the Na20-Zr02-P20, system with all three methods of synthesis used here (Table 1) in 30 min-2 h at ca. 194°C. Thus, the rate of crystallization appears to have been enhanced by one to two orders of magnitude using this MH process, since the previously reported conventional hydrothermal proces~es~~,~~required 24-168 h. a-ZrP crystallized below pH~0.9 while y-ZrP crys- tallized above this pH up to a pH of 4.2 which is the highest pH used in these studies. a-ZrP crystallized under MH conditions is more crystalline [Fig. 1(a)] than that crystallized under CH conditions [Fig.l(b)], as indicated by the breadth d1A 22.07 6.32 3.70 2.63 2.06 1.70 1 1 I I I -7.711 A -7.583A 2.51 5A 2.640 A r2.398 A 3.561 A I t LlllllLl 54 2Ndegrees Fig. 1 Powder X-ray diffraction patterns of a-ZrP synthesized at ca. 194°C with 2 h of treatment under (a) microwave hydrothermal conditions and (b)conventional hydrothermal conditions kept constant except for the presence and absence of micro- waves in the above two syntheses. This result again shows the catalytic effect of microwaves in the crystallization of ZrP-layered phases. The three-dimensional network phase of NaZr,(PO,), did not crystallize at all in this pH range, although Yamanaka and Tanaka14 reported the formation of this phase over the range 180-225 "C at ca. pH 0.8 in CH runs.This appears to confirm our earlier findings' that this MH process leads to the preferential formation of layered phases. The formation of layered a-TiP [Ti( HP04),-2H20] and y-TiP [Ti(HP04)2-2H,0] phases, however, did not occur in the Na,O-TiO,-P,O, system using any of the methods. In this system, the three-dimensionally linked network phase of d1A 22.07 6.32 3.70 2.63 2.06 1.70 I 1 I I I I 1 10.8 A 3.134A I /I I4.347A 2.602A 4 14 24 34 44 54 26'cJegrees Fig.2 Powder X-ray diffraction patterns of a novel TIP layered phase (sample 13 in Table2) prepared at 194'C and heated at different temperatures for 4 h: (a) as prepared (unheated), (b) heated at 105 "C and (c) heated at 300 "C J.MATER. CHEM., 1994, VOL. 4 1905 Table 3 Selective caesium-exchange behaviour of layered Ti phos- (a1 phate phases sample Cs exchange, K,/ml g-’ a-TiP (sample 11 from Table 2) 30f25 y-Tip (sample 12 from Table 2) 2384 & 454 13.2 A TIP (sample 15 from Table 2) 2551 k476 NaTi,( P04)3was formed preferentially. In the absence of Na, a-TiP was formed (sample 11, Table 2) at a pH of ‘zero’. When the pH was adjusted to the range 0.97-4.23 with LiOH, y-TiP and a new TIP phase with a basal spacing of ca. 13.2 A was obtained [Table 2; Fig. 2(a)]. Thus, the highly hydrated Li’ ion promoted the formation of layered phases but not the Na+ ion in the pH region studied. The new layered phase does not appear to be one of the stacking polytypes of y-TiP because when thc sample was heated at 105°C for 4h, it collapsed to 11.4 A [Fig,2(b)], and when heated at 30,O “C for 4h, it collapsed to 10.8 A [Fig.2(c)] rather than 9.1 A which is characteristic of the anhydrous form of y-TiP.” Thus a new layered TIP with a basal spacing of 13.2 A whicb collapses upon dehydration to give a basal spacing of 10.8 A has been formed under the MH conditions. This basal spacing is ca. 1.7 A larger than that of the anhydrous form of y-Tip.” The new TIP has a plate-like morphology [Fig. 3(a)] as in the case of a-TiP and y-Tip. The a-TiP made by the MH process (sample 11 in Table 2) has a plate-like morphology where the plates are formed by small aggregates [Fig.3(b)]. This type of aggregation is characteristic of the MH process. The network structure of NTP (sample 1 in Table2) shows a distinct morphology of intergrown disks [Fig. 3(c)] analogous to that of ZSM-5 zeolite which has a porous network struc- ture.I6 Other NTP phases (e.g. sample 9 in Table 2) exhibit a morphology of spherulitic aggregates (not shown). Thus, the MH process appears to lead to distinct morphologies Which are different from the conventional hydrothermal meth~d,’~’~~ depending upon the processing parameters. We have investigated the selective caesium exchaqge proper- I 5Pm 1 ties of the layered phases including the new 13.2 A phase by equilibrating a 30 mg sample for 24 h with 25 ml of 1moll-’ NaCl containing lop4mol I-’ CsCl (equivalent ratio of Na to Cs =10 000).The selective caesium uptake is expressed in terms of Cs distribution coefficient, Kd which is defined as the ratio of the amount .of Cs sorbed per gram to the amount of Cs remaining in a ml of solution. The new 13.2 A phase shows the highest Kd (Table 3) compared to the well known a-or y-Ti phosphates. Thus, the new phase may be useful for the selective separation of caesium from waste solutions and especially from acidic solutions where these layered titanium phosphates are The authors acknowledge the financial support of the Materials Research Laboratory Consortium on Chemically Bonded Ceramics. References S. Komarneni, R. Roy and Q. Li, Mater. Res. Bull., 1992,27, 1393.S. Komarneni, Q. Li, K. Stefansson and R. Roy, J. Mater. Res., 1993,8, 3176. Fig. 3 Scanning electron micrographs of (a)novel 13.2 A TIP layered R. Roy and 0.F. Tuttle, Physics and Chemistry of the Earth, 1956, phase, (b) layered a-TiP and (c) NaTi,(PO,), phase of network 1, 138. structure R. M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982, p. 360. First International Symposium on Hydrothermal Reactions ed. S. Somiya, Gakujutsu Bunken Fukyu-Kai, Tokyo, 1983, p. 965. S. Komarneni, R. Roy, E. Breval, M. Ollinen and Y. Suwa, Adv. Ceram. Mater., 1986, 1, 87. L. B. Fischer, Anal. Chew., 1986,58,261. 1906 J. MATER. CHEM., 1994, VOL. 4 8 9 H. M. Kingston and L. B. Jessie, Anal. Chem., 1986,58,2534. A. Clearfield, Chem. Rev., 1988,88, 125. 15 G. Alberti, U. Constantino and M. L. L. Giovasnotti, J. Inorg. Nucl. Chem., 1979,41, 643. 10 11 12 13 14 Inorganic Ion Exchange Materials ed. A. Clearfield, CRC Press, Boca Raton, 1982, p. 290. J. B. Goodenough, H. Y. P. Hong and J. A. Kafalas, Muter. Rex Bull., 1976, 11,203. R. Roy, E. R. Vance and J. Alamo, Muter. Res. Bull., 1982,17,585. J. Alamo and R. Roy, J.Am. Ceram. SOC., 1984,63, C78. S. Yamanaka and M. Tanaka, J. Inorg. Nucl. Chem., 1979,41,45. 16 17 P. A. Jacobs and J. A. Martens, Synthesis of High-Silica Ahminosilicate Zeolites, Elsevier, Amsterdam, 1987, p. 390. S. Komarneni and R. Roy, in Scientific Basis for Nuclear Waste Management, ed. D. G. Brookins. Materials Research Society, Pittsburgh, PA, 1983, vol. 6, pp. 77--82. Paper 4102609H; Received 3rd May, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401903

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(12), 1907-1912 Photoisomerization of lndolinespirobenzopyrant in Anionic Clay Matrices of Layered Double Hydroxides Hideyuki Tagaya,*" Shigemitsu Sato,a Tsuneo Kuwahara,b Jun-ichi Kadokawa: Karasu Masa" and Koji Chiba" " Department of Materials Science and Engineering, Yamagata University, Yonezawa, Yarnagata 992, Japan R&D Center, TDK Corp., Saku, Nagano 389-02 Japan Sulfonated indolinespirobenzopyran (SP-SO3 -) was intercalated into the Mg/AI or Zn/Al layered double hydroxides (LDHs) in the presence of toluene-p-sulfonic acid (PTS). The new intercalates exhibited reversible photoisomerization between SP-S03- and merocyanine (MC). The presence of PTS was critical for the reversible photoisomerization. MC was very stable and the rate constant of decolorization was <9 x s-'.SP interacted with non-polar portions between the layers although MC interacted with the polar interior surface of the LDH. There has been considerable interest in the study of photo- chromic molecules because they are candidates for useful photoresponsive materials.lP3 Spiropyrans (SP) form merocy- anines (MC) reversibly on irradiation and they constitute an important class of photochromic compound^.