摘要:

Structural chemistry of SrMn, -,Fe,O, x z 0.3 Peter D. Battle,*" Courtenay M. Davison," Terence C. Gibb*' and Jaap F. Vente" ahorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QR 'School of Chemistry, University of Leeds, Leeds, UK LS2 9JT A polycrystalline sample of nominal composition SrMn,~,Fe,~,O, was prepared by standard ceramic techniques and investigated at room temperature by iodometry, 57Fe Mossbauer spectroscopy and neutron powder diffraction. The former two techniques established 6 =0.11kO.01. Neutron diffraction showed that the sample wasobiphasic. The major component [98.3(5)% by mass] was a 10H perovskite [space group P~~/wwzc; a= 5.45035(5), c =22.3735(2) A], with the Sr-0 layers adopting a cccchcccch stacking sequence. A full structural refinement showed the composition to be SrMn,~,2Feo~2802~8,. The minor phase was a pseudo-cubic perovskite.The Mossbauer data suggest that the oxygen vacancies in the major phase are predominantly found in the coordination shell of Fe3+ cations. The sample did not show long-range magnetic order at room temperature. The crystal structure of all perovskites ABX, can be considered to consist of pseudo-close packed layers of stoichiometry AX, with the smaller cation, B, occupying octahedral holes between the layers. Successive AX, sheets can stack in either cubic close packed (ccp) or hexagonal close packed (hcp) arrange- ments, and several mixed stacking sequences are possible.' Consequently, the coordination polyhedra around B can link together by sharing either a common face or a common vertex.Only the former type of linkage is present in BaMnO,, and only the latter in CaTiO,. In other compounds, for example 6H BaTiO,, both types of linkage occur in a periodic manner. Many factors, including the electronic structure of the cation B and the size ratio, rA:rB, play a role in determining the structure adopted by a particular compound.2 As a result of this, compounds which might be expected to be structurally similar, for example SrMnO, and SrFeO,, can adopt very different crystal structures. At room temperature, the former3 has a 4H perovskite structure in which Mn209 dimers are linked together by vertex sharing, whereas the latter4 is a cubic perovskite containing only vertex-sharing Fe06 groups.The large number of phase transitions observed as a function of temperature and pressure in many perovskite system^^.^ sug-gests that the stability difference between the different stacking sequences is often small. Furthermore, the sensitivity of the structure to cation size and electron configuration suggests that a compound AB1 -xBx'03 containing two different trans- ition-metal cations might adopt a structure which, although still based on the stacking of AO, layers, is different from that of both ABO, and AB'O,. The magnetic and electrical proper- ties of perovskite oxides are controlled in part by the structures they adopt and, in view of the current interest in the magneto- resistance of Mn-containing perov~kites,~ we have begun a study of the compounds SrMn, -xFe,O,.We describe below a structural study, carried out using neutron diffraction and Mossbauer spectroscopy, of a composition close to SrMn,.,Fe, 303-6. The way in which we have chosen to write the formula of this compound hints at another important parameter in the structural chemistry of perovskites, i.e. the vacancy concentration in the anion sublattice. Experimental A black polycrystalline sample of overall composition SrMn, 7Fe0.303-6 was prepared by firing a well ground, pel- letized, stoichiometric mixture of SrCO,, MnO, and Fe,O, (Johnson Matthey 'Specpure' reagents) in a platinum cruc- ible at 1250 "C for 72 h in air, then cooling it at a rate of 100°C h-' to 500°C before removing it from the furnace.As the formation of the final product took place slowly, this heating cycle was repeated eight times, with intermediate regrinding and pelletizing, to ensure that the composition was homogeneous, X-Ray diffraction data collected in Bragg-Brentano geometry using Cu-Ka radiation were used to assess the purity of the product, and the oxygen content was deter- mined by iodometric titration. Neutron powder diffraction data were collected at room temperature on the diffractometer Dla at the Institut Laue Langevin, Grenoble. The sample was contained in a 16 mm diameter vanadium can whilst the bank of ten detectors was scanned through the angular range 6<28/<160 degrees an steps of 0.05'. The mean neutron wavelength was 1.956 A. Mossbauer data were collected in the temperature range 78 <T/K <290 using a 57Co/Rh source matrix held at room temperature; isomer shifts were determined relative to the spectrum of metallic iron.Results The initial characterisation of our sample by X-ray diffraction and chemical analysis suggested that it was a monophasic hexagonal material of composition SrMn,~,Feo~,02,90. The unit-cell dimensions were characteristic of a 10H perovskite structure. There are a number of possible stacking sequences' for close-packed SrO, layers which result in a 10H structure, and we analysed the observed neutron diffraction in order to determine which sequence is present in this particu- lar compound. The following scattering lengths were used: b,, =0.702, b,, = -0.373, b,, =0.954 and bo = 0.5805 x m.Preliminary refinements suggested that SrMn,~,Fe,,,O, is isostructural with Ba5W3Li,0151' and the data were consequently analysed using the space group P63/mmc, with three crystallographically distinct sites for each of Sr, Mn/Fe, and 0 (Fig. 1). The SrO, sublattice consists of layers stacked in a mixed cubic (c) and hexagonal (h)sequence which can be described as cccchcccch.' The resulting crystal structure can be seen to contain blocks of pseudo-cubic perovskite, three octahedra wide, separated by B209 dimers (B=Mn/Fe). During the early stages of refinement it became clear that the face-sharing octahedra, which form the dimers, are occupied only by Mn whereas the vertex-sharing octahedra in the perovskite blocks are occupied by a disordered distri- bution of Mn and Fe.However, our initial refinements, in which the Mn:Fe ratio was constrained to be 7:3, were unsatisfactory and we therefore allowed the composition of J. Muter. Chem., 1996, 6(7), 1187-1190 1187 0 0 0 0 0 0 0 0 1OH SrMno~,FeO,,O,, Fig. 1 10H crystal structure of SrMn, 72Feo 280287. Open circles represent Sr atoms. the hexagonal phase to vary. There was a marked improvement in the agreement indices as the phase became Mn rich. The difference between the observed and calculated diffrac- tion profiles at this stage consisted mainly of positive intensities with a 28 distribution which suggested that they were due to the presepce of a cubic second phase with a unit-cell parameter ~~3.85A.We hypothesised that this minority phase [ca. 1.7(5)%], which had not been detected during our preliminary X-ray study, might be near-stoichiometric SrFeO, consist-ent with the Fe deficiency in the hexagonal phase. All sub- sequent analysis of the neutron diffraction data was carried out by treating the sample as a two-phase mixture. The peak shape was constrained to be the same for both phases. In these final refinements we were able to establish that the oxygen vacancies in the primary phase occur only on the O(3)site in the centre of the blocks of pseudo-cubic perovskite (Fig. 1). The group of profile parameters being varied at this stage comprised two scale factors, a counter zeropoint, four peak- shape parameters (pseudo-Voigt function), five background parameters (shifted Chebyshev function), two unit-cell param- eters for the hexagonal phase and one for the cubic phase, and a preferred orientation correction for the hexagonal phase.The other parameters (all associated with the hexagonal phase) involved in the refinement were the nine variable atomic coordinates, the occupancy factor of O(3),the overall isotropic temperature factor, and the Mn:Fe ratios on the sites Mn/Fe( 1) and Mn/Fe(2). The latter sites were constrained to be fully occupied but no constraint was imposed on the Mn :Fe ratios, The second phase was finally treated as stoichiometric SrFeO, because the low concentration gave rise to very weak Bragg peaks which could not be used sensibly in a detailed structure refinement.In reality the phase is likely to be oxygen deficient, non-cubic and, perhaps, to contain some Mn. It could be postulated that it is the residue of a cubic 'high- temperature' phase with the same overall composition as the principal component, but this seems unlikely because the observed X-ray pattern improved substantially with successive anneals and the ultimate crystallinity of the hexagonal phase was excellent. Although we are unable to draw any meaningful conclusions about the second phase from these data, its inclusion in our analysis, albeit with a crude structural model, smoothed the refinement of the structure of the primary phase.The resulting agreement factors were as follows: R, =6.91YO, R, =5.19%, R,( 10H)=7.40%, x,d2 =1.004, DW -d =0.31 (lower limit of 90% confidence level=1.9013). The final observed and calculated diffraction profiles are shown in Fig. 2. The structural parameters of the 10H phase, the composition of which refined to be SrMn0.72Fe0.2802.87, are listed in Table 1 and the resulting bond lengths and bond angles are given in Table 2. The 57Fe Mossbauer spectrum of SrMno~7Feo~,0, observed-at room temperature is shown in Fig. 3, along with the spectrum calculated from a four-doublet fit. This fit is to some extent arbitrary, as the disordered nature of the crystal struc- ture will cause broadening of the resonances, and indeed the symmetrical Lorentzian line profile used here may be inappro- priate.However, a minimum of four doublets is required to represent the subtlety of the absorption profile, and the com- puted linewidths of all four components are similar. The parameters resulting from the fit are listed in Table 3. They are quoted to only two decimal places as they are undoubtedly average values representing a range of environments. The values of the isomer shifts are consistent with three components (1-3) from Fe3+ cations, while component 4has a substantially lower isomer shift and can be assigned to Fe4+ ions. The 1.7% by mass of the second phase will contribute <6% to the area of the spectrum, and with manganese substitution this value will decrease.There is no possibility of identifying this weak component separately. The asymmetry in the intensity of the spectrum (assuming that the recoilless fraction is the same at all sites) leads to an estimate of 24% of Fe4+ which is effectively independent of the other features of the model. If it is assumed that all the Mn is tetravalent, then this equates with an overall stoichiometry of SrMno.7Feo.302.89, in good agreement with that determined iodometrically. It also establishes that there must be ca. 20% of Fe4+ in the primary phase, although the precision of this estimate is limited by the low precision associated with our determination of the concentration of the second phase. In order to reconcile this Fe4+ concentration 20 40 60 80 100 120 140 160 2eldegrees Fig.2 Observed, calculated and difference neutron powder diffraction 87profiles for SrMn, 72Fe0 2802 at room temperature. Reflection positions are marked for both the primary and the secondary phases. Table 1 Crystallographic data for 10H SrMn, 72Fe0.2803-6u atom site X Y Z occupancy Sr(1) 2(d) 113 213 314 1 113 0.0503(2) 1Sr(2) 4~) 213 0.1544(1) 1Sr( 3) 4(e) 0 0 Mn/Fe(l) 2(a) 0 0 0 0.558/0.442( 5) Mn/Fe(2) 4(f) 113 213 0.6040(4) 0.531/0.469( 3) Mn(3) 4(f) 113 213 0.1936(3) 1 O(1) 6(h) 0.1801(3) 0.3602(7) 114 1 O(2) 12(k) 0.4985(2) -.0.0030( 5) 0.14816(8) 1 O(3) 12(k) 0.8323(3) 0.6647( 1) 0.0492( 1) 0.890(4) aspace group P6,lmmc. a=5.45035(5) A, c=22.3735(2) A, I/= 575.59( 1) A3.U,,,=0.0063(3) A'.mass frac: 98.3(5)%. 