^ In a non-polar solvent like toluene, SP is stable. When SP is exposed to UV light, SP undergoes ring opening to MC as shown in Scheme l.5,6In a non-polar solvent, MC is fairly unstable and isomerizes to SP immediately. In a polar solvent, the MC form is stable. The thermal reversion of photoinduced MC to the starting SP is influenced by the surrounding Many matrices such as polymer filrn~,~,~~ monolayers,11*12 liquid crystals,' smectite clay^'^-'^ biomolecules'7 and micelles'* are the subjects of intensive research. Many layered solids act as host lattices and react with a variety of guest molecules to give intercalation compounds in which the guest is inserted between the host layer^.'^-'^ When photochromic flugides were supported on a smectite clay in the dark the normal photochromic product was formed thermally.25 Pyridinium SP (Py+-SP) also intercalates between the inter- layers of the clay.26 However, in all of these reports, control of the photochromic reaction was not sufficient.Recently we have achieved co-intercalation of sulfonated SP (SP-S03-) and toluene-p-sulfonic acid (PTS) in the interlayers of an Mg/A1 layered double hydroxide (Mg/Al LDH).27 The intercalates exhibited reversible photoisomerization between IUPAC-recommended name: 1',3',3'-trimethyl-6-nitrospiro[2H-chromene-2,2'-indoline]. SP *.vls~~ uv SPS03-Scheme 2 Structure of SP-S03-SP-S03-and MC with a high stability of the MC forms.The LDHs consist of positively charged metal oxide/hydroxide sheets with intercalated anions, as shown in Fig. 1.28 Their general composition may be represented as [M"1-xM111x(OH)2]X+[A,,,"-rnH,O]"-, where A"- is an exchangeable anion. Fig. 1 shows the typical Mg/AI LDH which intercalates carbonate ions. In the case of the Mg/Al LDH, a variety of inorganic and organic anions have been exchanged into the LDH when x =0.26-0.65.29,30We now report the results of a detailed study of the photochomic properties of SP-SO,-in LDHs.Furthermore, we suggest mechanisms for the high stability of MC forms. Experimental Organic solvents were of analytical grade. If necessary, they were dried and fractionated prior to use. SP, SP-S03-and T c0:-c0:-c0:-c0:-7.8A 1 &$!.0-CH3 0 Oxygen MC Mg or Al Scheme 1 Photoisomerization of SP Fig. 1 Structural model of Mg/A1 LDH all of other materials used were from commercial suppliers and used as received. Preparation of LDHs Mg/AI LDH and the Zn/Al LDH were prepared by the reaction of a mixture of Mg(NO,), or Zn(N0,)2 with Al( .28.29 The reactions were carried out at room tem- perature for 2 h in Na,CO, solution, the pH of which was adjusted to ca.10 with NaOH. The Mg :A1 and Zn :A1 ratios were 0.30 and 0.25, respectively. The products were dried at 80'C for 48 h to give the LDH carbonates as white powders. The LDH carbonates were calcined at 500°C for 3 h to give the Mg/Al or Zn/Al oxides. LDH films were formed by drawing a glass slide out of a 1% suspension of the LDH. The formation of the LDH carbonates and the desorption of carbon dioxide by calcination were confirmed by X-ray diffraction measurements. Intercalation In the intercalation, 50 ml of a PTS (0.25 mmol), SP-SO,- (0.11 mmol) aqueous suspension and 0.1 g of the powdered calcined LDH were placed in a 100ml conical flask and stirred at 60 "C for 1 h under a nitrogen atmosphere.Reaction products were filtered and washed with acetone. For the LDH film, the film on the glass was soaked in an aqueous solution containing the guest molecules (SP-SO,- : 0.005-0.04 mmol per 50 ml, PTS: 0.3-1.2 mmol per 50 ml). After the reaction the film was washed with distilled water. Characterization of the Intercalates Powder X-ray diffraction spectra were recorded on a Rigaku powder diffractometer unit using Cu-Ka (filtered) radiation at 40 kV and 20 mA. Thermal analyses [thermogravimetry/ differential thermal analysis (TG/DTA)] were performed on a Seiko SSC5000 thermal analysis system using a heating rate of 10°C min-'. IR measurements were performed using a Horiba FTIR spectrometer. Photochromic Properties of Intercalates UV irradiation was carried out using a 400 W high-pressure mercury lamp.Visible light irradiation was carried out using a 500 W xenon lamp. Cut-off filters were used, if necessary. The power densities of the radiation were not measured. Absorption and fluorescence spectra were recorded using a Shimadzu UV-2200A spectrophotometer and a Hitachi spec- trofluorophotometer, respectively. Results and Discussion Intercalationof SP-S03-An XRD pattern of the Mg :A1 (0.70:0.30) LDH carbonate is shown in Fig. 2(a): The layer distance of the Mg/Al LDH carbonate is ca. 7.8 A. Ion exchange of carbonate ions with organic anions in the LDH is not easy. Therefore, deintercal- ation of carbonate ions was carried out by thermal treatment at 500 "C.The XRD pattern of calcined Mg/A1 LDH is shown in Fig. 2(h). There are no clear peaks because the calcined LDH was amorphous. Thermal analyses indicated that weight losses at 200-500 'C corresponded to evolution of water and carbon dioxide [Fig. 3(b)].28No large weight loss was observed for the calcined LDH [Fig. 3(u)J, confirming that the carbonate ion had been deintercalated. It is well known that smectite clays adsorb coloured organic J. MATER. CHEM.. 1994, VOL. 4 )25.5A IIu 5 10 15 20 25 2Bldegrees Fig. 2 XRD patterns of (a) the Mg/Al LDH, (b)the calcined Mg/A1 LDH, (c) the PTS intercalate and (d)the PTS/SP-SO, -co-intercalate r I J 100 200 300 400 500 600 T1°C Fig.3 Thermal analysis of (a)the calcined Mg/Al LDH, (h)the Mg/A1 LDH and (c) the PTS/SP-S03 co-intercalate~ compounds. SP in ethanol reacted with the calcined Mg/Al LDH; however, layer expansion was not observed by XRD measurements. Sulfonated SP was allowed to react with the calcined Mg/AI LDH in an aqueous !elution at 60 'C for 1 h. The layer distance expanded to 7.8 A. When the amount of intercal$ed anion was small, the interlayer spacing expanded to 7.8 A which indicates that the planes of the guests are parallel to the plane of the host layers.29 We have already confirmed that in such cases the interlayer spacing was independent of the size of guests.31 The size of the expansion indicated that the amount of intercalated SP-SO,- was small.The colour of the intercalate was red. The MC form was fairly stable and no isomerization to SP occurred even when the intercalate was irradiated by visible light. J. MATER. CHEM., 1994, VOL. 4 Co-intercalation of PTS and SP-SO,- As mentioned above, an intercalate containing the stable MC form was obtained. The LDH surface is covered by hydroxy groups and it has a high polarity.32 For smectite clay, there are two different kinds of microscopic environment, e.g. non-polar and polar regions.26 As already described, SP is stable in non-polar solvents and MC is stable in polar solvents. Therefore, we considered that to attain reversible photochro- mism, the creation of non-polar regions between LDH layers was important.We tried to co-intercalate PTS and SP-SO,-. The methylphenyl portion of PTS was expected to provide a non-polar region. PTS alone reacted with the calcined Mg/A1 LDH. In the intercalation in $n alkaline solution, the interlayer distance expanded to 7.8 A, indicating that the amount of intercalated PTS was small. In the int5rcalation in a weak acid solution the layer expande: to 17.7 A, as shown in Fig. 2(c). The length of PTS is ca. 8.5 A. Thc thickness of the brucite layer of t!e Mg/A1 LDH was 4.77A.33 The layer expansion was 12.9A, significantly larger than the length of the PTS anion. These indicated that the plane of PTS in the LDH layers was perpendicular to the plane of the host layers, as proposed by Dre~dzon.~~ In the reaction of the mixture of PTS and SP-SO,-with the calcined Mg/Al LDH, a yellow compound was obtained.The layer dist$nce of the Mg/A1 LDH expanded to between 7.8 and 25.5 A depending on the reaction conditions and whether the Mg/Al LDH was a powder or a film. Thermal analysis of the PTS/SP-S03- intercalate showed a larger weight loss than that of the Mg/Al LDH carbonate [Fig. 3(c)]. We have already reported that an organic anion in LDH interacts moderately with the positive charge of the layers2* However, the interaction was stronger than that of an anion with a proton and weaker than that of an anion with a sodium cation. Therefore, the evolution temperature of an organic anion in the Mg/AI LDH layer was higher than that of an acid, and lower than that of sodium salts.Weight losses continued above 600 "C, indicating the strong inter- actions between the Mg/A1 LDH and the PTS and SP-S03-guests. PTS is a strong acid. The interlayer spacing of the PTS intercalates was larger than with the SP-S03 -intercalate. The IR spectra of the PTS intercalate and PTS/SP-S03- co-intercalate were similar [Fig. 4(b) and (41,indicating that the amount of intercalated PTS was larger than that of SP-so3 -. Photochromic Properties of Intercalation Compounds The fluorescence spectrum of powdered PTS/SP-SO,-co-intercalate in the Mg/A1 LDH is shown in Fig. 5. In a non-polar solvent, toluene, the fluorescence maximum of the SP was 527 nm. On the other hand in a polar solvent, ethanol, the solution was red, indicating that the MC form was stable in ethanol.The fluorescence maximum of the MC form was 620nm. The fluorescence maximum of the yellow powdered co-intercalates was 574 nm as shown in Fig. 5(b). This maxi- mum was between that of SP (527 nm) and MC (620 nm). When the sample was irradiated with UV the co-intercalate changed to red. At the same time the fluorescence maximum shifted to 610 nm as shown in Fig. 5(4. The colour of the intercalate changed from red to yellow again on irradiation with visible light. At the same time the fluorescence maximum returned to 574nm. To confirm the reversibility between the SP and MC forms, repeated alternate irradiation with UV and visible light was carried out. As shown in Fig.6, the fluorescence intensity at 610 nm increased 3000 2000 1000 wavenum berkm-' Fig. 4 IR spectra of (a) the calcined Mg/A1 LDH, (b) the PTS intercalate, (cj the SP-SO,-intercalate and (dj the PTS/SP-S03-co-intercalate 500 600 700 wavelengthhm Fig. 5 Fluorescence spectra of (a) SP in toluene, (b)SP in ethanol, (c) the PTS/SP-S03-co-intercalate as reacted and (dj the PTS/S P-SO,-co-intercalate after UV irradiation on UV irradiation and decreased on visible light irradiation during each cycle. The same results were observed with the powdered Zn/Al LDH. The same photoisomerization was also observed on photoirradiation of the Mg/A1 LDH film. Fig. 7 shows the absorbance of the Mg/A1 LDH film with PTS/SP-SO, -co-intercalate.The absorption maximum was at 535 nm, i.e. close to the maximum of MC, 526nm, in methanol. There are, so-called, H and J aggregates for SP the absorption maxima of which are near 480 and 615 nm, re~pective1y.l~ The absorption maximum of the PTS/SP-S03-co-intercalates after UV irradiation was J. MATER. CHEM., 1994, VOL. 4 U 3 I Fig. 6 Reversible photochromism of the PTS/SP-SO,-co-intercalate in (a) the Mg/Al LDH and (b)the Zn/Al LDH \ (a., 300 400 500 600 700 wavelengthhm Fig. 7 Absorption spectra of (a) SP in toluene, (b)SP in toluene after UV irradiation, (c) SP-SO,-in methanol and (d) the PTS/SP-SO,- co-intercalate after UV irradiation different from those of the H and J aggregates. It was therefore concluded that the MC in the LDH layers did not aggregate.The absorbance at 535nm decreased with time after UV irradiation (Fig. 8). The half-life of photoinduced MC in toluene solution is only a few seconds at room temperature. To compare with solution, SP was trapped in a polyolefin polymer matrix. The half-life of the photoinduced MC form in the polyolefin was 3 min at 25 "C. Compared to these short lifetimes, the photoinduced MC in the Mg/A1 LDH was stable. When PTS (0.6mmol per 50ml) and SP-SO,-(0.01mmol) had reacted for 2 h, the amount of SP-SO,-intercalated into the Mg/A1 LDH was 3.2%. The half-life of thermal decolorization for the photoinduced MC in the Mg/A1 LDH was >200 h. This indicated that the rate constant of decoloration was t9x lo-' s-'.We believe that this rate constant is the smallest of those reported for non-H and -J aggregates. To make clear the relationship between reaction conditions and the stability of the photoinduced MC, we prepared PTS/SP-SO, -co-intercalates under various system- atic reaction conditions. As shown in Table 1, the amounts of intercalated SP-SO,- were almost the same (3.2-3.5%) at reaction times >0.5 h. However, the stability of the photoind- uced MC (half-life) increased with the reaction time (nos. 2-5). This suggests that rearrangement of the intercalated SP-SO, -to a stable conformation might proceed with longer reaction i400 500 600 700 waveleng t hlnm Fig. 8 Typical decoloration of the PTS/SP-SO, -intercalated Mg/AI LDH film after UV irradiation.Absorption spectra at (u) r=O and (b)t=m. times. The amount of SP-S03-intercalated increased from 3.5 to 7.0% (nos. 6-8) with increasing concentration of SP-SO,-in the reactant. The absorbance after UV irradiation increased with higher concentrations of SP -SO3-in the reactant; however, the stability of the SP-S03 form did not change. A high concentration of PTS in the reactants sup- pressed the amount of intercalated SP-SO,- (nos. 9, 10).The half-life of the photoinduced MC decreased slightly at higher concentrations of PTS. Mechanism of High Reversibility and High Stability of Photoinduced MC As already mentioned, the LDH surface has a high polarity. Therefore, the intercalate containing SP-SO, gave only the MC form and did not isomerize to the SP form thermally or photochemically.The colour of the PTS/SP-SO, -co-intercalate was yellow. This indicated that within the LDH layers, SP-S03- does not interact strongly with the LDH surface and might exist near a non-polar region such as the methylphenyl portion of PTS, as shown in Fig. 9. UV irradiation of PTS/SP-SO,- co-intercalates produced ring opening to the high-polarity MC form. This MC form might interact with the LDH surface and remain near the LDH surface. Interaction of the MC with the LDH surface was moderately strong; therefore, a high stability of the photoin- duced MC form was attained. To help confirm the mechanism, a PTS intercalate without SP-S03-was soaked with a toluene solution containing SP.The colour of the Mg/Al LDH surface was u hite, indicating that SP did not interact with the Mg/Al LDH surface. When this was UV-irradiated, the Mg/Al LDH surface changed to pink. This indicated that the SP in toluene underwent ring opening and the resulting high-polarity MC interacted with the Mg/A1 LDH surface. When this was irradiated with visible light the pink colour disappeared. Repeated cycling of the colour by UV and visible light irradiation was attained. These results suggest the importance of both the non-polar portions, toluene, and the polar portion, the LDH surface. When the toluene was removed by evaporation, the LDH surface changed to pink, and even when the surface was irradiated with visible light, the pink colour did not disappear because of the absence of non-polar regions.Conclusions New intercalates of SP-S03-and PTS into Mg/Al or Zn/Al LDHs exhibited reversible photoisomerization between J. MATER. CHEM., 1994, VOL. 4 191 1 Table 1 Photochromic properties of PTS/SP-S03 -intercalates reactant (mmol per 50 ml) reaction absorbance (arb. units) thermal weight of time/ decolouration, intercalated no. SP-SO3-PTS h t=O t=22 h t=86 h t,:,/h SP (Yo) 1 0.005 0.3 1 -----2 0.0 1 0.6 0.25 0.36 0.08 0.07 10 2.3 3 0.01 0.6 0.5 0.51 0.33 0.25 80 3.4 4 0.01 0.6 1 0.45 0.35 0.30 >200 3.5 5 0.01 0.6 2 0.33 0.30 0.27 >200 3.2 6 0.01 0.6 2 0.29 0.26 0.23 >200 3.5 7 0.02 0.6 2 0.63 0.54 0.50 >200 5.5 8 0.04 0.6 2 0.77 0.64 0.6 1 >200 7.0 9 0.01 1.2 2 0.14 0.12 0.10 >200 1.3 10 0.02 1.2 1 0.43 0.37 0.3 1 >200 3.8 SO3-SO3-SOB-SO3-SO3-I I I I IQ Q Q Q CH3 CH3 CH3 CH3 SO, SO,-SO, /////////////////////////// Fig.9 Possible mechanism for the reversibl le photoisomerization and high stability of MC SP-SO,-and MC. The high stability of the MC indicated the presence of a very suitable environment for MC in the layered solids in which MC interacted with the polar interior surface of the LDH. For many matrices such as polymer films, monolayers and liquid crystals it is difficult to confirm the co-existence of polar and non-polar regions in the vicinity of the matrix. On the other hand, many layered solids intercalate more than one guest.This suggests the possibility of attaining other variable regions between the layers as obtained in this study. The authors are grateful to H. Morioka and A. Ogata for their technical collaboration. References 1 H. Durr and H. Bouas-Laurent, Photochromism: Molecules and Systems, Elsevier, New York, 1990. 