1188 J. Muter. Chem., 1996, 6(7), 1187-1190 Table 2 Bond lengths/A and selected bond angles/degrees in SrMno,72Feo~2802,87 Sr(1)-O( 1) Sr(1)-0(2) Sr(2)-O( 2) Sr(2)-O(3) Mn/Fe( 1)-0(3) Mn/Fe(2)-O(2) Mn/Fe(2)-0(3) Mn(3)-O( 1) Mn(3)-O( 2) Mn( 3)-Mn( 3) 2.728(4) (6 x) 2.777(2) (6 x) 2.688(3) (3 x) 2.722(3) (3 x) 1.928(3) (6 x) 1.871 (5) (3 x) 1.985(6) (3 x) 1.919(5) (3 x) 1.862(4) (3 x) 2.52( 1) Sr(2)-0(3) Sr(3)-O( 1) Sr(3)-O( 2) Sr(3)-O( 3) O(3)-Mn/Fe( 1)-O( 3) 0(2)-Mn/Fe (2)-0(2) 0(2)-Mn/Fe (2)-0(3) O(3)-Mn/Fe(2)-O(3) O( l)-Mn(3)-0( 1) O(l)-Mn(3)-0(2) 0(2)-Mn(3)-0(2) 2.725(3) (6 x) 2.733(4) (3 x) 2.729(3) (6 x) 2.835(4) (3 x) 90.6( 1) 94.6( 4) 89.53(8) 86.0( 3) 81.5(2) 92.49( 6) 92.9(2) Ba5W3Li201515were reported many years ago.The latter is isostructural with SrMn0.72Fe0.2802,87 whereas the former has a structure wherein Mn,O, dimers share vertices with trimers (Mn30,,) of face-sharing octahedra, thus leading to an hchchhchch stacking of BaO, layers, a sequence with a much greater proportion (60%) of hexagonal stacking than we have observed in SrMno,72Feo~2802~87. Cs4NiCdF,, has been shown’ to have a third 10H sequence, chhhcchhhc, which also contains 60% hexagonal stacking. The structure of SrMno.72Feo.2802.87 thus contains a relatively large component of cubic perovskite, a property which can be attributed to the relatively small size of the A-site cation.2 This can be seen in Fig.1, where it is possible to recognise not only segments of the SrMnO, struc- ture, i.e. Mn209 dimers sharing vertices with neighbouring octahedra, but also blocks of pseudo-cubic perovskite remi- niscent of SrFeO,. SrMn0,72Fe0.2802.87 is thus a compound of the type AB,-,B,’O,-, which adopts the structure of neither AB03-6 nor AB’03-6, but an intermediate structure contain- ing an intergrowth of elements of both end members. The occurrence of only Mn in the face-sharing octahedra is consist- ent with this descrip$on, The average Mn-0 bond distance in the +mers (1.891 A) is very similar to that found previysly (1.889 A) in SrMnO,,, as is the Mn-Mn distance (2.52 A, cf 2.50A). This supports the assumption that the Mn in the dimers is tetravalent, as does the value of 4.15 calculated for the bond valence of Mn(3).l6>l7 The location of the anion vacancies only on the site 0(3), i.e.a site in the centre of the Fe-rich region of the structure, is also consistent with the structural chemistry of SrFeO, -64 and SrMnO, . Throughout the above discussion we have described our sample using the chemical composition determined in the diffraction study. We believe that the large contrast between the scattering lengths of Mn and Fe makes neutron scattering a particularly sensitive analytical tool in this case. In addition to making it feasible to refine the cation stoichiometry, neutron diffraction was also the only technique which detected the presence of a pseudo-cubic impurity at 1.7% by mass.The disorder caused by the presence of anion vacancies and a near- random distribution of Mn and Fe over the vertex-sharing octahedral sites reduced the high resolution normally associ- ated with 57Fe Mossbauer spectroscopy, although it was still possible to deduce an accurate ratio for the Fe oxidation states. The Mossbauer spectra also show that the sample is paramag- netic at room temperature, an observation which is consistent with the absence of magnetic Bragg peaks in the neutron diffraction pattern. The isomer shifts and relative areas of the four doublets used to fit the spectra are consistent with the presence of Fe3+ and Fe4+ as distinct oxidation states in this compound; there is no evidence for the presence of an averaged or mixed valence state.The results described above suggest that the system SrMn, -xFe,03 -could be an abundant source of interesting structural chemistry. We are currently investigating the tem- perature dependence of both the structural and the electronic properties of these compounds as a function of composition, J. Muter. Chem., 1996,6(7), 1187-1190 1189 4-3-2-101 2 3 veiocity/mm s-l Fig. 3 Observed and fitted Mossbauer spectra of SrMno,7Feo,30,-, at room temperature. The maximum absorption is 3%. Table 3 Mossbauer parameters for SrMno,,Feo.,03-d at room temperature component 6/mm s-l d/mm s-l r/mm s-l area (%) 1 0.37 0.28 0.34 32 2 0.31 0.79 0.30 33 3 0.26 1.21 0.27 11 4 -0.06 0.29 0.27 24 with the stoichiometry (cu.7% Fe4+) determined by neutron diffraction, we are led to the conclusion that some reduction to Mn3+ occurs, presumably in the vicinity of the vacant anion sites. Alternatively, the precision associated with the occupancy factor of the O(3) site (Table 3) might have been overestimated. Assuming random placement of vacancies on O(3) sites and an occupancy of 0.89, then 63% of the Mn/Fe( 1)and Mn/Fe(2) sites have six 0 neighbours, and 37% have one or more vacancies. If the vacancies on O(3) are not random but are preferentially associated with a pair of Fe3+ cations, which is consistent with charge considerations, then a greater fraction of the iron will have a nearby vacancy. It is possible that components 2 and 3 in Table 3, which show a significant quadrupole splitting and represent 44% of the Fe, are associ- ated with a vacancy, while components 1 and 4 are fully coordinated to oxygen and show only a small quadrupole splitting. Thus there is some evidence in favour of a non-random distribution of anion vacancies, but this should not be regarded as proven.Discussion The structural chemistry of SrMno.72Feo.2802.87 shows a number of interesting features. The very adoption of the 10H structure is quite unusual. It does not occur in the BaMn, -xFex03-system,14 although 10H BaMnO, -x5 and and we hope to publish the results of these studies in the 8 J M Dance, J Darnet, A Tressaud and P Hagenmuller, Z Anorg Allg Chem ,1984,508,93near future 9 H M Rietveld, J Appl Crystallogr ,1969,2, 65 We are grateful to EPSRC for financial support 10 A C Larson and R B von-Dreele, General Structure Analysis System (GSAS), Los Alamos National Laboratones, Report References A L Patterson and J S Kasper, In International Tables for X-ray Crystallography, Kynoch Press, Birmingham, 1959, vol I1 J B Goodenough and J A Kafalas, J Solid State Chem, 1973, 6,493 P D Battle, T C Gibb and C W Jones, J Solid State Chem, 1988,74,60 J B MacChesney, R C Sherwood and J F Potter, J Chem Phys, 1965,43,1907 T Negas and R S Roth, J Solid State Chem ,1971,3,323 B L Chamberland, A W Sleight and J F Weiher, J Solid State Chem , 1970,1,506 M R Lees, J Barratt, G Balakrishnan and D M Paul, Phys Rev B, 1995,52, R14303 LAUR 86-748,1990 11 A J Jacobson, B M Collins and B E F Fender, Acta Crystallogr Sect B, 1974,30,816 12 B C Tofield, C Greaves and B E F Fender, Muter Res Bull, 1975,10,737 13 R J Hill and H D Flack, J Appl Crystallogr, 1987,20,356 14 V Caignaert, M Hervieu, B Domenges, N Nguyen, J Pannetier and B Raveau, J Solid State Chem ,1988,73,107 15 T Negas, R S Roth, H S Parker and W S Brower, J Solid State Chem, 1973,8,1 16 N E Brese and M O’Keeffe, Acta Crystallogr Sect B, 1991, 47,192 17 I D Brown and D Altermatt, Acta Crystallogr Sect B, 1985, 41,244 Paper 6/01020B, Received 12th February, 1996 1190 J Muter Chem, 1996, 6(7), 1187-1190

ISSN:0959-9428    

DOI:10.1039/JM9960601187

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Powder neutron diffraction study of LiMnV04 ~~~~ ~ ~ M. Sato,' S. Kano,' S. Tamaki,b M. Misawa,c Y. Shirakawad and M. Ohashi" 'Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, Niigata 950-21, Japan bDepartmentof Physics, Faculty of Science, Niigata University, Niigata 950-21, Japan 'Department of Chemistry, Faculty of Science, Niigata University, Niigata 950-21, Japan dDepartmentof Materials Science and Processing, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan "Institutefor Material Research, Tohoku University, Sendai 980-77, Japan Powder neutron diffraction data have been recorded for LiMnVO,. The crystal structure was determined by Rietveld analysis. The Rietveld refinement confirmed that the compound adopts an orthorhombic spinel-related structure.The framework structure of the compound is comprised of nearly regular, edge-sharing octahedra of MnO,. The lithium and vanadium atoms in LiMnVO, occupy, as partially mixed species, the two kinds of tetrahedral interstitial sites constructed by cubic close-packed oxygen atom arrays; this structural feature is quite similar to those of CrV0, and LiCuVO,. The magnetic susceptibility and the metal-oxygen bond distances estimated from the refined structure of LiMnVO, clearly indicate that the valence states for the transition-metal ions are divalent for manganese ions and pentavalent for vanadium ions. The lithium oxide spinels, Li,Mn,O, (O<x<2), have been widely investigated for cathode materials in rechargeable lith- ium batteries because of their high cell voltage, long shelf life and wide operating-temperature range.' In a spinel of general formula A[B,]X, with prototypic symmetry Fd?rn, the struc- ture is characterized by the [B2]X4 framework, of which the anion arrays are stacked with cubic close packing (ccp)., This framework, which provides a three-dimensional interstit-ial space for lithium ion transport, remains intact during the lithiation Vanadium oxides, e.g.V2OS7 and LiV308,* are also attractive cathode materials for lithium batteries because of the high valence state of the vanadium ions. Their crystal structures are not spinel-type, but rather a sort of bronze-type structure in which where a nearly two- dimensional framework responsible for lithium insertion is realized; although a spinel-type compound, LiV,O,, exists it shows a rather poor performance as a cathode material, i.e.low operating cell voltages of around 2.5 V us. Li+/Li., Therefore, if suitable syntheses can be determined, oxide sys- tems consisting of manganese and vanadium ions in a rigid framework may be interesting candidates for cathode materials in lithium batteries. While making a survey of the pseudo-binary systems of LiMn20, and LiV204, we reported previously a new com- pound formulated as LiMnVO, with a spinel-related ~tructure.~ The previous powder X-ray diffraction measurements on LiMnVO, have shown it to be orthorhombic and a spinel- related structure has been assumed.However, X-ray diffraction is very insensitive to lithium positions and the scattering is dominated by the manganese and vanadium. As a result, it is possible that not only uncertainty over the lithium positions but also relatively large distortions of VO, tetrahedra and MnO, octahedra could occur, which would still be accommo- dated by the X-ray diffraction. The sensitivity of neutrons to lithium and oxygen positions makes neutron scattering an ideal tool for the observation of such ambiguities. In this work, we report the results of a neutron diffraction study on LiMnVO,, and discuss its structural features in comparison with those of CrVO," and LiCuVO,," which possess spinel-related structures. Experimental LiMnVO, was prepared by a conventional solid-state reaction of reagent-grade Li,C03, Mn(CH3COO),.4H20 and V,O, powders. Stoichiometric amounts of the powders were ground thoroughly, pelletized under a pressure of 30 MPa, and fired in an alumina crucible at 500-750 "C for 24 h in air with several intermittent grindings.The phase purity of the product was confirmed by the powder X-ray diffraction method. The powder X-ray diffraction analysis for the LiMnVO, sample showed a trace amount of a cubic spinel phase remaining in the product, in addition to a new LiMnVO, compound. The composition analysis for the product was carried out by a flame spectroanalyser (Hitachi 180-50) for the lithium content and by a induced coupled plasma spectroanalyser (Seiko SPSl5OOV) for the other compo- nents, except for oxygen.Powder neutron diffraction patterns for the Rietveld analysis were recorded using the KPD diffractometer (Tl-3) installed at JRR-3M Guide Hall in the Japan Atomic Energy Researct Institute (JAERI). An incident neutron wavelength A= 1.767A was obtained from the (311) reflection of five germanium single crystals. The powder sample (of size 3 cm3) was enclosed in the cylindrical vanadium vessel and mounted on a double-axis diffractometer. The data were collected on thoroughly ground powders by a multi-counter-scanning mode in the 28 range 10-121.90" with a step width of 0.10" and a monitoring time of 260s at room temperature. The powder pattern obtained was analysed by the Rietveld method using the RIETAN94" profile refinement program.? The magnetic susceptibility was measured using the Faraday method using a home-made computerized magnetometer which was calibrated with a sample of high purity Mohr's salt [Fe( NH,)2(S04)2.6H,0].Magnetic susceptibility data were collected at an appropriate applied magnetic field over the temperature range 77-773 K. Results and Discussion Data analysis was carried out on the room-temperature data set. As previously reported, other than the weak impurity peaks observed by X-ray diffraction, no additional reflections were observed, indicating that the unit cell of LiMnVO, is an t Structural refinement data as output from the RIETAN94 program are available as supplementary data (SUP 57140) from the British Library.For details of the Supplementary Publications Scheme, see Information for Authors, Issue 1. J. Muter. Chem., 1996, 6(7), 1191-1194 1191 orthorhombic system with space group Cmcm aFd approximate cell parameters a =5 75, b =8 70 and c =6 35 A The impurity peaks observed at 28=20 70 and 42 00" were found to corre- spond to a cubic spinel phase Considering t$e cell constants for the spiFel compounds LiMn,O, (a=8 247 A)13 and L1V204 (a=8 241 A),', these peaks may be attributable to the (1 1 1) and (3 I 1) reflections, respectively, of