2 M. Irie, Petrotech, 1989,12, 359. 3 G. H. Brown, Photochromism, Wiley Interscience, New York, 1971. 4 M. Irie, T. Tamaki, T. Seki and J. Hibino, Photochromic Spiropyrans, Bunshin, Tokyo, 1993. 5 Y. Onai, K. Kasatani, M. Kobayashi, H. Shinohara and H. Sato, Chrm. Lett., 1990, 1809. 6 V. A. Kronganz, S. N. Fishman and E. S. Goldburl, J. Phys. Chem., 1978,82,2649. 7 V. Nadolski, P. Uznanski and M. Kryszewski, Makrornol. Chem., Rapid Commun., 1984,5, 327.8 E. J. C. Kellar, G. Williams, V. Kronganz and S. Yitzchaik, J. Muter. Chem., 1991, 1, 331. 9 S. Z. Gardlund, J. Polym. Sci., Polym. Lett. Ed., 1968,6, 57. 10 K. Horie, N. Kenmochi and I. Mita, Makromol. Chem., Rapid Commun., 1988,9,267. 11 D. A. Holden, H. Ringsdorf, V. Deblauwe and G. Smets, J. Phys. Chem., 1984,88,716. 12 R. Vilanove, H. Hervet, H. Gruler and F. Rondelez, Macromolecules, 1983,16, 825. 13 H. Tomioka and T. Itoh, J. Chem. Sue., Chem. Commun., 1991, 532. 14 T. Seki, K. Ichimura and E. Ando, Langmuir, 1988,4, 1068. 15 T. Seki and K. Ichimura, J. Chem. SOC., Chem. Commun., 1987, 1187. 16 T. Seki and K. Ichimura, J. Photopolym. Sci. Tech., 1989,2, 147. 17 U. Pfeifer, H. Fukushima, H.Misawa, N. Kitaniura and H. Masuhara, J. Am. Chem. Soc., 1992,114,4417. 18 S. Tazuke, S. Kurihara, H. Yamagishi and T. Ikeda. J. Phys. Chem., 1987,91,249. 19 D. W. Bruce and D. O’Hare, Inorganic Materials, John Wiley, Chichester, 1992, p. 165. 20 W. Y. Liang, in Intercalation in Layered Materials, ed. M. S. Dresselhause, Plenum Press, New York, 1986, p. 21. 21 R. Schollhorn, in Inclusion Compounds, ed. J. L. Atwood, 1912 J. MATER. CHEM.. 1994, VOL. 4 J. E. D. Davies and D. D. MacNicol, Academic Press, New York, 28 T. Kuwahara, 0. Onitsuka, H. Tagaya, J. K'idokawa and 22 1984, vol. 1, p. 249. H. Tagaya, T. Hashimoto, M. Karasu, T. Izumi and K. Chiba, 29 K. Chiba, J. Inclusion Phenom. Mol. Recognit. Chevi., in the press. H. Tagaya, S. Sato, H. Morioka, J. Kadokawa, hl. Karasu and Chem. Lett., 1991,2113. K. Chiba, Chem. Muter., 1993,5, 1431. 23 H. Tagaya, K. Saito, T. Kuwahara, J. Kadokawa and K. Chiba, 30 H. Tagaya, S. Sato, K. Chiba, K. Takahashi, T. k'okoyama and 24 Catal. Today, 1993, 16,463. H. Tagaya, K. Ara, J. Kadokawa, M. Karasu and K. Chiba, J. Muter. Chem., 1994, 4, 551. 31 M. Endo, Kagaku Kogaku Ronbunshuu, 1993,19,923. E. Narita, T. Yamagishi and T. Tonai, Nippon kagaku Kaishi, 1992,291. 25 J. M. Adams and A. Gabbut, J. Inclusion Phenom. Mol. Recognit. Chem., 1990,9, 63. 32 33 F. Cavani, F. Trifiro and A. Vacari, Catal. Toduy, 1991,11, 173. R. W. G. Wykoff, Crystal Structure, John Wiley, New York. 1963, 26 H. Takagi, T. Kurematsu and Y. Sawaki, J. Chem. Soc., Perkin Trans. 2, 1991, 1517. 34 vol. 1, p. 268. M. A. Drezdzon, Inorg. Chem., 1988,27,4628. 27 H. Tagaya, T. Kuwahara, S. Sato, J. Kadokawa, M. Karasu and K. Chiba, J. Mater. Chem., 1993,3, 317. Paper 4/02886D; Received 16th Muy, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401907

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 12), 1913- 1920 Investigations of Structure and Protonic Conductivity in the so-called Tin Zeolites Gary B. Hix,*aRobert C. T. Slade," Kieran C. Mollof and Bernard Ducourantc a Department of Chemistry, University of Exeter, Stocker Road, Exeter, UK EX4 4QD Department of Chemistry, University of Bath, Claverton Down, Bath, UK BA2 7AY Laboratoire des Agregats Moleculaires et Materiaux Inorganiques, URA CNRS 79, Universite Montpellier 2, Place Eugene Bataillon, 34095 Montpellier Cedex 5, France 'Tin zeolites', composites containing a zeolite and SnO,, have been produced from parent zeolites (mordenite, zeolites X, Y and A) by thermal and microwave methods. Materials characterisation employed X-ray powder diffractometry (XRD), 29Si and 27AI MAS NMR, "'Sn Mossbauer spectroscopy, and ac conductivity investigations. Both XRD and MAS NMR studies show degradation of the zeolite framework in materials prepared by the thermal route.The extent of the damage depends upon the tin salt used in synthesis and is higher for those zeolites with higher Al contents. Mossbauer spectra are dominated by a central peak arising from microdispersed SnO,, but a second Sn'" environment is also present in materials prepared thermally from zeolites X, Y and A. All the 'tin zeolites' exhibit protonic conductivities greater than those of the parent zeolites, with thermally prepared samples exhibiting conductivities up to an order of magnitude greater than corresponding materials prepared by the microwave method. The introduction of tin(1v) oxide into mordenitei,2 and zeolite Y3 has been reported in the literature.The emphasis of those investigations of 'tin zeolites' was on the ionic (H' ) conduction in the materials, with structural investigations limited to X-ray powder diffraction studies. Samples were made by melting varying quantities of tin@) salts (SnCl, or SnSO,) into the zeolite frameworks and oxidising them in situ by thermal treatment, giving dispersed SnOz with an enhanced specific surface area relative to pure SnO,. For the samples in which mordenite was used as the zeolite component, the use of small quantities of tin(11) salt in the syntheses was reported to result in tin ion exchange only (the oxidation state of the exchanged ions was not reported), and increasing the amount of tin salt was said to produce SnO,., Preparations involving zeolite Y were reported to give both SnO, and ion e~change.~ All products were therefore composite in nature.The ionic (protonic) conduction in these so-called 'tin zeolite' materials has been studied by ac and dc techniques.'-, Measurements concerning the mordenite-based materials were carried out in a water-moistened atmosphere, at 100% relative humidity (RH), and conductivity crz lop2S cm-' at 100"C. Studies using dc techniques showed that the conducting species to be the proton, the conduction being assigned to a proton-hopping mechani~m.~ Measurements on zeolite Y-derived samples at 75% RH gave 0% lop2S cm-I at 116 "C; the conduction mechanism was assigned in that case as being a polyatomic vehicle conduction of proton^.