the cubic spinel compounds, probably forming a solid solution such as L1(Mn,V)204The following scattering lengths were used l5 Li, -1 900, Mn, -3 730, V, -0 382, 0,5 803 fm An initial site assignment was referred to the results obtained by the previous X-ray diffraction study,g I e ,4c sites for Li, 4a sites for Mn, 4c sites for V, 8f site for 0(1) and 8d site for 0(2), respectively, in Cmcm space group Since a small amount of the impurity spinel phase remained in the sample, the recorded pattern was analysed while assuming a two-phase mixture In the two- phase refinement mode of RIETAN94, a matrix refinement of the LiMnVO, structure, with the variables of scale factor, lattice parameters, fractional coordinates and individual iso- tropic thermal parameters, was undertaken, while the refine- ment of the impurity spinel phase was made using the same variables as those for LiMnVO, except for an overall isotropic thermal parameter, where the site assignment was based on the cubic space group Fd%, 8a sites for Li, 16d sites for the mixed species of Mn and V with an equimolar ratio, and 32e sites for 0 The R factor of the weighted pattern fitting, R,,, was reasonably reduced to ca 660% However, the isotropic thermal parameter for tht lithium site was converged to a relatively large value, 2 4 A2, and that for th? vanadium site was converged to a large negative value, -2 8 A2 These results apparently indicate the possibility of partial disorder of lithium and vanadium in the two 4c sites Therefore, the following refinement was performed on this assumption, resulting in a improved convergence with R,, =5 74% and normal isotropic thermal parameters for the two 4c sites The lithium and vanadium atoms are distributed on the two 4c sites with mutual displacement ratios of 14% The proportion by mass of the impurity phase in LiMnVO, was determined on the basis of the scale factors finally obtained for LiMnVO, and the impurity From the analysis proposed by Hill and Howard,16 the proportion of the impurity phase was found to be 252 mass% At present, it is unknown whether a mixture of LiMn20, and LiV204 or a solid solution of Li(Mn,V),O, is the true composition of the impurity spinel phase, because of the very similar lattice parameters and of the fairly low concentration of the impurity in the product In Table 1 the crystallographic data together with the data collection conditions are given The positional parameters finally obtained are listed in Table 2 Table 3 shows the bond distances and angles for the compound Fig 1shows the results of the pattern fitting of LiMnVO,, where the reflection peaks corresponding to the impurity spinel phase in LiMnVO, are shown as the lower series of bars The crystal structure, determined using the crystallographic parameters refined for LiMnVO,, is illustrated in Fig 2 The structure is quite similar to that of CrV0410 which has the same space group and the same atomic locations except for lithium, and it also rather resembles that of LICUVO,,~~ these structures are also shown Although LiCuVO, crystallizes in an orthorhombic system with space group Imma, which is different from that of both LiMnV0, and CrVO,, these three materials have several common structural features In LiMnVO, and CrVO,, almost regular, edge-sharing M0, (M =Mn, Cr) octahedra run along the c axis There are two kinds of M-0 bond distances in the ,MO, octahedroq, i e one is the short M-O( 1) bond (2 144 A for Mn, 1?57 A for Cr) an! the other is the long M-0(2) bond (2 183 A for Mn, 2 039 A for Cr), the average lengths of the M-0 bonds are 1192 J Muter Chem , 1996, 6(7), 1191-1194 Table 1 Neutron data collection and analysis of LiMnVO, neutron data collection wavelength/,& 1767 28 rangeldegrees 10-121 90 step scan increment (28) 0 10 count time/s step 260 contnbuting reflections 124 total number of observations 1120 sample can diameter 10mm, length 40 mm, vanadium can model refinement spGce group Cmcm a14 5 7465( 2) blh 8 7057( 5) CIA 6 3425( 3) volumejA3 317 3 Z 4 calculated density/g cm 3 701 scattering lengthlfm Li -1 900, Mn -3 730 V -0 382, 0 5 803 program RIETAN94 function minimized M=C[w(Y,-ly],w=l/Y, no of parameters refined 36 background 8-term polynomial peak shape pseudo-Voigt function reliable factors' (%) R,, 5 74 RP 4 45 RE 4 21 R, 4 49' RF 3 08' 'Defined in ref 12 'Those for the impurity spinel phase are R,= 3 71% and RF, 3 06% The unit-cell parameter of the impunty phase is a=8224(2)A Table 2 Positional parameters for LiMnV04 atom site f xla Ylb z/c B/A2 Li(1) V(l) Mn V(2) Li(2) 0(1) 4c 4c 4a 4c 4c 8f 086(7) 014 10 086 014 10 00 00 00 00 00 00 -0336(3) -0 336 00 0361(8) 0 361 0245(1) 025 0 25 00 025 0 25 00334(9) 12(8)12 1 O(3) 0 3(2) 03 lO(2) O(2) 8g 10 0259(1) -00212(7) 025 0 8(2) 2 170 A for Mn and 2 012 A for Cr, which are almost equal to the distances estimated from the effective ionic radii17 of Mn2+ (VI fold), Cr3+ (VI fold) and 02-(I1fold) The oxygen atoms in LiMnVO, and CrV0, form cubic close packed (ccp) arrays running in the direction parallel to the (1 1 0) plane A pair of edge-sharing tetrahedra are formed in the interstitial space constructed by the ccp arrays of oxygen atoms In LiMnVO,, lithium and vanadium atoms are located in equivalent positions with a displacement ratio of 14% in these tetrahedra, while one of the corresponding two types of tetrahedra is missing in the case of CrVO, The CuO, octahedron in LiCuVO, is quite distorted toward a square-planar coordination owing to the Jahn-Teller Cu2+ ion In spite of such a distortion, the coor- dination environment around the vanadium atoms is very similar to those in LiMnVO, and CrV0, In addition, all the oxygen atoms in LiCuVO, also form ccp arrays However, the lithium atoms in LiCuVO, occupy octahedral sites but not tetrahedral ones, as in the case of LiMnVO, This is probably because the Jahn-Teller distortion results in more space being available to the octahedral site than in the case without the distortion of the CuO, octahedron In view of the cation distribution in interstitial sites constructed by ccp arrays of oxygen atoms, the following formulae can be suggested (A,),,, CBlO,,O, for LiMnVO,, Wtet CBlO,tO, for CrVO, and Table 3 Bond distances (in A)and angles (in degrees) for LiMnVO," distances M( l~---O(l~ 1.964( 14) x 2 M( 1)' -O(2) 2.124(27) x 2 Mn-O(1) 2.144(9) x 2 Mn-0(2) 2.183(5) x 4 M(2)-0(1)...1.704(46)x 2 M (2)- O(2!,, 1.722(45)x 2 O(1)-O( 1y 2.747( 11) 0(1)-0(2) 3.079( 10) 0(2)-0(2)' 3.192( 1) angles O(ly-M( l,)-O( 1); 132.94( 18) O(2)-M( 1)' YO(2)"" 81.3( 12) O( 1)" -M( 1)'-0(2) 107.8(5) O(1)-Mn-O( 1)' 180.0 O(1)-Mn-O(2) 90.7(2) O( 1)-Mn-O(2)' 89.2(2) O(2)- Mn- O(2)' 93.9( 3) O(2)- Mn- O(2)~' 86.0( 3) 0(2)- Mn -0(2)"' 180.0 O(1)-M(2)-0( l)k 107.4( 41) 0(2)g -M (2)-O(2)" 106.9(40) O(1)-M( 2)-O( 2)"' 110.6( 2) symmetry codes: (none)x,y,z; ' 1/2 +x,1/2 +y, -2; ii 1/2+x,1/2 -y, -z; iii 1/2-x,1/2,+y,z; iv x,y,1/2-z; x,-y,-z; vI 1/2+x,1/2-y,1/2+2; Vii 1-x,y,z; vlll -x,y,z; ix x,y,1/2 -z; -1/2+x,1/2 +y,z; xi -x, -y, -z.a M( 1) and M(2) represent mixed species for the Li-rich 4c site and the V-rich 4c site, respectively. -50001 I I a0 -c. I I I I I I I# I I I1 I I 111 I I 11111 I1111111111 1111111111111 1111 I1 11111 1111 111111 1111111 I 1 I1 1 1 1 1 Ill Ill I1 I1 I IIILIIII~II Fig. 1 Powder neutron diffraction pattern fitting for LiMnVO,. The calculated and observed intensities are shown as solid line and dots, respectively. The upper and lower bars are the diffraction positions for LiMnVO, and the impurity phase, respectively.The trace is a plot of the difference between calculated and observed intensities. Fig. 2 Structural models of (a)LiMnVO,, (b)CrVO, and (c) LiCuVO,. The positional parameters are taken from ref. 10 for CrVO, and from ref. 11 for LiCuVO,. In LiMnVO,, the large tetrahedra are those of the Li-rich 4c species [M( l)], the small tetrahedra are those of the V-rich 4c species [M(2)], and the octahedra are those of the Mn 4a species. In CrVO, and LiCuVO,, the tetrahedra and octahedra are those of V04 and MOs (M =Cr, Cu and Li), respectively. (A)tet[B2IoctO4for LiCuVO, with a normal spinel distribution, where A is a tetrahedral cation and B an octahedral cation. The similarities and differences between LiMnVO, and the spinel structure are as follows: (1) the oxygen atoms in LiMnVO, form ccp arrays like the spinel structure, but they are slightly distorted; (2) the manganese atoms reside in half of the B sites of the spinel structure but in an ordered arrangement; and (3) one half of the A sites which are occupied by lithium and vanadium correspond to the A sites of the spinel structure.Fig. 3 shows the environment around the vanadium atoms for three compounds. The Li-rich and V-rich tetrahedra in LiMnVO, are linked together by sharing two O(2) atoms. The Li-rich [denoted as M(l) in Fig. 33 tetrahedron is fairly distorted from an ideal tetrahedron, where the position of the central atom is shifted toward the opposite side of the V-rich [denoted as M(2) in Fig. 31 tetrahedron.Such distortion seems to be due to a coulombic repulsion between the two cations since the Li-rich tetrahedron has a less positive charge than the V-rich tetrahedron. LiMnV0, showed typical paramagnetic behaviour in the temperature range 77-773 K as shown in Fig. 4.The extrapo- lated Curie temperature is -84K, implying an antiferromag- netic interaction between magnetic ions. The effective mag- netic moment calculated from the x-' us. l/T plot is 5.62 pB. Three combinations of possible valence states of manga-nese and vanadium ions in LiMnVO, can be considered, i.e. Mn2+-V5+, Mn3+-V4+ and Mn4+-V3+. The theoretical effective magnetic moments, calculated by taking into account Fig. 3 Environment around lithium and vanadium atoms in (a) LiMnVO,, (b) CrV04 and (c) LiCuVO,.M(l) and M(2) in LiMnV04 are the Li-rich and V-rich atomic species, respectively. The atomic distances in CrVO, and LiCuVO, are taken from ref. 10 and ref. 11, respectively. (Symmetry code is given in Table 3.) 250--200 F5 150-0 I 200 400 600 800 TIK Fig. 4 Temperature dependence of the inverse molar magnetic suscepti- bility of LiMnVO, J. Mater. Chem., 1996, 6(7), 1191-1194 1193 Table 4 Bond-valence sums for Mn ions in LiMnVO, 1 atom J bond-valence parameter' RV bond-valence sum K Mn2+ O2 1790 2 15 Mn3+ O2 1760 198 Mn4+ O2 1753 1 94 'The values are taken from ref 18 the spins of d electrons only for each magnetic transition ion, are 5 91, 5 19, and 4 79 pB for Mn2+-V5+, Mn3+-V4+ a nd Mn4+-V3+ combinations, respectively The observed value is in good agreement with that for the Mn2+-V5+ combination The concept of bond valence provides another powerful method for the prediction of valences of metal ions in crystals According to Brown and Altermatt,18 the valence vZJof a bond between two atoms z andj can be defined so that the sum of all the valences from a given atom z with valence K, obeys K=c'CJ=cexpC(RtJ-dd,J)/blY J J where b is taken to be a universal constant equal to 0 37 A,dtJ is the observed distance and R,, the bond-valence parameter Table 4 shows the calculated bond-valence sums for Mn atoms in LiMnVO, All the bond-valence sums calculated are nearly equal to 2 no matter which bond-valence parameters are used These results are also consistent with the valence combination deduced from the magnetic susceptibility data The electrical conductivity measurement for LiMnVO, showed typical semi- conducting behaviour with r~ =1x S cm-'at 300 K From the preliminary measurement of thermoelectric power at room temperature, the main charge carriers were found to be holes These facts indicate that the unpaired electrons of Mn2+ ions in LiMnV0, are localized in 3d orbitals LiMnVO, does not show any lithium intercalation or deintercalation by chemical or electrochemical methods, although it contains manganese and vanadium components which are usually effective for lithium intercalation For the manganese component, this may be because the manganese ions in LiMnV0, are not in a high valence state Moreover, in spite of the vanadium ions possessing a high valence state, it is likely that vanadium ions in a rigd tetrahedral coordi- nation cannot take part in lithium intercalation In fact, the V5+ ions in LiCuVO, are not responsible for the chemical lithium intercalation, which is performed via the reduction of only Cu2+ ions l9 Conclusions The crystal structure of LiMnVO, was reexamined by Rietveld analysis on the basis of powder neutron diffraction data The results obtained are summarized as follows (1) The structure is closely analogous to those of CrVO, and LiCuVO, The atomic arrangement is related to that of a spinel structure The manganese atoms occupy the nearly regular octahedral sites which are constructed by cubic close-packed arrays of oxygen atoms, while the lithium and vanadium atoms are located in a pair of edge-sharing tetrahedral sites with a displacement ratio of 14% in these sites (2) From the magnetic susceptibility data and the valence-bond sum calculations, the valence states of the manganese and vanadium atoms are found to be divalent and pentavalent, respectively (3)LiMnVO, exhibits neither lithium intercalation nor demter- calation reactions, probably because it has divalent manganese atoms which are non-active for lithium intercalation, and ngid VO, tetrahedra in spite of the presence of high-valent vanadium atoms We are grateful to Dr Y Tsuchiya, Niigata University, for making available the magnetic susceptibility measurement facilities, to Mr K Uematsu, Niigata University, for his help with the chemical analysis of the samples, and also to Mr K Nemoto, IMR, Tohoku University, for his assistance with the neutron diffraction experiments References 1 I Faul and J Knight, Chem Znd, 1989,24,820 2 F S Galasso, in Structure and Properties of Inorganic Solids, Pergamon, Oxford, 1970, ch 8 3 W I F David, M M Thackeray, L A De Picciotto and J B Goodenough, J Solid State Chem ,1987,67,316 4 B Zachau-Chnstiansen, K West, T Jacobsen and S Atlung, Solid State Zonics, 1990,40/41, 580 5 T Ohzuku, J Kato, K Sawai and T Hirai, J Electrochem SOC, 1991,138,2556 D6 M M Thackeray, A De Kock, M H ROSSOUW, Liles, R Bittihn and D Hoge, J Electrochem Soc ,1992,139,363 7 C R Walk, in Lithium Batteries, ed J-P Gabano, Academic Press, London, 1983, p 265 8 S Panero, M Pasquali and G Pistoia, J Electrochem Soc , 1983, 130,1225 9 M Sat0 and S Kano, Chem Lett, 1994,427 10 M J Isasi, R Saez-Puche, M L Veiga, C Pic0 and A Jerez, Muter Res Bull, 1988,23, 595 11 R Kanno, Y Kawamoto, Y Takeda, M Hasegawa, 0 Yamamoto and N Kinomura, J Solid State Chem ,1992,96,397 12 F Izumi, in The Rietveld Method, ed R A Young, Oxford University Press, Oxford, 1993, ch 13 13 Natl Bur Stand (US) Monogr 25,1984,21,78 14 L A De Picciotto and M M Thackeray, Muter Res Bull, 1986, 21,583 15 International Tables for Crystallography, ed A J C Wilson, Kluwer Academic Publishers, Dordrecht, 1992, vol C, p 384 16 R J Hill and C J Howard, J Appl Crystallogr ,1987,20,467 17 R D Shannon, Acta Crystallogr Sect A, 1976,32,751 18 I D Brown and D Altermatt, Acta Crystallogr Sect B 1985, 41,244 19 R Kanno, K Kawamoto, Y Takeda, M Hasegawa and 0 Yamamoto, Solid State Ionics, 1990,40/41, 576 Paper 5/07904G, Received 4th December, 1995 1194 J Muter Chem, 1996, 6(7), 1191-1194

ISSN:0959-9428    

DOI:10.1039/JM9960601191

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

~~ Layered structures of hydrated vanadium oxides. Part 5:-Single-crystal structure of RbJZ05 and phase changes of rubidium intercalate Takeshi Yao,a Yoshio Oka*band Naoichi Yamamotoc "Division of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606, Japan bDepartment of Natural Environment Sciences, Faculty of Integrated Human Studies, Kyoto University, Kyoto 606, Japan 'Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606, Japan Rbo.,V205, an anhydrous phase of the rubidium intercalate, has been synthesized in hydrothermal VO(OH),-RbCl system. It crystallizes in the monoclinic system C2/m: a = 11.596(2), b = 3.6908(9), c = 9.723(1)A, fi = 100.93( 1)" and 2 = 4.A single-crystal study (R/R,= 0.069/0.079) revealed that the V205 layers are isostructural with those of K0.5V205 in Part 4 which consist of zigzag chains of edge-sharing VO, octahedra.The interlayer Rb atom forms an RbOs rectangudar prism with the apical oxygens of the VO, octahedra in contrast to the KO7coordination in Ko.,V405. Rb0.,V20, with 9.55 A spacing was oxidized by hydrogen peroxide to a hydrated phase, Rbo~,V205~0.8H20 with 10.85 A spacing. This compound was reduced by rubidium iodide to a less hydrated phase, Rbo~,V205~0.5H20 with 10.41 A spacing, rather than to Rb0.,V20,. Interlayer sites for monovalent cations in anhydrous phases are suggested to be correlated with cationic sizes and contents. ., phase vanadium bronzes such as 6-Ag,V205 In Part 4' we reported the crystal structure of the anhydrous potassium intercalate (K intercalate) Ko.sV205, where our previously proposed model of the double-sheet type V20, layer2v3 was proved to be the same as the V205 layer of the 6 Phase conver- sions between hydrated and anhydrous phases were also studied for the K intercalate.' The anhydrous phase Ko.sVz05 is changed into the hydrated phase K0.,V2O5-H2O by the oxidation of the V205 layer or, in other words, by the partial extraction of K+ ions.The reverse process occurs by the reduction of the V205 layer or by the uptake of K+ ions using a potassium iodide solution. This suggests that the hydration process depends strongly on the interlayer cation content. For further understanding of the structures and reactivities of the alkali-metal intercalates we need more information about other intercalates, especially those containing larger cations.In the present study we have achieved the hydrothermal synthesis of single crystals of the rubidium intercalate (Rb intercalate) in an anhydrous phase, Rb0.5V205. The structural analysis dis- closed an interlayer cationic site which was different from that found in K0.5V205. The phase changes of the Rb intercalate were also found to be somewhat different from those of the K intercalate. Experimenta1 Sample preparation The anhydrous and hydrated Rb intercalates were prepared using hydrothermal systems of VO(OH),-RbCl and VOS0,-Rb2S04, respectively. The preparation methods and sample characterization were essentially the same as those used in Part 4 for the K intercalate.' In brief, a suspension of 0.25-0.30 g VO(OH)2 powder in RbCl solution (80 ml, 0.1 mol 1-I) or a solution of VOSO, and Rb2S0, (80 ml, V and Rb each 0.015mol I-') was sealed in a Pyrex ampoule and was treated in an autoclave at 220-280°C for 24-48 h.The com- positions of the anhydrous and hydrated phases were Rb0.48(1)V205and Rbo~30~l~V205-0.8H20,respectively, and therefore they were formulated as Rbo.,V20, and t Part 4 = ref. 1. Rb0.,V2O, .0.8H20, respectively. When treated at 280 "C, single crystals of the anhydrous phases were obtained which exhibited a thin plate-like shape. Powder X-ray diffraction patterns were recorded by using monochromated Cu-Ka radiation.Layer spacings of the layered phases were calculattd from diffraction angles of 001 reflections to an error of 0.01 A. Single-crystal structure determination Data collection was performed on a Rigaku AFC-7R diffractometer using Mo-Ka radiation. The crystallographic and experimental parameters are listed in Table 1. The mono- clinic space group C2/m was chosen and the lattice parameters were determined from 22 reflections of 21.2 < 28/degrees < 27.1. The intensity data with I >3o(I) were used in the structural analysis. An empirical absorption correction of the $-scan method was applied resulting in min./max. transmission fac- tors = 0.53/1.00. The structural analysis was performed by using the TEXSAN software package.' An initial model based on the 6-phase V205 layer of Part 4' was applied successfully to determine the V and 0 atomic sites and subsequently the Rb atom was located in a differential Fourier map.The occupancy of Rb site was determined as 0.956(8) leading to x=0.478(4) Table 1 Crystallographic data and experimental parameters for Rbo 5VP5 composition crystal dimensions/mm3 sp$ce group 44 bI4 CIA Pltegrees VIA3 Z DJg cmP3 scan technique scan width Awldegrees maximum 28ldegrees no. of reflections (I >O) no. of reflections [Z > 3o(I)] no. of variables absorption correction RIRW Rbo 48V205 0.25 x 0.15 x 0.01 C2/m 11.596(2) 3.6908( 9) 9.723( 1) lOo.93( I) 408.6(1 ) 4 3.651 28-1.21+0.30 tan 8 80 1451 857 49 I) scan 0.069/0.079 J.Muter. Chem., 1996, 6(7), 1195-1198 1195 in Rb,V205, in good agreement with x =0.48( 1) determined by chemical analysis; full occupancy gives a stoichiometric composition of Rbo,5Vz05. The structure refinements finally led to RIR, =0.069/0.079. Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC). See Information for Authors, J. Mater. Chem., 1996, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 1145/5. Results and Discussion Crystal structure of Rbo.sVz05 The crystal structure of Rbo,,V205 is depicted in Fig. 1.Selected bond distances for the V-0 and Rb-0 coordination polyhedra and V-V distances are listed in Table 2. The VZO, Fig. 1 Crystal structure of Rb,,sV20s viewed along the 6 axis. Rb and V atoms positioned at y=O and 0.5 are indicated by full and empty circles, respectively. Table 2 Selected bond distances/A in Rbo,SV,Os V( 1)06 octahedron V( 1 )-O( 1)a,b 1.906(2) V(1)-0(2) 2.049(9) V( 1)-0(3)e 1.800(9) V(l)-0(3)d 2.388(9) V( 1)-0(4) 1.61(1) V(2)--0(1) 1.97(1) V(2)06 octahedron V(2)-0(2)'sf 1.914(2) V(2)-0(2)d 2.57(1) V(2)-0(3) 1.840(9) V(1)-0(5) 1.61( 1) Rb-0(4)',fJ,h 2.934(7) RbO, rectangular prism Rb-O(5)".bsis's 2.954(8) metal-metal V( 1)-V(2Y,b 2.977( 3) Symmetry codes: "x-4, -y-$, z; "-4, -y+), z; 'x-1, y, z; d-x+1, -y, -z; ex++, -y+& 2; fx++, -y-4, 2.; Z-Xf), y-4, z+l; *-x++, --y+), z+l; l-x+ j, -y-& z+l; '-x+ j, -y++, z+ 1.\ a Fig. 2 RbO, rectangular prism coordination layers of the double-sheet typezs3 consist of zigzag chains of edge-sharing V( 1)06and V(2)06 octahedra running along the b axis, which are isostructural with those of Ko.sVz05 in Part 4.' As shown in Fig. 2, the interlayer Rb atom in an equatorial plane at z=OS is surrounded by, eight apical oxygevs of V06 octahedra [0(4) x 4 at 2.934 A, O(5) x 4 at 2.954 41 forming an Rb08 rectangular prism (3.081, 3.691, 3.399A along the a, b and c axes, respectively). This type of coordination is formed as a result of the relative location of adjacent Vz05 layers expressed by the monoclinic angle b= 100.93'.In the case of KO,~VZO~,~ the interlayer K atom is coordinated by seven apical oxygens, i.e. four oxygens of one Vz05 layer and three oxygens of the opposite layer in the manner of a KO, monocapped trigonal prism. The K atom is attracted to the four-oxygen side and consequently shifts slightly from an equatorial plane to z=O.476. This type of coordination corresponds to the monoclinic angle ,B =92.24' (or 87.76' relative to the p value of Rbo,,V205). There are two main types of A-0 coordination for monovalent At ions in A,V205, namely, A08 rectangular prism and A07 monocapped trigonal prism. Table 3 presents the relationship between the ionic radius of A' and the A-0 coordination for isomorphous A,Vz05 (x 0.5) compounds.There is a clear boundary at the size of K+ ion, i.e. A07 for A+ smaller than Kt and A08 for A' larger than K'; both types of coordination are found in polymorphs of Ko,,VzO, (Table 3). Note that the top member, cu0,85vzo5,6 has different types of coordination, i.e. A06 octahedra and A04 rectangular planes. Note that Strobel13 reported a v'-phase RbO,,V,0, synthe- sized by the cathodic reduction of the reaction products of Rb2C03 and VZO,. The compound crystallizes in the mono: clinic system C2/m with a= 11.63(4), b=3.664(8), c=9.75(3) A and p= 101.2( 5). Although its structure remains undetermined, it is quite possible that V'-R~~,~V,O~ is the same as Rbo,,V20,. Galy14 sorted the layered bronzes A,V205 into several groups where V'-R~~,~V~O~ is grouped into the v phase together with A=Ca, K and NH4 and therefore Rb0,,V205 should be designated v-RbO,,V,O5. Among the v-phase members, Table3 Ionic radius" (R,) of A+ ion, layer spacing (dWl), space group (SG), monoclinic angle (B) and coordination number (CN) of A for A,V205 with monovalent A ions ~~~ ~ _____~~ A,V*OS R, of A+/,& dWJA SG /?/degrees CN of A ref.c~o.ssv2os 0.96 8.24 Cm 111.804:6' 6 Na0.56V205 1.02 8.92 C2/m 90.91 7 7 A&.68V205 1.12 8.74 C2/m 90.48 7 4 KOSV2OS 1.38 9.50 C2/m 92.24 7 1 Ko.5V20sb 1.38 9.32 Cmcm 90 Xf 89 RbOSVZOS 1.49 9.5s C2/m 100.93 X this work T10.,,VzO,' 1.50 9.46 C2/m 100.90 8 10 "Taken from ref. 11 for Na', Ag', K+, Rb', T1' and ref. 12 for Cu'. bOrthorhombic system.'Analysed from powder X-ray diffraction data. dRectangular plane. 