^ Since water molecules act as the 'vehicles' for the charge carriers (H'), the water content of the sample affects the conductivity directly. There is a consequent variation of conductivity with relative humidity, with higher relative humidities leading to higher conductivities.Hydrous tin@) oxide itself (Sn0,-nH,O) is formed as a white precipitate on hydrolysis of tin@) salts. It is an ion- exchange material which has been shown to be a fast protonic conductor, with a room-temperature conductivity CJ = 4 x S cm-l.' This type of material has been termed a particle hydrate,6 consisting of charged particles with partly protonated oxide-hydroxide surfaces separated by a weakly acidic aqueous region.We now report an in-depth study of Sn0,-containing composites produced by two different synthetic routes and derived from mordenite, and from zeolites A, X and Y. The first route is that of Knudsen et d.,'resulting in materials which 'have been ion-exchanged' and also contain a separate tin oxide phase. The second method employed the microwave heating and hydrolysis of an organotin compound. A compari-son of conductivity and microstructures in these materials is presented. Experimental Materials synthesized using the salt-melt procedure are desig- nated T-series materials (e.g. T-Sn-X refers to the material made by the salt melt route with zeolite X), whilst those made following microwave procedures are designated M series.Tin@) sulfate, tin(r1) chloride and triphenyltin chloride (BDH Chemicals, AnalaR grade) were used as supplied with- out purification. Zeolites (mordenite, zeolites A, X and Y) were supplied by Laporte Inorganics. Analyses (XRF) of the parent zeolites are given in Table 1. T Series Tin oxide-containing zeolites were synthesized following the methods described by Knudsen et a/.' Tin@) sulfate (1.4 g) was intimately ground with sodium zeolite (4.0 g), and then heated at 400 "C for 20 h. The products were washed for 5 x 24 h in 200 cm3 of deionised water (with filtration and fresh water daily), and then stored in an atmosphere of 50% RH (over a saturated solution of sodium hydrogensulfate).Tin(r1) chloride dihydrate (3.0 g) was intimately ground Table 1 Chemical analyses (XRF) and Si:Al ratios of the parent zeolites used in this study Si :A1 SiOz A1,0, Na,O zeolite (%) (Yo) ("/.I XRF NMR Na mordenite 81.80 10.90 6.88 6.15 6.11 H mordenite Na-A 90.90 43.08 7.31 36.23 <100ppm 8.05 9.15 1.01 8.91 1" Na-X 49.75 31.82 18.43 1.35 1.32 Na-Y 62.72 22.94 12.11 2.49 2.41 " Only one site type in the "Si NMR spectrum. with sodium zeolite (4.0 g) and heated at 100°C under dynamic vacuum for 3 h to remove any excess of water. The mixtures were heated at 200 "C for 20 h in air, oxidising the tin, and then at 400°C for 90min, to remove the tin(1v) chloride formed. The products were washed and stored as above.A preparation using tin(I1) chloride dihydrate was also carried out using hydrogen mordenite. This sample has been designated T-Sn-MorH. Where appropriate in the further discussion, the anion characteristic of the tin-containing reagent in the preparation is denoted by (C1-) or (SO,"). Thus T-Sn-X(Cl-) denotes a material prepared from zeolite X and tin@) chloride. M Series This synthesis involved hydrolysis of an organotin species to form tin(1v) oxide, following Ashcroft et aL7 A solution of triphenyltin chloride ( 1.0 g) in ethanol (30 cm3) was placed in a flask with the sodium zeolite (5.0 g). The flask was covered and then subjected to nine 1 min bursts of microwave heating (at 100% power) in a standard Samsung 600 W microwave oven.Between bursts of microwave radiation, the solutions were allowed to cool, and ethanol that had been lost by evaporation was replaced. Care was needed with this pro- cedure due to the flammability of ethanol. The materials were recovered by filtration, washed and stored as above. XRD Powder diffraction profiles were recorded on a Philips diffractometer (PW 1050/25 goniometer; Cu-Ka radiation, i= 1.54178 A). Data were collected for 10 s in steps of 0.1" of 20. Observed patterns were compared with computer-generated simulations.* MAS NMR 29Si(59.584 MHz) and 27Al(78.152 MHz) MAS NMR spectra at ambient temperature were recorded on a Varian VXR 300 spectrometer by the SERC Solid State NMR Service (Durham).Relaxation delays in recording 29Si spectra were 30s, determined as sufficiently long to avoid all saturation effects. The corresponding delay for 27Al spectra was 0.5 s. 29Si spectra were deconvoluted assuming gaussian line- shapes, thereby giving spectral parameters for the different Si sites. '19Sn Mossbauer Spectroscopy T-series samples were finely ground and suspended in an inert matrix. Once mounted, the samples were maintained at 77 K by a continuous-flow liquid-nitrogen cryostat linked to a digital temperature controller. Temperature stability was held to within kO.1 K throughout the experiment. Spectra were collected on a constant-acceleration Mossbauer spectrometer (Bath), using a Ca11gmSn03 source. The drive unit was con- trolled by a digitally generated saw-tooth waveform to pro- duce constant acceleration.Velocity calibration was based upon the spectrum of cr-Fe, with CaSnO, being used as the zero-velocity reference. Spectra of M-series materials were similarly recorded at Montpellier. Data were acquired through a multi-channel scaling analyser as two spectra related through a mirror plane. Folding these two spectra improved the counting statistics and removed the geometric effect. Experimental Mossbauer spectra were computer-simulated (with a combination of Lorentzian lines) and the calculated J. MATER. CHEM.. 1994, VOL. 4 spectrum was subsequently refined, using the 'General Mossbauer Fitting Program' written by K. Ruenbauer and T. Birchall.' Isomer shifts are expressed with respect to the source.Impedance Analysis Microcrystalline samples of the modified zeolite systems and the parent zeolites were compressed in a 13 mm die at 5 tonne cm-2 to give pellets of cu. 1 mm thickness. To aid the binding of the pellets, a small quantity of water was added prior to pressing. Pellet faces were coated with conduc- tive silver paint (Acheson Electrodag 915), and copper or nickel disc electrodes were attached using silver paint as ad- hesive. Samples were then allowed to equilibrate at ambient temperatures in a desiccator over saturated aqueous sodium hydrogensulfate (RH =50%). Sample assemblies were mounted in a brass cell which held six samples simultaneously. Impedance spectra (admittance or impedance plane) were collected in the range 5 Hz-1 MHz (oscillation voltage 100 mV) using a Hewlett-Packard 4192A LF impedance analyser controlled by an IB M-compatible computer, which used software embedding EQUIVCRT mod- elling software." Temperature was controlled in the range 293 < T/K <353 by immersion of the cell in a controlled-temperature water-ethylene glycol bath, allowing at least 30 min for equilibration at each temperature (this period was found by experience to be in excess of the minimum necessary to give temporal stability of impedance spectra).RH within the cell was controlled by means of saturated aqueous sodium hydrogensulfate solution placed in a sponge at the bottom of the cell. Results and Discussion Analyses and Si :Al Ratios Analyses (XRF) of the samples, and Si :A1 ratios determined thereby, are given in Table 2.Application of Loewenstein's rule, which disallows Al- 0-A1 linkages in zeolite frame- works," also allows the Si:Al framework ratios to be calcu- lated from the relative intensity contributions of different sites in the 29Si MAS NMR spectra via eqn. (1). Si:Al= 114,mII (n1/4)1~.~ (1) m=0,4 m=0,4 where 14,mis the relative intensity of a line assigned to a Q4(mAl) silicon site (rn is the number of -OAl linkages Table 2 Chemical analyses (XRF) of the tin zeolite samples Si : A1 ratio SiOz A1,0, Na,O SnO, material (%) (Yo) (%) (Yo) XRF NMR T-Sn-A(CI -) 29.13 25.25 12.36 11.36 0.98 1.08 T-Sn-A( 24.56 21.30 11.92 16.30 0.98 1.03 T-Sn-X(C1-) 33.68 23.45 9.81 11.04 1.24 1.32 T-Sn-X(SO4'-) 30.74 21.10 5.49 18.14 1.22 1.34 T-Sn-Y (Cl- ) T-Sn-Y(SO,,-) 44.52 54.48 16.40 13.77 7.92 3.89 8.94 19.46 2.35 2.30 2.58 2.78 T-Sn-Mor( Cl -) 54.48 7.23 2.51 12.25 6.39 7.34 T-Sn-Mor( SO,'-) 50.73 7.06 3.86 18.04 6.12 7.16 T-Sn-MorH(C1F) 45.79 4.03 0.08 24.24 9.52 7.39 M-Sn-A 33.83 28.45 16.24 6.32 1.01 -~ M-Sn-X 36.11 23.10 13.38 5.82 1.32 1.39 M-Sn-Y 51.60 19.13 9.5 1 2.96 2.32 2.43 M-Sn-Mor 71.88 9.98 5.59 2.82 6.11 6.25 (Cl-) indicates samples made using SnCl,, (SO,2-) indicates samples made using SnSO,.J. MATER. CHEM., 1994, VOL. 4 around the Si atom).The values thus obtained are also given in Table 2. T Series It can be seen (Table 2) that the samples made using SnC1, contain less tin than the samples made using SnSO,. This is to be expected since the preparation using SnC1, involves a step in which SnCl, is removed as vapour by heating the samples to over 380°C. The reduction in sodium content, with respect to the parent zeolites, indicates that some ion exchange has taken place. However, the amount of tin in the samples is greater than would be required to compensate for the observed Na loss; the remainder of the tin is present as oxide. The Si : A1 ratios calculated from XRF data are closer those of the parent materials than are those obtained from NMR spectra. The values obtained from XRF account for all of the silicon and aluminium present in the whole sample, whereas the NMR method allows calculation of the %:A1 ratio for the framework only.The higher 'Si: A1 ratio' resulting from NMR data is a consequence of leaching of aluminium from the framework during processing. M Series The tin contents (Table 2) are lower than those of the corre- sponding T-series materials. The reductions in sodium contents of samples are consistent with the proportion of zeolite in the final material, i.e. there has been no ion exchange. The Si:Al ratios determined by XRF and from NMR are essentially those of the parent materials. There is no evidence for leaching of aluminium from the zeolite frameworks. XRD Profiles TSeries Data for materials prepared from the same parent zeolite differ markedly depending on whether SnC12.2H,0 or SnSO, was used as the other reagent (Fig.1-4). Materials which had SnS0, as the tin precursor exhibited sharp narrow XRD lines associated with the zeolite frame- work. These lines were, however, reduced in intensity with respect to the zeolite parents; this is due to (i) reduction in the amount of crystalline zeolite present (by degradation or fragmentation of the zeolite by acid leaching of framework aluminium), (ii) 'dilution' of the zeolite by introduction of the tin oxide phase (which gives additional broad features). The latter is predominant. 10 20 30 40 50 60 70 80 28/degrees Fig. 1 X-Ray powder diffraction profiles of (a) T-Sn-Mor(Cl-), (b)T-Sn-Mor (c) T-Sn-MorH and (d) M-Sn-Mor AmA.-(a,)_I ~ 1 IsI'I'I'I1 ! ' l ' l ' t ' T 7 1 ' l ' i 10 20 30 40 50 60 70 80 2 @degrees Fig.2 X-Ray powder diffraction profiles of (a) T-Sn-A(Cl-), (b)T-Sn-A(SO?-) and (c) M-Sn-A I I ' I ' I ' I ' I ' I ' 1 ' I ' 10 20 30 40 50 60 70 80 2€J/degrees Fig. 3 X-Ray powder diffraction profiles of (a) T-Sn-X(C1-), (b)T-Sn-X(S042-) and (c) M-Sn-X (a) I ' I ' I ' I ' I ' I ' I ' 1 7 1 7 ' I ' I ' I ' I ' 10 20 30 40 50 60 70 80 2Wdegrees Fig. 4 X-Ray powder diffraction profiles of (u) T-Sn-Y(C1-), (b)T-Sn-Y(S0,2-) and (c) M-Sn-Y Samples derived from SnCl2.2H,O show, in contrast, much greater reductions in XRD line intensities, the extent of the reduction varying with the zeolite parent.In the most extreme examples, those of T-Sn-X and T-Sn-Y (Fig. 3 and 4, respect- ively), there are no observable zeolite lines, only very broad lines due to small particles of tin oxide being seen. Since these samples have a lower tin content than the analogous samples made with SnSO,, the loss of intensity in the zeolite lines is attributed to disruption of the framework, which is also evident in 27Al and 29Si NMR spectra (see below). Cell parameters for the zeolite framework in the samplFs giving a zeolite-related XRD profile showed very little (< 0.1 A) variation with respect to those of the parent zeolites (Table 3). M Series The XRD profiles (Fig. 1-4) show lines attributable to the zeolite frameworks, but none due to the presence of SnO,.This is due to the low percentage and small particle size of SnO, in the samples, and would also account for the non- occurrence of the intensity reductions observed for the T-series samples. These observations indicate that the struc- tural integrity of the zeolite framework has been maintained throughout the synthetic procedure. Calculated cell param- eters vary only slightly with respect to those of the pristine zeolite parents (Table 3). NMR Spectra M Series The 27Al spectra of these materials indicated exclusively tetrahedrally coordinated Al, as in the parent zeolites. Peak positions are given in Table 4. The 29Si spectra were also almost identical in all spectral parameters to those of the zeolite parents.The 29Si spectrum of M-Sn-X is shown in Fig. 5, in which deconvolution into constituent gaussians is also illustrated. The microwave preparative procedure has no Table 3 Unit-cell parameters for tin zeolites material a/A b/A CIA )T-Sn-Mor(C1~ 18.24(6) 20.77( 6) 7.58(3) T-Sn-Mor(SO4'-) 18.13( 2) 20.46( 3) 7.50(1) T-Sn-MorH (C1 -) 18.28( 3) 20.71(3) 7.52( 1) T-Sn-A( C1 -) 24.61( 1) 24.61 ( 1) 24.61(1) T-Sn-A(SO,'-) 24.59( 3) 24.59 (3) 24.59( 3) T-Sn-X( C1- )" T-Sn-X( SO,'^ ) 25.02( 13) 25.02( 13) 25.02( 13) T-Sn-Y (Cl- )" T-Sn-Y ( -) 24.94(4) 24.94( 4) 24.94( 4) M-Sn-Mor 18.11(2) 20.5 1( 1) 7.52( 1) M-Sn-A 24.61( 1) 24.61 (1) 24.61 (1) M-Sn-X 25.00( 1) 25.00( 1) 25.00( 1) M-Sn-Y 24.95( 1) 24.95( 1) 24.95( 1) (Cl-) indicates samples made using SnCl,, (SO4'-) indicates samples made using SnSO,.No zeolite lines in the XRD profile. Table 4 Observed 27Al MAS NMR peak positions for tin zeolites __~ material 6 (tetrahedral) S (octahedral) T-Sn-Mor(C1-) 54.1 0.2 T-Sn-Mor(S0,' -1 54.7 -0.4 T-Sn-MorH(C1-) 54.1 -0.4 T-%-A( C1- ) 57.5 -0.1 T-%-A( SO,' -) 54.3 -0.2 T-Sn-X( C1- ) 59.0 -2.5 T-Sn-X(SO,'-) 59.1 -3.9 T-Sn-Y (Cl-) 59.3 -3.1 T-Sn-Y(SO4'-) 58.7 -1.5 M-Sn-Mor 54.6 --M-Sn-A 58.3 M-Sn-X 59.4 -M-Sn-Y 60.0 -(C1-) indicates samples made using SnCl,, (SO,*-) indicates samples made using SnSO,. J. MATER. CHEM., 1994, VOL. 4 Fig. 5 Proton-decoupl6d high-resolution 29Si MAS NMR spectrum of M-Sn-X, recorded at ambient temperature, with 21 spin rate of ca.5 kHz and n/2 rf pulses. The recorded spectrum is giben as the dotted line (...) and fitted gaussians are shown by the solid lines. The difference between the observed and calculated spectrum is given as a solid line below the spectrum. effect (observable by NMR spectroscopy) upon the zeolite frameworks, which is fully consistent with the XRD results. TSeries The 27Al spectra of all T-series samples are dominated by the signal of tetrahedrally coordinated A1 (in the zeolite frame- work), but also indicate the presence of some octahedrally coordinated aluminium, i.e. a peak at 6~0typical of A]"' octahedrally coordinated by 0 atoms [cf. Al( H20)63+(as), which is the reference]. Peak positions are given in Table 4.The octahedral A1 is that acid-leached from the framework,I2 and is consequent on the heat treatment of SnSO, and SnCl,, producing SO, and HC1 (from the reaction of SnC1, with water), respectively. For a given zeolite host, the intensity due to octahedral A1 in the 27Al spectrum of the material made using SnCl, is greater than that in the spectrum of the corresponding material made using SnS0, (Fig. 6). This implies that, as is also evident in XRD studies (see earlier), the extent of framework damage is higher if SnC1, is used in the synthesis. Chemical shifts in the deconvoluted 29Si spectra of all samples in the T series (summarised in Table 5) are easily assigned to specific Q4(mAl) units.13 The positions of the lines vary slightly (<2.5 ppm) from those of the host zeolites (in this study and also reported in the literat~re),'~ and this is attri- buted to cation effe~tsl~,'~ brought about by tin ion exchange, to distortions in local geometry and to susceptibility differences.The damage to the zeolite frameworks is evident in 29Si MAS NMR spectra in the observation of (i) additional peaks in the spectra (except for T-Sn-Mor samples). (ii) a general upfield shift in the intensity distribution of the spectra (Si nuclei have, on average, fewer A1 nearest neighbours) and (iii) broadening of constituent lines. Where aluminium atoms are widely separated in the framework (e.g. in mordenite, Si :A1zz 5.8) their partial removal leaves the framework essen- tially intact, but results in observable changes in the relative intensities of lines in