'Octahedron. /Ref. 9 gives additional coordination of KO6 trigonal prism for 20% of the K atoms, 1196 J. Muter. Chem., 1996, 6(7), 1195-1198 Ca0.,V2O5 with a divalent cation is the only one to have been characterized by a single-crystal study.15 It was revealed that two-thirds of the Ca atoms form a CaO, trigonal prism and the rest form a CaO, rectangular prism. This cation distri- bution over the interlayer sites is somewhat different from those observed for A+ cations in other v-phas? members (Table 3). It is presumed that the Ca2+ ion (1.00 A radius"), being smaller than the K+ ion, should prefer CaO, c?ordi- nation which determines both the layer spacing of 9.072 A and the monoclinic angle of 101.87", and the rectangular prism site which is formed concomitantly is occupied to some extent.In fact, the layer spacing of Ca0.,V2O5 is rather smaller than those of the other v-phase members of A+ cations (Table 3). On the other hand, in Rbo.5V205 the larger Rb+ ion occupies exclusively RbO, sites resulting in a phase similar to Cao.,V205; the Rb+ ion must be too large to occupy the trigonal-prism site and in fact the trigonal-prism site was proved to be vacant in this study. Rb0.5V205 is the first example of a single-crystal structure for a v phase with A+ cations, and it is found to be isostructural with T10.48V205 which was characterized by the X-ray Rietvelt method." Hydrated and anhydrous phases of the Rb intercalate Phase changes caused by redox reactions were examined for the Rb intercalate in a similar manner to that described in Part 4 for the K intercalate.' The results are shown in Fig.3 by the changes in the X-ray diffraction patterns. The anhydrous phase Rbo5V205 was oxidized by soaking in a hydrogen peroxide solution, and was transformed into a hydrated phase accompapied by an expansion of the layer spacing from 9.55 to 10.85 A [Fig. 3(u)]. This hydrated phase is formulated as Rbo~3V205-0.8H20,and it seems to be the same as the phase obtained from the hydrothermal VOS04-Rb2S04 system.2,16 Next the hydrated phase Rbo.3V205.0.8H20 of the VOS04-Rb2S04 system was reduced by soaking in a rubidium iodide solution, leading to another hydrated phase, Rb,V20, .nH20 (xz 0.4, n z0.5) [Fig.3(b)], with a contraction of the layer spacing from 10.85 to 10.41 A. Note that the Rb interylate shows two more hydrated phases with 10.70 and 10.15 A spacings which appeared in the VO(OH),-RbCl system but have not been characterized yet because no single- phase sample was obtained. This is not the case for the K intercalate, which has one hydrated phase. The oxidation process leading to RbO.,V2O5.O.8H2O is parallel to that of the K intercalate' but the reduction process stops at Rbo.4V205*0.5H20,part of the way to Rb0.5V205, in contrast to that of the K intercalate which leads to the ultimate phase K0.5V205-' As reported in Part 1,2 the anhydrous phase Rbo.3V205 was obtained by heating Rbo~,V2O5.O.8H20 to 150°C; it shows 9.80 A spacing which is significantly larger than the 9.55 A spacing found for Rb0.,V2O5, and it returns immediately to Rb0.,V2O5.0.8H2O when cooled in air.The difference in layer spacing must be attributed to the difference in Rb-0 coordi-nation, namely RbO, for Rb0.3V205 and RbO, for Rb0.5V205, for the following reason. As reported previously,'6 the layer spacings of Ao.3V205 for A=Na, K, Rb and Cs derived from A0.3V205*nH20increase linearly with the ionic radii of the A+ ions, which suggests the same type of A-0 coordination for all A0.3V205 compounds, particularly for K0.3V205 and Rb0,,V2O5. Ko.3V205 must havc the AO, coordination judging from the layer !pacing (9.49A) which is equal to that of Ko.54, 205(9.50 A),' and consequently AO, coordination is expefted in Rbo.3V205. In fact, the orthorhombic KO.~V&)~ with the AO, coordination exhibiots a spacing of 9.32 which is smaller than that of 9.50 A for K0.5V205 with A0, coordination (Table 3); this cFrresponds to the relatio!ship between the spacings of 9.55 A for Rbo.,V205 and 9.80 A for 001 003 treated in a hydrogen peroxide solution J C 002 I\ c 004 005 A I I I I I I I I I I I 10 20 30 40 50 001 (b) 004 005 002 Fig.3 Phase changes of the Rb intercalate demonstrated by the changes in X-ray diffraction patterns: (a) from Rbo.,V205 to Rbo.3V205-0.8H20formed in 3% hydrogen peroxide solution at room temperature for 3 min; (b) from Rb0,,V2O5.0.8H20 to Rbo,V205~0.5H20 formed in 0.1 mol dm-3 rubidium iodide solution at 85 "C for 28 h Rb0.3V205.The phase changes of the Rb intercalate are summarized in Fig. 4. Conclusion The structural and phase changes of the Rb intercalate have been investigated and the results are compared with those for the K intercalate in Part 4.' The anhydrous phase Rb0.,V2O5 adopts a structure related to that of K0.5V205, but the inter- stitial Rb site has Rb0, coordination, different from the KO, coordination. It is considered that the A-0 coordination in A0.5V205 changes from A07 to AOs with increasing size of the A+ ion and the boundary is at A =K. The phase conversion J. Muter. Chem., 1996, 6(7), 1195-1198 1197 4 dehydration1 Fig.4 Diagram of the phase relation for the hydrated and anhydrous phases of the Rb intercalate into the hydrated phase Rbo3V205 0 8H20was achieved by oxidizing the V205 layer However, the reverse process from RbO3V2O508H20 led to a less hydrated phase, Rb, 4V205 0 5H20,instead of Rb, 5v205 The Rb-0 coordi-nation of the anhydrous phase changes with the Rb content, namely Rb08 for RbO5V2O5 and Rb07 for Rb,,V205 The Rb intercalate consequently exhibits various layer spacings, depending more strongly on the Rb content and on the extent of hydration than the K intercalate The present work is supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan References 1 Y Oka, T Yao and N Yamamoto, J Muter Chem, 1995,5,1423 2 T Yao, Y Oka and N Yamamoto, J Muter Chem , 1992,2,331 3 T Yao, Y Oka and N Yamamoto, J Muter Chem , 1992,2,337 4 S Anderson, Acta Chem Scand, 1965,19, 1361 5 TEXSAN Crystal Structure Analysis Package, Molecular Structure Corp ,The Woodland, TX, 1985,1992 6 J Galy, D Lavaud, A Casalot and P Hagenmuller, J Solid State Chem, 1970,2,531 7 Y Kanke, K Kato, E Takayama-Muromachi and M Isobe, Acta Crystallogr Sect C, 1990,46,536 8 Y Kanke, K Kato, E Takayama-Muromachi and M Isobe, Acta Crystallogr Sect C, 1990,46, 1590 9 J-M Savariault and J Galy, J Solid State Chem ,1992, 101, 119 10 M Ganne, A Jouanneaux, M Trournoux and A LeBail, J Solid State Chem ,1992,97,186 11 R D Shannon and C T Prewitt, Acta Crystallogr Sect B, 1969, 25,925 12 L H Ahrens, Geochim Cosmochim Acta, 1952,2, 155 13 R Strobel, J Solid State Chem ,1987,66,95 14 J Galy, J Solid State Chem , 1992, 100,229 15 A Kutoglu, Z Kristallogr ,1983, 162,263 16 Y Oka, T Yao and N Yamamoto, Seramikkusu Ronbunshi (J Ceram SOC Jpn ),1990,98, 1365 Paper 6/00349D, Received 16th January, 1996 1198 J Muter Chem, 1996, 6(7), 1195-1198

ISSN:0959-9428    

DOI:10.1039/JM9960601195

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Precursor dependence of the nature and structure of non-stoichiometric magnesium aluminium vanadates Fathi Kooli,".b Inmaculada Crespo,' Cristobalina Barriga," Maria A. Ulibarri" and Vicente Rives*b "Departamento de Quimica Inorganica e Ingenieria Quimica, Universidad de Cdrdoba, Facultad de Ciencias, 14004 Cdrdoba, Spain bDepartamento de Quimica Inorganica, Universidad de Salamanca, Facultad de Farmacia, 37007 Salamanca, Spain Hydrotalcite-like compounds containing (a)Mg", Al"' and V"' in the layers and carbonate in the interlayers, or (b)Mg" and Al"' in the layers and decavanadate in the interlayers, have been synthesized and characterized using powder X-ray diffraction, thermal analysis, FTIR spectroscopy, temperature-programmed reduction, and specific surface-area determination.The crystalline phases formed after calcination in air at 700 "C have been identified using these same experimental techniques. It has been found that the nature of the precursor determines the nature of the Mg-V-0 phases formed; thus, when starting from (a), Mg,( VO,), containing isolated [VO,] units is formed, while when starting from (b),MgV20,, containing pairs of edge-sharing [VO,] octahedra, is formed preferentially. Such a reactivity appears to depend on the nature of the support, and not on the stoichiometry (ie.,Mg/V atomic ratio) of the starting materials. Heterogeneous catalytic oxidation has been one of the most rapidly developing areas of catalysis in recent years. Selective oxidation of short-chain paraffins is an alternative route to the use of alkenes for the synthesis of valuable organic chemicals.The catalysts used most widely in these reactions are based on mixed systems constituting supported (or unsupported) metal oxides, and, within this group, supported vanadium-containing catalysts have exhibited good performances. If the supports are acidic (Le., silica, titania), low selectivities to the desired products are attained, while the use of basic supports (mag- nesia, lanthana, etc.),where the corresponding metal vanadates are formed, leads to better selectivity results.' Different crystal- line phases can be formed in the V-Mg-0 system, depending on the V/Mg ratio and on the oxidation state of vanadium. For Vv-containing systems, four crystalline phases have been identified: orthovanadate Mg,( V04)2,pyrovanadate (a and p) Mg,V,07, and metavanadate MgV,O,.Several authors have claimed that the catalytic properties of the V-Mg-0 system in the oxidative dehydrogenation of propane depend on the preparation method and on the vanadium the high selectivity being related to the presence of isolated, tetra-hedrally coordinated V-containing species. These systems have also been studied for the oxidative dehydrogenation of butane, for which the composition and preparation method of the layers, are also Some of us have been previously reported on hydrotalcites containing V"' in the layers.21*22 In the present paper, we report on the preparation of hydrotalcite-like materials containing vanadium (as V"'), together with Mg" and Al"', in the layers, and carbonate in the interlayer, and other materials containing Mg" and Al"' in the layers, but decavanadate in the interlayer.Upon calcination in air, magnesium vanadates are formed, and the nature of the crystalline phases formed has been correlated to the nature of the precursors. Experimental Synthesis of MAVL samples A series of samples (hereafter MAVL) was prepared with carbonate anions in the interlayer and Mg", Al"' and V"' in the brucite-like layers, with different V/Al ratios, but with a constant Mg/( V +Al) =3, following the method described by Kooli et al." As an example, a detailed description of the procedure used to prepare the sample with V/Al= 1 is given: MgCl, -6H20 (5.09 g) and AlCl, 6Hz0 ( 1.06 g) were dissolved in 50 ml of distilled water. VCl, (0.656 g) was dissolved separ- ately in 50ml of distilled water.These solutions were added dropwise to 100ml of sodium carbonate solution (0.5mol catalyst are also important for the catalytic performan~e.~,~ drn-,). The pH was kept constant close to 9, by using a The large selectivity observed has been ascribed, also to the presence of isolated tetrahedral [VO,] species, linked to [MgO,] octahedra and forming V-0-Mg units; the lack of V-0-V species, where oxygen can be released easily through reduction of Vv to VIV, decreases the activity for combustion, in comparison with vanadia. Although these catalysts are usually prepared by impreg- nation of the support with aqueous solutions of vanadium precursors, or through reaction in the solid state between vanadia and magnesia,, in the present paper we report on the use of alternative precursors, such as hydrotalcite-like mate- rials.These, also known as anionic clays or as layered double hydroxides, consist of brucite layers where a partial Mg"/M"' substitution has been achieved, the positive charge in excess being cancelled by anions in the interlayer, where water molecules also The interlayer anions can be exchanged by polyo~ometalates.~-~~ Systems with two divalent and one trivalent, or one divalent and two trivalent cations in the Metrohm Dosimat 725 apparatus, and NaOH solution (1.5 mol drn-,). The mixture was stirred for 1 h at room tempera- ture, and then washed with hot water.The slurry was treated hydrothermally in a stainless-steel digestion bomb lined with Teflon, at 120 "C for 48 h, then filtered and dried. For samples possessing different V/Al ratios, the relative amounts of V and A1 salts were changed accordingly. Another sample (MAVI) was obtained from an Mg,AIhydrotalcite (the synthesis of which is described below), after exchanging the interlayer carbonate anions with (V10028)6-. The parent hydrotalcite (sample MA) was prepared following the method described by Rei~hle:~, Mg(N03)z .6H20 (47 g) and Al(NO,), *9H@ (27.5 g) were dissolved in 150 ml of distilled water. The mixture was added dropwise to 150ml of a solution containing NaOH (23.5 g) and Na2C0, (11.5 8); the final pH was close to 11.The slurry was aged overnight under stirring at room temperature, then separated by centrifu- gation, washed with distilled water, and treated hydrothermally J. Muter. Chem., 1996, 6(7), 1199-1206 1199 in a stainless-steel digestion bomb lined with Teflon at 120°C for 24 h. The resulting slurry was centrifuged and dried over- night at 80°C. NaVO, (0.9g) were dissolved in 50ml of distilled water, the pH was adjusted to 4.5 by adding a solution of 0.5 mol dmP3 HCl. The vanadium solution was added dropwise to a suspension of sample MA (1% by mass). The pH of the mixture was maintained at 4.5 using a Metrohm Dosimat 725 and 0.5mol dm-3 HCl solution. The orange- yellow suspension was aged for 2 h, and then hydrothermally treated for 24 h at 120 "C.The solid was separated by centrifu- gation and washed with distilled water, and then dried in open air oven at 60°C. All chemicals were from Fluka (Germany), with adequate purity. Characterization Chemical analyses for Mg, A1 and V contents were carried out in a Perkin-Elmer 3100 apparatus. Powder X-ray diffraction (PXRD) patterns were obtained using a Siemens diffractometer with Cu-Ka radiation (graphite monochromator), using steps of 0.02" (28) and a time constant of 0.8 s. Thermogravimetric curves (TG) were recorded under air or in a nitrogen flow, using a Polymer Laboratory TG-1500 apparatus, working at a heating rate of 10°C min-l. Differential thermal analysis (DTA) of the samples was performed in air or in a nitrogen flow in a Perkin-Elmer DTA 1700 instrument, coupled to a Perkin-Elmer 3600 Data Station.The FTIR spectra were recorded in a Perkin-Elmer 1730 spectrometer, in the 4000-400 cm-' range, using KBr pellets. A Micromeritics Flowsorb I1 2300 instrument was used to determine the specific surface area values (A) of the samples by the single-point method, after degassing in situ with flowing nitrogen at 120 "C. Temperature-programmed reduction (TPR) analysis was car- ried out in a Micromeritics TPR/TPD 2900 instrument, at a heating rate of 10 "C min-l, and using ca. 15 mg of sample and a Hz/Ar (5% v/v) mixture (from Sociedad Espaiiola del Oxigeno, Spain) as a reducing agent (60 ml min-l); experimen- tal conditions for TPR runs were chosen according to data reported elsewherez4 in order to achieve good resolution of the component peaks.Results and Discussion Nature of the precursors Chemical analysis. Results of elemental chemical analyses for the samples prepared are included in Table 1. The exper- imental Mg/M'" (M"' =Al"' +V"') ratio for sample MA is coincident, within experimental error, with the expected value (i.e., that existing in the starting mixed aqueous solutions). However, as vanadium is added and the V/A1 ratio in the starting solution is increased, the Mg"/M"' ratio decreases steadily, and is always lower than 3. At the same time, the V/A1 ratio in the solids obtained is always lower than in the starting aqueous solutions.Changes in the ratio of trivalent cations (V/Al in the present case) concentration in the precipitated solid, compared to the values in the parent solutions, have also been reported for hydrotalcite materials containing Ni, A1 and Cr, with different values for the Cr/Al ratio." The experimental Mg/(Al +V) values are in all cases lower than those expected from the concentration of these cations in the parent solutions. This difference arises from the experimental conditions used to prepare the samples: precipitation of Mg" requires higher pH values than those required to precipitate Al"' and VII1, therefore pH values of 10-12 are usually required to prepare hydrotal- cites.* In the present case a pH value of 9 was used to precipitate the solid, as the use of higher pH values in preliminary studies, oxidation of V"' to VIV was observed.The Mg/Al ratio in sample MAVI is also lower (but only by ca. 10%) than in the parent solid (sample MA) used to prepare this sample; such a results could arise from a partial dissolution of Mg2+ during the exchange process in acidic medium (pH=4.5) used to prepare this sample.16 Powder X-ray diffraction. The PXRD patterns of representa- tive MAVL samples (MAVL1, MAVL3, and MAVL6) and sample MAVI are shown in Fig. 1, together with the pattern corresponding to sample MA. This last pattern is typical of a material with the hydrotalcite structure. The value of the c parameter (see Table 1) corresponds to three times the thick- ness of the 'sandwich' formed between the centre of two consecutive brucite-like layers and the interlayer, where car- bonate anions together with water molecules exist.This thick- ness can be calculated from the position of the diffractjon peak corresponding to the (003) plane, recorded at 7.6 A. Other maino peaks correspond to planes (006), 3.78 A, and (009), 2.32A. These values coincide with those reported in the literature for tarbonate-containing hydrotalcite material^.^ The peak at 1.53 A in the doublet recorded close to 282560", is due to diffraction by the (001) plane, and corresponds to one half of parameter Such a parameter depends on the radius of a7yZ5 the hydroxy group in the brucite-like layers, and on the ionic radii of the metal cations in these layers.Taking into account the ionic radii in octahedral coordivation of the catipns existing in ou! samples, i.e. Mg" (0.86 A), Al"' (0.675 A) and V"' (0.78 A),z6 an increase in the value of parameter a should be expected as the vanadium content (replacing Al"' ions) is increased, as observed experimentally in these samples (Fig. 2), thus confirming that V"' is incorporated into the brucite-like layers. A similar effect has been reported previously19 for Ni,Al,Cr-containing hydrotalcites, when aluminium is substi- tuted by larger cations, such as chromium. The slight increase in the value of parameter a when passing from sample MA to sample MAVI may be due to the slight decrease in the Mg" content (the ionic radius of Mg" is larger than that of Al"').The increase in the vanadium content gives rise in all MAVL samples to materials with the layered hydrotalcite structure (Fig. 1), and the positions of the PXRD peaks due to diffraction by (001) planes do not shift significantly from one sample to Table 1 Chemical analyses results, lattice parameters and specific surface areas of the samples studied sample Mg (mass%) A1 (massYo) V (mass%) V/Al' Mg/( V +Alp &A C"A A/m2 g-' MAVLl MAVL2 22.4 21.1 7.5 4.0 7.5 17.0 0.53 (0.5) 2.24 (3) 2.17 (3) 1.79 (3) 3.06 3.09 23.6 23.0 141 103 MAVL3 MAVL4 MAVL5 MAVL6 20.6 20.7 21.0 19.5 3.O 2.1 1.5 0.0 19.9 21.8 25.4 25.3 3.51 (5) 5.48 (8) 9.10 (1 1) - 1.69 (3) 1.68 (3) 1.56 (3) 1.60 (3) 3.09 3.10 3.10 3.11 23.3 23.1 23.0 23.1 106 112 80 69 MA M AVI 21.7 11.9 9.5 5.8 0 28.5 0 2.61 2.52 (2.5) 2.29 (2.5) 3.06 3.04 22.8 35.5 23 74 'Atomic ratio (expected values in brackets).bRounded values. 1200 J. Muter. Chem., 1996, 6(7), 1199-1206 I I I I 1 I I I 10 20 30 40 50 60 20ldegrees Fig. 1 Powder X-ray diffraction patterns of samples MA, MAVL1, MAVL3, MAVL6 and MAVI. *, diffraction peaks due to A1 sample holder; #, Mg-Al-V co-product (see text). 3.12 I 1 3.11 3.10 3.09 ? 3.08 3.07 3.06 3.05 i I i I I I I 0 5 10 15 20 25 30 V (mass%) Fig.2 Change in a parameter of the MAVL series of samples with changing V content another, differences being within the experimental uncertainty of the peak position determination.It should also be noted that the value of parameter c decreases slightly when V"' ions exist in the brucite-like layers; this variation is due to the decrease in the magnesium content (see Table 1) and in the Mg"/M"' ratio, thus giving rise to stronger electrostatic inter- actions between the interlayer carbonate anions and the bru- cite-like layer^.^,^^ The crystalline phase existing in sample MAVI (Fig. 1) is also layered, but does not correspond to samples MA yr MAVL. Sharp reflections are recorded at 5.90 and 3.90A, which can be ascribed to reflections (006) and (009) of a layered matFria1, for which (003) would be recorded at 11.75 k0.05 A, a p9sition that does not coincide with the broad peak close to 10 A.Previous studie~'~,'~,'~-'~,~~ indicate that this broad feature is probably due to the formation of Mg and/or A1 polyoxovanadate during the ion-exchange process, the particular nature of which depends on the divalent and trivalent cations in the host layer^.'^.^^ The positions of the (001) peaks are in agreement with the presence of (V10028)6- species in the interlayer space, with its main C2 axis parallel to the layer^.^*'^,'^-'^ Temperature-programmed reduction. The formation of a hydrotalcite-like phase was confirmed by PXRD for all samples synthesized. No evidence was found for the formation of by- products, such as V205, in addition to the salt phase charac- terized by the broad diffraction line close to 28= 10" in the case of sample MAVI.So, we should expect that all vanadium exists as V"' in sample MAVL, but as Vv in sample MAVI. In order to confirm this, the temperature-programmed reduction profiles of all samples were recorded. This technique has proved to be suitable for this purposes.29 The TPR curves of samples MAVL3 and MAVI are shown in Fig. 3. The curve for sample MA is not shown, as no consumption of hydrogen was ~bserved.~' Curves for the MAVL samples were essentially identical to that shown for sample MAVL3. Hydrogen consumptions were ca. 170 and 5500 pmol H2 (g sample-') for samples MAVL3 and MAVI, respectively. Taking into account that the stable state of vanadium after reduction is V"'29 and the vanadium content (Table 1) for these samples, these results indicate that the average oxidation states of vanadium in these samples are +3.1 and +4.97 for samples MAVL3 and MAVI, respectively.These values are acceptable as +3 and +5 within experimental error; the slightly higher consumption for sample MAVL3 is probably due to a partial oxidation of V"' species in the edges or in the external layers of the crystallites.2' These results confirm the oxidation states of vanadium in these samples. Specific surface area measurements. Specific surface areas for all samples studied are included in Table 1. A general decrease in the specific surface area is observed for the MAVL samples as the vanadium content is increased, from a maximum value of 141 m2 g-' for sample MAVLl to 69m2 g-' for sample MAVL6, which does not contain Al.In contrast, the value for sample MA is only 23m2 g-', within the range reported elsewhere for Mg,Al hydrotalcites submitted to hydrothermal treatment similar to that performed here.30 These specific surface area values are lower than those reported by Lopez Nieto et for Mg-Al-V-0 catalysts prepared by the impregnation of calcined (450 "C) Mg,Al hydrotalcite with vanadium-containing solutions, or similar systems prepared by impregnation of y-A1203 with V-Mg citrate solutions.32 However, the values are of the same order as for V-Mg-0 oxidative dehydrogenation catalysts prepared by impregnation of Mg(OH), with aqueous NH4V03,33 and are markedly larger than those for pure and biphasic Mg van ad ate^.^^ The 200 400 600 800 TPC Fig.3 Temperature-programmed MAVL3 and MAVI reduction profiles of samples J. Muter. Chem., 1996, 6(7), 1199-1206 1201 change in the values for the MAVL samples may be related to the presence of an amorphous, high specific surface area impurity in these samples, although the corresponding PXRD profiles do not provide any evidence of the presence of such an amorphous material, ie the profiles are 'flat' below the diffraction peaks of the layered material Most probably, such a change in the specific surface area can be related to the crystallinity of the layered material All PXRD profiles in Fig 1 were plotted on the same y-axis scale, and so a broader peak should be related to a less crystallized material If the sharpness of the peaks displayed by the profiles of samples MA and MAVL are compared, it can be observed that the peaks of sample MA are very sharp, intense and well defined, and the noise level is very low However, profiles for samples MAVL are rather noisy, but the noise level decreases on passing from sample MAVLl to sample MAVL6, ze as the specific surface area decreases With regard to sample MAVI, its specific surface area (74 m2 g-l) is ca three times larger than that of its precursor solid, sample MA Similar values have been reported for decavanad- ate-intercalated Mg,A1 hydrotalcites l635 36 Such an increase is not due to an increased accessibility of the gallery region to nitrogen molecules, as reported for other samples37 because of the increase in the gallery height containing decavanadate anions, as nitrogen molecules do not penetrate the interlayer region, therefore, the specific surface area measured corre-sponds to the external surface area of the particles, its increase being also probably due to a partial loss of crystallinity during the exchange treatment, as can be concluded from the PXRD diagram in Fig 1 with a noise level similar to that of the profiles for samples MAVL FTIR spectroscopy.The FTIR spectra of sample MAVI and of representative MAVL samples are included in Fig 4 All spectra show a broad, intense band centred at ca 3600 crn-', due to the stretching mode of hydroxy groups in the layers and of water molecules, the broadness of this absorption I I I 110% T MAVI indicates that these species are hydrogen bonded3' The medium intensity absorption close to 1640 cm-' is due to the deformation mode of water molecules (dHZ0), and its larger intensity for sample MAVI suggests a larger water content in this sample than in