the 29Si spectra.Fig. 7 illustrates the greater framework disruption for the T-series products relative to those of the M series and also the greater disruptive effect of using SnCl, as a reagent. The 29Si spectrum for T-Sn-X( SO4,-) [prepared from SnSO,, Fig. 7(u)] is changed relative to that for the parent zeolite (to J. MATER. CHEM., 1994, VOL. 4 1917 * A\ 100 50 0 -50 * .. I‘ 100 50 0 -50 6 Fig. 6 Proton-decoupled high-resolution 27Al MAS NMR spectra of (a) T-Sn-Y(Cl-), (b)T-Sn-Y(SO,’-) and (c) M-Sn-Y (no octahedral Al), recorded at ambient temperature with spin rates of cu. 5 kHz and employing 71/6 rf pulses. *indicates a spinning side band.which that for M-Sn-X, Fig. 5, is a close approximation), but observable concentrations of Q3 sites [Si(OT),O- or silanol peak positions remain essentially unchanged. The 29Si spec- groups], with a chemical shift of ca. -100 ppm.17 Leaching trum for T-Sn-X(Cl-) [prepared from SnCl,, Fig. 7(b)],on can result in fragmentation of the crystallite, giving a variety the other hand, bears little resemblance to that for the of Q” (n=1, 2, 3 or 4) sites, all of which will exhibit different parent zeolite. chemical shifts. The additional lines observed are broad, In the composites formed from zeolite A (the parent richest indicating the presence of a variety of similar (but differently in aluminium) dealumination results in the production of distorted) fragment environments. Table 5 29Si MAS NMR parameters for tin zeolites Si(OA1) Si( 1 Al) Si(2AI) Si(3A1) Si(4A1) material -6 W/HZ I(%) -6 W/HZ I(%) -6 W/HZ I(%) -6 W/HZ I(%) -6 W/HZ I(%) T-Sn-A(SO,’-) 93.6 262 12 89.5 151 75 95.8 71 1 86.2 69 2 83.8 287 10 T-Sn-A(CI-) 100.5 74 2 95.4 210 20 90.8 238 74 98.3 82 3 85.2 91 2 T-Sn-X(SO,’-) 103.0 74 1 99.3 182 4 95.3 192 12 89.9 236 34 85.7 146 82.6 274 45 4 T-Sn-X( C1- ) 104.1 239 3 98.5 466 31 94.0 219 11 89.7 270 22 85.1 253 15 108.2 635 13 79.5 393 5 T-Sn-Y (SO,’-) 113.1 571 7 99.9 347 39 94.5 190 29 89.3 235 14 83.7 238 1 106.2 281 10 T-Sn-Y(Cl-) 114.1 588 12 100.9 300 35 95.3 234 24 90.1 283 9 80.8 283 1 106.9 353 19 T-Sn-Mor( T-Sn-Mor(Cl-) T-Sn-MorH ) 113.9 113.9 112.7 288 300 310 50 44 52 107.0 107.1 107.2 368 347 394 45 52 42 M-Sn-A - _ _ ~--- 89.4 100 M-Sn-X 103.4 170 3 99.4 181 8 94.5 183 20 89.5 174 36 85.1 110 33 M-Sn-Y 105.9 220 7 100.1 273 37 94.7 186 40 89.6 190 15 84.8 187 2 M-Sn-Mor 113.0 274 36 106.5 379 58 99.7 318 7 --.- J. MATER. CHEM., 1994, VOL. 4 Table 6 '19Sn Mossbauer parameters for tin leolites diso/ AEq/ FWI-lM/ relative material" mm s-lb mm s-lb mm s-l intensity M-Sn-A 0.16 0.55 1.04 1.o M-Sn-X 0.17 0.55 1.09 1.o M-Sn-Y 0.16 0.6 1 0.07 1.o M-Sn-Mor 0.14 0.5 1 1.27 1.o T-Sn-Mor( SO4,-) T-Sn-Mor( C1- ) T-Sn-MorH (Cl- ) 0.04 0.06 0.03 3.25 0.59 0.60 0.67 1.80 1.03 1.02 1.21 1.09 1.oo 1.oo 0.94 0.06 T-Sn-A(SO4,-) 0.05 0.14 0.59 1.34 1.04 0.88 0.91 0.09 T-Sn-A(C1-) 0.13 0.19 0.57 1.39 110 0.97 0.7 1 0.29 -50 1 ~ ~ -70 ~ ' 1 -90 ~ ~ " ~ ~"' -1 20 ~ ~I ' -1 30 ' ~ ~ ' "T-Sn-X(SO4'-) ~ ~ ~ ~0.05 0.11 ' ~ 0.55 1.35 ' 1 00 0 85 0.88 0.12 T-Sn-X( C1- ) 0.04 0.07 0.51 1.36 1 05 0 98 0.68 0.32 T-Sn-Y(SO,'-) 0.04 0.11 0.55 1.30 0 46 0.91 0.82 0.16 4.05' - 0 94 0.02 T-Sn-Y (C1 -) 0.06 0.09 0.53 1.33 0 99 0 98 0.85 0.15 ~ a indicates samples made using SnSO,, rCl-) indicates samples made using SnC1,.'diso values f0.03 mm s-', AEq values k0.06 mm s-', Fitted as a singlet.appearance similar to Fig. 8(a)] showed a single peak close to I zero velocity and similar to that attributed to tin(rv) ~xide.'~,'~ As in the related work of Berry et al. (on similar reactions with Laponite)," the peak was fitted as an unresolved doublet -60 -80 -1 00 -1 30 arising from a small quadrupole splitting (AE, ~0.5mm s-'). 6 These spectra would therefore be consistent with the presence of tin exclusively in finely dispersed SnO,.Fig. 7 Proton-decoupled high-resolution 29Si MAS NMR spectra of (a) T-Sn-X(SO,'-) and (b) T-Sn-X(C1-), both recorded at ambient temperature with spin rates of ca. 5 kHz and 42 rf pulses. Recorded T Series spectra are given as dotted lines (...) and fitted gaussians are shown For all the T-Sn-Mor samples [prepared from Na-mordenite by the solid lines.The differences between the observed and calculated and either tin@) chloride or tin(rr) sulfate] the spectra were spectra are given as solid lines below the spectra. as for the M series, arising from Sn exclusively in dispersed oxide. In the case of T-Sn-MorH(Cl-) [prepared from H-'I9Sn Mossbauer Spectroscopy mordenite and tin@) chloride] an additional doublet (AE,= 1.50mm s-') of low intensity (6%) was detected in the Spectral parameters resulting from computer-fitting of experi- spectrum [Fig. 8(b)]. The isomer shift (6,,=3.3 mm s-') for mental data are given in Table 6, with three typical spectra that doublet is consistent with residual tin@) species present illustrated in Fig. 8. as an impurity." The Mossbauer data for this component are different from either anhydrous (aiso=4.10, AEq=0.56 mm M Series s-l) or hydrated SnC1, (6,,,=3.68, AE,= 1.24 mm s-'), but For these samples (prepared by microwave-assisted hydrolysis closely resemble data for a frozen solution of SnCl, in of organotin chloride) the 'I9Sn Mossbauer spectra [with methanol (dis0 =3.36, AEq=1.76 mm s-'),~~suggesting that 100-h$ 96-v C .-0 .-2 92 6 2 aa -a4 1 -6 -2 2 6 -6 -2 2 velocity/mm s-' Fig.8 'I9Sn Mossbauer spectra of (a) T-Sn-Mor(SO4'-) and (b) T-Sn-MorH(C1-) and (c) T-Sn-X(C1-). Dashed lines indicate individual contributions to the observed spectra (see text), with the final fits being represented by solid lines. Spectral parameters are listed in Table 6.J. MATER. CHEM., 1994, VOL. 4 the Sn" species in T-Sn-MorH(C1- ) occupies an environment including several oxygen donors. The spectra of other T-series samples, made from tin(I1) sulfate or tin@) chloride and zeolite X, Y or A, also contained a peak at close to zero velocity. For those materials, satisfac- tory fits could not be obtained on the basis of a single unresolved doublet. The spectra were, however, satisfactorily simulated as the sum of two doublets, the first being the 'unresolved doublet' arising from Sn in dispersed oxide (AEq20.5 mm s-l) and the second (lower-intensity) doublet having a larger quadrupole splitting (AEqz 1.3 mm s-I). -1 -2 -3 0----_ 0 -1 -J ' 6.. Fig. 9 Temperature-dependent conductivities for (a) T-Sn-Mor and M-Sn-Mor samples: 0, T-Sn-Mor (Cl-); 0,T-Sn-Mor (SO:-); 0, T-Sn(H)-Mor; X, M-Sn-Mor; (b) T-Sn-A and M-Sn-A samples: '3, T-Sn-A (Cl-); 0,T-Sn-A (SO,'-); 0, M-Sn-A; (c) T-Sn-X and M-Sn-X samples: 0, T-Sn-X (Cl-); 0,T-Sn-X (SO:-); 0,M-Sn-X; and (d)T-Sn-Y and M-Sn-Y samples: 0,T-Sn-Y (Cl-); 0,T-Sn-Y 0, M-Sn-Y.Lines shown are the linear regressions and correspond to the activation energies given in Table 7. Fig. 8(c) shows the spectrum of T-Sn-X(C1-) [a sample made using tin@) chloride], along with the contributions of both doublets. In the case of T-Sn-Y(S02-) only, a third signal of very low intensity (2%) was attributable to residual tin@) impurity (6,,,=4.05 mm SKI).No attempt was made to identify this species, and the site was simulated by a single component in the fit (Table 6).For comparison, Mossbauer data for SnSO, are diso=4.00, AEq=1.OO mm s-.21 An unresolved question is the nature of the Sn environment giving rise to the second doublet in T-series samples prepared from zeolites X, Y and A. The small isomer shift (6,,, ~0.1mm sK1) is consistent with SnIV bonded to oxygen, but the quadrupole splitting AEq z 1.3 mm s-l corresponds to an environment considerably more distorted than the octahedral site in SnO,, a site which is itself not perfectly regular. One possibility is that hydrolysed Sn" ions introduced by ion exchange are associated with/bonded to the S6Rs (single six- rings) in zeolites X, Y and A (mordenite does not have S6Rs), with Sn coordinated both to the zeolite framework and to hydroxy/water groups with distinct