samples MAVL The medium intensity absorption at 1370cm-', absent in the spectrum of sample MAVI, is originated by the v3 mode of carbonate species 38 Its shift from the position for free carbonate anions (ca 1450 cm-l) is due to the strong interactions with the neighbouring water molecules and hydroxy groups in the interlayers, moreover, the broad, weak shoulder recorded in the spectra of these samples at ca 3020 cm-l has been ascribed39 to the stretching mode of hydroxy groups of water molecules hydrogen-bonded to carbonate anions The shoulder close to 1500cm-l may be due to the presence of bicarbonate anions (formed because of the lowering of the pH during washing of the solids with distilled ~ater),~' or to carbonate adsorbed on the external surfaces of the layers 41 Absorptions at lower wavenumbers are due to lattice vibrations involving cations and hydroxy groups in the brucite-like layers, the intensities of the band at 720 cm-' and the shoulder at 550 cm-l increase with increas- ing vanadium content and so these absorptions should be related to the V-0 bonds in the layers 42 The absence of the absorption at 1370 cm-' in the spectrum of sample MAVI indicates that this sample does not contain carbonate However, the sharp, intense absorption at 960 cm has been associated with the symmetric stretching mode of terminal V=O groups, while the absorptions between 800 and 500cm-' may be related to antisymmetric and symmetric stretching modes of V-0-V chains in decavanadate l5 43 DTA and TG analysis.The TG diagrams of selected samples are shown in Fig 5 The curve for sample MA coincides with that previously reported7 30 35 44 for Mg,A1 hydrotalcite, with two main mass losses, the first one between room temperature and 270°C due to removal of water molecules (both physi- sorbed and from the interlayer), and the second one originated MAVI MAVLG (Nz) 3000 2000 1000 200 400 600 T/"C wavenurnberkm-l Fig. 5 TG diagrams of samples MA, MAVL6 (in air and in nitrogen) Fig.4 FTIR spectra of samples MAVL1, MAVL3, MAVL6 and MAVI and MAVI 1202 J Muter Chem, 1996, 6(7), 1199-1206 by removal of hydroxy groups from the brucite-like layers (as water molecules) and of carbonate anions (as COz) from the inter layer^.^^ The corresponding DTA profile for this sample (Fig. 6) shows two strong endothermic effects, centred at 260 and 430 "C, respectively. When V"' species exist in the brucite-like layers, the mass losses recorded in the TG diagram are less defined and, for the richest V"' sample (MAVL6) the second mass loss extends to 700 "C, without any well defined plateau.42 It is also observed that the total mass loss recorded for samples MAVL (35-30%0, depending on the vanadium content) is always lower than that recorded for sample MA (ca.46%). This fact is due to the presence of V"' in the layers, which become oxidized during the thermal treatment, giving rise to Vv-containing species (uide infra),which require the presence of an increased amount of oxide anions, thus decreasing the total mass loss. In contrast, if the analysis is performed in nitrogen, the two mass losses are again well defined and the oxidation of V"' to Vv can be avoided; consequently, a larger mass loss (37%) is observed. If the DTA profiles in Fig. 6 are examined, major differences can be also observed when the analysis are performed in air or in nitrogen for sample MAVL6.Therefore, two endothermic peaks at 200 and 415°C are recorded in nitrogen for sample MAVL6, similarly to the results obtained for sample MA. However, when the analysis is recorded in air, the second endothermic effect (due to removal of hydroxy and carbonate groups) is not recorded. Instead, two weak exothermic effects are recorded at 580 and 695 "C, probably due to a recrystalliz- ation process, leading to formation of well defined Mg-V-0 specie^.^'.^^ Cancellation of the second endothermic peak when the analysis is performed in air can be ascribed tentatively to oxidation of V"' in the layers to Vv (an exothermic process) with the simultaneous removal of hydroxy and carbonate groups (an endothermic process), which cancel each other out.A similar finding has been reported previously for Co,Al- and Mg,Mn-containing hydrotalcite material^.^^,^^ We can conclude that V"' species are unstable during calcination in air, the system rapidly undergoing an oxidation '1'1'1' MAVI 200 400 600 TI'C Fig. 6 DTA diagrams of samples MA, MAVL6 (in air and in nitrogen) and MAVT Table 2 Chemical formulae of the MAVL and MAVI samples sample formula MAVL1 [:Mg0.685A10.205V0.lO9(OH)2 I(co3)O. 1517 * 0.52H20 MAVL2 [Mg0.642A10. 1lV0.247(OH)21(c03 )O. 178 0.58H20 MAVL3 cMg0.627A10.083V0.289(0H )Zl(c03 )0.186* 0*62H20 MAVL4 [Mg0.628A10.057V0.3147(OH )21(co3)0.186 0.65H20 MAVLS ~M~0.608A10.038v0.352~oH~2~(co3~0.195*0.64H20 MAVL6 CMg0.6MV0.383 (0H)21(c03 )O. 191 0.65H20 process to yield Vv species; however, such an oxidation cannot proceed when the analysis is performed in nitrogen.The TG and DTA profiles of sample MAVI are also shown in these figures. The TG curve (Fig. 5) shows that mass loss starts even at room temperature, and so it must be due to removal of physisorbed water molecules; physisorbed water is usually removed around 90 "C, while intercalated water is lost between 120 and 200 "C. Dehydroxylation of the brucite-like layers occurs between 240 and 560 "C. The total mass recorded (26%) is again lower than the value obtained for sample MA (46%0), in agreement with the fact that for the carbonate- containing sample, dehydroxylation and decarboxylation occurs, while for sample MAVI only dehydroxylation takes pla~e.'~.~~The DTA curve is also similar to those previously reported in the literature for similar comp~unds,~~,~~ with an intense endothermic peak at 190°C related to release of interlayer water.This peak is recorded at a temperature slightly lower than that for a similar process in Mg,A1 hydrotalcite, thus suggesting some sort of hydrophobic character for the interlayer decavanadate species that would favour release of water molecules at lower temperatures. Two minima are recorded at 320 and 450"C, probably due to removal of differently held hydroxy group^,^^.^^ as carbonate removal is not possible in this sample. Finally, the broad feature between 600 and 700°C can be related to formation of binary and/or ternary Mg-Al-V oxides, as confirmed by X-ray diffra~tion.~~ The chemical formulae of all solids prepared, calculated from the elemental chemical analysis data summarized in Table 1, and from recorded mass losses during TG analysis, are given in Table 2.Nature and structure of the calcined solids: non-stoichiornetric Mg,AI vanadates Powder X-ray diffraction and specific surface area. From the TG and DTA studies reported above, it is concluded that calcination of the samples at temperatures close to 700°C is enough to yield thermally stable materials, as no thermal effect is recorded above this temperature. Nevertheless, the samples were calcined at increasing temperatures, and the phases stable at each temperature were identified by PXRD and FTIR spectroscopy. With regard to sample MAVI, the PXRD patterns indicate that the layered structure is stable up to 250°C; at this temperature the (001) lines due to the decavanadate-pillaJed phase are lost, and a new, broad reflection centred at 4.7 A is recorded, probably due to depolymerization of decavanadat? to (v309)3-species." The position of the peak close to 1.52 A due to reflection by planes (110) remains unchanged, indicating that brucite-like layers still exist in this material, containing Mg" and Al"' cations.This behaviour is similar to that reported previously by Twu and Dutta," who indicated that above 100 "C a partial depolymerization of decavanadate takes place, leading to lower nuclearity polyvanadates; the PXRD diagrams indicated rather poorly crystallized materials, probably due to removal of interlayer water molecules, a process usually leading to broadening of the peaks.In order to check whether the J. Muter. Chem., 1996, 6(7), 1199-1206 1203 layered structure is still stable at 250"C, a portion of the matenal calcined in air at this temperature was submitted to hydrothermal treatment in order to improve its crystallinity Theo resulting powder exhibits a rather sharp PXRD line at 9 7 A, corresponding to a gallery height (tating into account the thickness of the brucite-like layers) of 4 9 A, consistent with the presence of (V309)3- species So, the process could be described as a depolymerization of decavanadate anions, through reaction with interlayer molecular water molecules (V10028)~-+3H20-+3(V309)3- +(HV04)2- +5Ht Although this reaction is rather slow at room temperature, it may be favoured by the restricted interlayer space, with a large charge density, together with the calcination Moreover, TG results indicate that water molecules are removed between 25 and 210 "C, and so such a water removal could be associated with chemical processes undergone by the decavanadate ion At 350°C the layered structure collapses and only poorly crystallized MgO is identified by PXRD, Al"' ions probably form a solid solution within the MgO lattice, thus accounting for the low crystallinity and deviation of the peaks from the exact positions expected for MgO When calcination is per- formed above 550°C, PXRD indicates the presence of Mg2V207 as the major product, together with A1203, in agreement with previous results by L6pez-Salinas and Ono,15 who propose that reaction between interlayer vanadate and the Mg,A1 mixed oxide takes place at this temperature, leading to formation of different Mg vanadates It should be noted that Mg2V207 is synthesized at 700 "C or above in the absence of hydr~talcite,~~ thus suggesting that the interlayer space of hydrotalcite might provide a reactive environment for polyoxovanadates at relatively low temperatures It cannot be ignored, however, that reaction of Mg pyrovanadate with MgO to yield Mg orthovanadate only takes place at high temperatures in the presence of potassium50 and maybe that traces of sodium in our samples (synthesized in a strongly, basic medium, with NaOH and Na2C0, aqueous solutions) may play a similar role Finally, calcination at 700°C leads to formation of metavanadate and spinel, as identified by PXRD, according to the following reaction Mg,V,O, +Al2O3 +MgA1204 +MgV2O6 The hydrotalcite structure of samples MA and MAVL is stable up to 200 0C,2122 30 above this temperature, dehydroxyl- ation takes place with the simultaneous collapse of the layered structure, leading to formation of amorphous solids between 300 and 500°C If vanadium is present, its interaction with other components of the solids takes place between 580 and 695°C (exothermic effects due to crystallization in the DTA profile of sample MAVL6 recorded in air) Therefore, in order to identify the species existing in the stable solids, samples were calcined in air at 700°C for 3 h and the solids obtained were subinitted to study These samples are labelled (former name)-700 The PXRD patterns of representative samples are shown in Fig 7 For sample MAVL1-700 the only peaks recorded in the PXRD diagram correspond to poorly crys- tallized MgO As the vanadium content is increased (samples MAVL3-700 and MAVL6-700), peaks due to MgO become weaker, while peaks due to Mg3(V04)2 develop, this phase being the only one detected by PXRD for sample MAVL6-700 The specific surface areas of the calcined samples are 136, 38, 16 and 31 m2 8-l for samples MAVL1-700, MAVL3-700, MAVL6-700 and MAVI-700, respectively The value obtained for sample MAVLl is within the range reported in the literature for Mg,Al hydrotalcite matenals calcined at this same tempera- ture3OS1 As the vanadium content is increased, the specific surface area decreases, probably due to crystallization of well defined phases, Mg3(V04),, in agreement with the PXRD results The value for sample MAVI-700 is 31 m2 8-l and, probably due to the different crystalline phases existing in this I I #' 1 I I * 10 20 30 40 50 60 29Idegrees Fig.7 Powder X-ray diffraction patterns of samples MAVL1, MAVL3, MAVL6 and MAW calcined at 700 "Cin air 0,MgO, *, Mg3(V0&, #, MgV,O,, MgA1204+Y sample (if compared to those existing in samples obtained after calcination of MAVL samples), this value cannot correlate in any defined way with the specific surface areas for the other samples FTIR study. The FTIR spectra of the calcined products confirm the nature of the phases existing, as concluded from PXRD studies above The spectra for these same selected samples are shown in Fig 8 The broad absorption close to 3500 cm-', which is rather intense for sample MAVL1-700 and whose intensity decreases as the V content is increased (it is absent for samples MAVL6-700 and MAVI-700), should be assigned to vOH of adsorbed water molecules The observed change