SnIV -0 bond kngths.It must, however, also be borne in mind that the fr'imework disruption in these samples could well result in SnIV in regions of variable tin coordination geometry, such that the two- doublet fit merely mimics a variety of closely similar Sn" environments. ac Conductivity Studies Impedance spectra consisted of a high-frequency arc and a sloping rise of reactance with respect to resistance at lower frequency. Such spectra are typical of ionic conductors when blocking electrodes are used. The conductance of the sample was extracted as the intercept of the low-frequency line with the resistance (real) axis.The temperature dependences of the conductivities showed Arrhenius-like behaviour over the experimental temperature range (see Fig. 9). Empir tcal acti- vation energies E, (corresponding to linear regressior, fits) for protonic conduction are given in Table 7. E, values can only be regarded as empirical parameters, as there mill be a variation of the water content (and number of charge carriers) of the samples with temperature. All samples were, however, studied in the same environmental conditions, enabling facile comparisons. T Series The observed conductivities of the materials are in the range 3 x (at 293 K, for T-Sn-Mor made from SnSO,) to 3 x S cm-' (at 353 K, for T-Sn-A made from SnCl,), with significant enhancement with respect to the untreated Table 7 Activation energies for protonic conduction in 'tin zeolites' material EJkJ mol-' T-Sn-Mor(SO,'-) 70&4 T-Sn-Mor(C1-) 65k7 T-Sn-MorH (CI-) 72k3 T-Sn-A(SO,'-) 36f3 T-Sn-A(C1-) 54f3 T-Sn-X(SO4'-) 37k2 T-Sn-X(C1-) 14+2 T-Sn-Y(SO,'-) 14*4 T-Sn-Y (C1- ) 18f8 M-Sn-A 50+3 M-Sn-X 41&5 M-Sn-Y 12f 1 M-Sn-Mor 39k2 (C1-) indicates samples made using SnCI,, ( SO,'-) indicates samples made using SnSO,.parent zeolite^.'^^*^^-^^ Th e conductivities of the materials based on mordenite are lower than those reported by Knudsen et a!.' This is likely to be due to the lower relative humidity at which the measurements were made in this study. A lower water content in the sample will result in fewer charge carriers (protons) being available and hence a lower conductivity.Samples made using SnC1, exhibit higher conductivities than those in which SnSO, was used. These samples have more extensively damaged frameworks, but generally contain less tin. It is therefore unlikely that the enhancement in conductivity is due solely to the introduction of SnO, as had been suggested by Knudsen et al.' The increased conductivity of the T-Sn-Mor samples (with respect to the parent zeolites) can be attributed to the effects of damage to the zeolite framework. In T-Sn-MorH the damage to the zeolite is more significant and the conductivity is further enhanced. The T-Sn-Y materials exhibit conductivities of the same order of magnitude as those reported by Krogh Andersen et al.( lop2 S cm-' at 116 3C).3 M Series The conductivities of these samples are lower than those of the related T-series samples. The activation energies are also lower than those of the corresponding T-series samples, and Mossbauer spectra of these samples showed that tin is present only in SnO, (see earlier). The lower conductivities for the M-series samples are consequent on very little disruption of the zeolite frameworks and also on low tin contents. Conclusions All of the T-series samples exhibit degradation of the zeolite framework. This is evident from both XRD and MAS NMR studies. The extent of the damage depends upon the tin salt used in synthesis and is higher for those zeolites with higher A1 contents.More damage was incurred by a given zeolite when SnC1,-2H2O was used in the synthesis than when SnS0, was used. In their papers on 'tin mordenites' Knudsen et a[.'%, did not report any such degradation of the host zeolite. The synthesis of the M-series samples caused no apparent damage to the zeolite frameworks. This is expected since the amount of HCl generated during the hydrolysis of triphenyltin chloride will be small compared with that evolved on hydroly- sis of SnCl, (present when SnCl, is used in thermal syntheses). Evidence from Mossbauer spectroscopy indicates clearly that these materials are composites. All spectra contain a dominant contribution from a central peak (an unresolved doublet) arising from dispersed SnO,.In samples prepared thermally from zeolites X, Y and A, a second doublet attribu- table to a second, unidentified, SnIV site is observed. Comparison of the conductivities of corresponding mate- rials from the two series shows there is an increase in the observed conductivity when the zeolite framework has been disrupted. We thank Dr. D. J. Jones of 1'Universite Montpellier 2 for useful discussions on the fitting of Mossbauer spectra, and J. MATER. CHEM., 1994, VOL. 4 G. Edwards for assistance with synthetic aspects. We thank Laporte Inorganics for provision of the zeolite samples. We thank the SERC National Solid State NMR service (Durham) for recording NMR spectra and for subsequent deconvol- utions. We thank SERC for a studentship for G.B.H.We thank NATO for a travel grant enabling joint studies at Exeter and Montpellier. The authors thank the referees for constructive criticism of this paper. References 1 N. Knudsen, E. Krogh Andersen. 1. G. Krogh Andersen and E. Skou, Solid State Ionics, 1988,28-30, 627. 2 N. Knudsen, E. Krogh Andersen. 1. G. Krogh Andersen and E. Skou, Solid State Ionics, 1989,35, 51. 3 E. Krogh Andersen, I. G. Krogh Andersen, N Knudsen and E. Skou, Solid State Ionics, 1991,46, 89. 4 A. Ono, J. Mater. Sci., 1984, 19, 2691. 5 L. Glasser, Chem. Rev., 1975,7521. 6 W. A. England, M. G. Cross, A. Hamnett, P. J Wiseman and J. B. Goodenough, Solid State Ionics, 1980,1, 231 7 R. C. Ashcroft, S. P. Bond, M. S. Beevers, M. A M. Lawrence, A. Gelder, W.R. McWhinnie and F. J. Berry, Polyhedron, 1992. 11, 1001. 8 R. von Ballmoos, Collection of' Simulated XRD Powder Patterns for Zeolites, Butterworth, Guildford, 1984. 9 K. Ruenbauer and T. Birchall, Hyperfine Interactions, 1979,7, 125. 10 B. A. Boukamp, Solid State Ionics, 1986,18 & 19. 136; 1986,20, 31. 11 G. Engelardt and D. Michel, in High Resolution Solid State NMR of Silicates and Zeolites, John Wiley, New York, 1987, p. 150. 12 C. A. Fyfe, G. C. Gobbi, W. J. Murphy, R. S. Ozubuko and D. A. Slack, J. Am. Chem. Soc., 1982,86,3061. 13 M. Magi, E. Lippmaa, A. Samoson, M. Tarkamand and G. Engelhardt, J. Phys. Chem., 1984,88, 1518. 14 G. R. Hays, W. V. van Erp, N. C. M. Alma, P. A. Couperus, R. Huis and A. E. Wilson, Zeolites, 1984,4, 377. 15 G. Engelardt and D. Michel, in High Resolution Solid State NMR of Silicates and Zeolites, John Wiley, New York, 1987, pp. 256-257. 16 M. Magi, E. Lippmaa, A. Samoson, G. Engelhardt and A- R. Grimmer, J. Phys. Chem., 1984, SS, 1518. 17 J. M. Thomas, C. A. Fyfe, S. Ramdas, J. Klinowski and G. C. Gobbi, J. Am. Chem. SOC., 1982,86,3061. 18 N. N. Greenwood and T. C. Gibb, in Miisshauer Spectroscopji, Chapman and Hall, London, 1972, p. 374. 19 F. J. Berry and A. G. Maddock, Radiochim. Acta. 1977,24, 32. 20 F. J. Berry, R. C. Ashcroft, M. S. Beevers, S. P. Bond, A. Gelder and W. R. McWhinnie, Hyperfine Interactions, 1991,68,261. 21 J. N. R. Ruddick, Ret.. Silicon, Germanium, Tin and Lead Compounds, 1976,2,115. 22 D. N. Stamires, J. Chem. Phys., 1962,36, 3174. 23 M. Lal, C. M. Johnson and A. T. Howe, Solid Slate Ionics, 1981, 5,451. 24 E. Krogh Andersen, I. G. Krogh Andersen, K. E. Simonsen and E. Skou, in Solid State Protonic Condirctors II, ed. J. B. Goodenough, J. Jensen and M. Kleitz, Odense University Press, 1983, pp. 155-160. 25 S. Y. de-Andersen, E. Skou, I. G. Krogh Andersen and E. Krogh Andersen, in Solid State Protonic Conductors 111, ed. J. B. Goodenough, J. Jensen and A. Potier, Odense University Press, 1985, pp. 247-257. Paper 4/02972K; Received I 8th Muy, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401913

出版商:RSC    

年代:1994

数据来源: RSC