in the intensity of this band could be related to the change observed in the specific surface areas of the samples The weak bands close to 1500 and 1400cm-' in the spectra of samples MAVL1-700 and MAVL3-700 (and almost indis- tinguishable in sample MAVL6-700) are due to adsorbed carbonate species However, the main differences in these spectra appear in the 1100-400 cm- range, where differences exist with the spectra of the corresponding uncalcined samples (Fig 4) The spectrum for sample MAVL1-700 shows broad bands at 830,700 and 470 cm- ',mainly originated by vibration modes due to Mg-0 and A1-0 moieties 30 45 When the vanadium content is increased, sharper and better defined bands are recorded at 850, 703 and 462cm-', with a weak shoulder close to 913 cm-' The intensities of these bands increase as the vanadium content increases, ze with the Mg,(VO,), content, as concluded from PXRD, these absorp- tion bands are therefore associated with the presence of Mg-V oxide phases, and coincide with bands reported for such oxides 42 A rather different spectrum is recorded for sample MAVI- 700 In this case, the main (composed) band is recorded at 804cm-', together with weaker bands at 529 and 471 cm-', and a sharp, well defined absorption at 1007 cm-l These 1204 J Muter Chem, 1996, 6(7), 1199-1206 I I I I MAVL6-700 MAVLB -700I V U 1\MAVL 1 - 700 I m 3000 2000 1000 wavenumbedcm-l Fig. 8 FTIR spectra of samples MAVL1, MAVL3, MAVL6 and MAVI calcined at 700°C in air bands correspond to the presence of spinel (MgA1,0,) and MgV,O,, the crystalline phases also identified by PXRD.TPR measurements.The hydrogen consumption during TPR analysis of samples calcined at 700 "C corresponds roughly to a decrease of 1.8k0.1 in the oxidation state of vanadium.PXRD data indicate the presence of Vv compounds in all samples studied; therefore it can be concluded that during reduction, Vv species undergo transformation to V'" species, which are not reduced further. However, despite similar hydro-gen consumptions, the patterns are worthy of discussion. TPR profiles for samples MAVL1-700, MAVL3-700, MAVL6-700 and MAVI-700 are shown in Fig. 9. For samples MAVL-700 a single peak is recorded; as expected, the area under the peak (directly related to the hydrogen consumption) increases smoothly as the vanadium content increases; the ordinate values have been normalized to represent consumption per mass unit of vanadium.In addition, the position of the peak shifts towards higher temperatures as the vanadium content is increased, from 570°C for sample MAVL1-700, to 680°C for sample MAVL3-700, and to 730°C for sample MAVL6-700. 5 -IMAW-700 0 200 400 600 800 TI'C Fig. 9 TPR profiles of samples MAVI, MAVL1, MAVL3 and MAVL6 calcined at 700 "C in air Such a shift towards higher reduction temperature as the V content is increased has also been reported by Lopez Nieto et when reducing vanadium oxide catalysts prepared by impregnation of calcined (450"C) Mg,A1 hydrotalcite with vanadium-containing solutions. In contrast, sample MAVI-700 shows a complex feature, extending from 675 to 775 "C,where up to three different maxima and a shoulder can be dis-tinguished.In all cases, however, the hydrogen consumptions correspond to a decrease of two units in the average oxidation state of vanadium. The behaviour shown by the calcined MAVL samples is not straightforward compared to the behaviour of the uncalcined ones, as in the latter case no hydrogen consumption was observed, due to the existence of non-reducible V"' species; TPR experiments thus confirm oxidation during the calcination process. Following the conclusions of the PXRD study on these samples, the shift towards higher temperatures of the position of the reduction maximum indicates that the V-containing crystalline species existing in these samples, Mg3(V04),, becomes less reducible as their crystallinity increases.It has been shown previou~ly~~.~~that the onset temperature for reduction of Mg-V-0 compounds increases as a-Mg2V207<Mg,( VO,), <MgV2O6. In the present case, it is noteworthy that magnesium vana-dates existing in calcined sample MAVI are less reducible (i.e. its reduction takes place at a higher temperature) than in the uncalcined material. The difference in the behaviour of calcined samples MAVI and MAVL6 (both containing similar vanadium loadings) when subjected to reduction can be ascribed to the different structures of the V-containing species existing in these samples. Mg3(V04)2species (existingin samples MAVL-700)are formed by an oxide ion packing, where Mg" occupies octahedral holes and Vv are located in a tetrahedral oxide en~ironment,~,while in MgV206 pairs of distorted [VO,] octahedra sharing an edge (i.e.[V,O,,] units) form zigzag layers, with Mg" ions octahedrally coordinated between the layers." In Mg2V20, (identified during calcination of sample MAVI below 700 "C), pairs of [VO,] tetrahedra share a corner, forming [V,O,] units.56 Therefore, the reduction kinetics of isolated [VO,] tetrahedra, [V20,,] dioctahedra and [V,O,] ditetrahedra should be different, thus accounting for the different shapes of the TPR curves. Note that selective oxidation of propane to propene on pure Mg vanadates decrease as the onset temperature for their reduction decreases, i.e. a correlation seems to exist between the ease of reduction and the selectivity in the named reaction.34 Concluding remarks Hydrotalcite-like materials containing Mg" and V"' ions in the brucite-like layers, and carbonate in the interlayer, have been prepared.Although the syntheses of these was described previously by some of us,21.22in that case the syntheses were carried out in nitrogen, while in this case no special experimen-tal conditions were used to avoid V"' oxidation. Upon calci-nation in air, the material undergoes oxidation, forming magnesium vanadates. The nature of the V-containing species existing in the solids calcined at 700°C (when the lattice has become relatively stabilized), depends strongly on the nature of the starting hydrotalcite-like material, forming Mg3(V04)2 containing [VO,] tetrahedra when V existed in the starting material asv"' in the brucite-like layers, but MgV,O, containing [VO,] octahedra when V existed as decavanadate in the interlayer region.Therefore, solids obtained after calcination in air at 700°C are formed by mixed oxides with different immediate environments around the Vv ions. The simultaneous presence of Al"' ions in sample MAVI is unavoidable, and their possible effect on the crystallization process cannot be neglected. In J. Muter. Chem., 1996, 6(7), 1199-1206 1205 addition, some sort of ‘limiting reactant’ can also be taken into account, I e the V/Mg ratio could in some way determine the nature of the products formed However, this does not seem to be the case, as the Mg/V atomic ratio is larger in sample MAVI-700 than in samples MAVL3-700 and MAVL6- 700, but the crystalline phases found in sample MAVI-700 20 21 22 23 C Barriga, F Kooli, V Rives and M A Ulibarri, in Proceedings of ACS Meeting, Anaheim, Spnng 1995, in press V Rives, F M Labajos, M A Ulibarri and P Malet, Inorg Chem, 1993,32,5000 F M Labajos, V Rives, M A Centeno, P Malet and M A Ulibarri, lnorg Chem , 1996,35, 1154 W T Reichle, J Catal, 1985,94, 547 (MgV,O,) correspond to those with lower Mg/V ratios than those formed upon calcination of samples MAVL3 and MAVL6 (only MgO is detected by PXRD for sample Therefore, we can conclude that the nature and structure of the starting V-containing material (V”’ in octahedral coordi- nation in the brucite-like layers in samples MAVL, or Vv in decavanadate in sample MAVI), determine strongly the nature of the Mg-V-0 species formed upon calcination at 700°C The possible effect of the co-existence of Al”’ remains unclear The synthesis method developed may provide materials with different basicity (depending on the MgO/A1,03 ratio) and MAVL1-700) 24 25 26 27 28 29 30 31 P Malet and A Caballero, J Chem Soc, Faraday Trans 1, 1988, 84,2369 R Allmann, Chimia, 1970,24, 99 J E Huheey, E A Keiter and R L Keiter, Inorganic Chemistry Principles of Structure and Reactivity, Harper Collins, New York, 1993,4th edn, ch 4, p 121 G W Bnndley and S Kikkawa, Am Mineral, 1979,64,836 M Doeuff, T Kwon and T J Pinnavaia, Synth Met, 1989,34,609 V Rives, M A Ulibarri and A Montero, Appl Clay Sci, 1995, 10,83 F M Labajos, V Rives and M A Ulibarri, J Muter Sci, 1992, 27,1546 J M Lopez Nieto, A Dejoz and M I Vazquez, Appl Catal A General, 1995,132,41 different active species ([VO,] or [VO,]) for oxidative dehydrogenation catalysts 32 33 X Gao, Q Xin and X Guo, Appl Catal A General, 1994,114, 197 D Slew Hew Sam, V Soenen and J C Volta, J Catal, 1990, 123,417 F K acknowledges a grant from Ministerio de Educacion y Ciencia (Madrid, Spain, ref SB 92-AE0474743) and M A U 34 35 X Gao, P Ruiz, Q Xin, X Guo and B Delmon, J Catal, 1994, 148,56 F Kooli, V Rives, M A Ulibarri and W Jones, Muter Res Soc acknowledges a grant from Universidad de Cordoba Financial Symp Proc ,1995,371,143 support from CICYT (MAT93-0787), DGICYT (PB92-0665) and Consejena de Cultura y Turismo de la Junta de Castilla y Leon is gratefully acknowledged 36 37 W Kagunya and W Jones, in Multzfunctional Mesoporous Inorganic Solids, NATO AS1 Series C, No 400, ed C A C Sequeiera and M J Hudson, Kluwer, Dordrecht, 1993, p 217 T Kwon and T J Pinnavaia, Chem Muter ,1989,1,381 38 M J Hernandez-Moreno, M A Uhbarri, J L Rendon and References 1 2 3 4 5 6 7 A Corma, J M Lopez-Nieto, N Paredes, M Perez, Y Shen, H Cao and S L Suib, Stud Surf Sci Catal, 1992,72,213 A Corma, J M Lopez-Nieto, N Paredes and M Perez, Appl Catal, 1993,97,159 A Corma, J M Lopez-Nieto and N Paredes, Appl Catal, 1993, 104,161 M A Chaar, D Patel, M C Kung and H H Kung, J Catal, 1987,105,483 H H Kung and M A Chaar, US Pat, 1988,4,772,319 M del Arco, M J Holgado, C Martin and V Rives, J Muter Sci Lett, 1987,6,616 F Cavani, F Trifiro and A Vaccari, Catal Today, 1991, 11, 173 and references therein 39 40 41 42 43 44 45 46 C J Serna, J Phys Chem Miner, 1985,12,34 E C Kruissink, L L van Reijden and J R H Ross, J Chem Soc, Faraday Trans I, 1991,77,649 0 Clause, M Gazzano, F Trifiro, A Vaccan and L Zatorski, Appl Catal, 1991,73,217 K El Malki, Ph D Thesis, Clermont-Ferrand, France, 1991 F M Labajos, Ph D Thesis, Salamanca, Spain, 1993 F Kooli, V Rives and M A Ulibarri, Muter Sci Forum, 1994, L Pesic, S Salipurovic, V Markovic, D Vucehc, W Kagunya and W Jones, J Muter Chem ,1992,2, 1069 M del Arco, C Martin, I Martin, V Rives and R Trujillano, Spectrochim Acta, Part A, 1993,49, 1575 M A Ulibarri, J M Fernandez, F M Labajos and V Rives, Chem Muter, 1991,3,626 152-153,375 8 9 10 11 12 13 14 15 16 17 18 19 A de Roy, C Forano, K El Malki and J P Besse, in Expanded Clays and Others Microporous Solids, ed M L Occelli and H E Robson, van Nostrand Reinhold, New York, 1992, p 108 M A Drezdzon, Inorg Chem ,1988,27,4628 T Kwon, G A Tsigdinos and T J Pinnavaia, J Am Chem SOC, 1988,110,3653 J Twu and P K Dutta, J Catal, 1990,124,503 W Jones and M Chibwe, in Pillared Layered Structures Current Trends and Applications, ed I V Mitchell, Elsevier, London, 1990, p 67 E Narita, P Kaviratna and T J Pinnavaia, Chem Lett, 1991,805 J Wang, Y Tiam, R C Wang and C A Clearfield, Chem Muter, 1992,4,1276 E Lopez Salinas and Y Ono, Bull Chem SOC Jpn ,1992,65,2465 M A Ulibarri, F M Labajos, V Rives, R Trujillano, W Kagunya and W Jones, Inorg Chem ,1994,33,2592 F Kooli, V Rives and M A Ulibarri, lnorg Chem ,1995,34,5114 F Kooli, V Rives and M A Ulibarri, Inorg Chem ,1995,34,5122 F Kooh, K Kosuge and A Tsunashima, J Solid State Chem, 1995,118,285 47 48 49 50 51 52 53 54 55 56 J M Fernandez, C Barriga, M A Ulibarri, F M Labajos and V Rives, J Muter Chem ,1994,4,1117 I M Crespo, M Sc Thesis, University of Cordoba, Spain, 1994 G M Clark and R Morley, J Solid State Chem ,1976,16,429 M C Kung and H H Kung, J Catal, 1992,134,668 F M Labajos, V Rives and M A Ulibarri, in Multzfunctional Mesoporous Inorganic Solids, NATO-AS1 Series C, no 400, ed C A C Sequeiera and M J Hudson, Kluwer, Dordrecht, 1993, p 207 A Guerrero-Ruiz, I Rodriguez-Ramos, J L G Fierro, V Soenen, J M Herrmann and J C Volta, in New Developments in Selective Oxidation by Heterogeneous Catalysis, ed P Ruiz and B Delmon, Elsevier, Amsterdam, 1992, p 203 X Gao, P Ruiz, Q Xin, X Guo and B Delmon, Catal Lett ,1994, 23,321 N Krsihnamachari and C Calvo, Can J Chem ,1971,49,1630 H N Ng and C Calvo, Can J Chem, 1972,50,3619 R Gopal and C Calvo, Acta Crystallogr ,Sect B, 1974,30,2491 Paper 6/00237D, Received 1 lth January, 1996 1206 J Muter Chem, 1996, 6(7), 1199-1206

ISSN:0959-9428    

DOI:10.1039/JM9960601199

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Synthesis and characterization of a novel microporous aluminophosphate AIP0,-JDF (ZAlPO, HOCH2CH2NH2)from alcohol systems Qiuming Gao,"" Shougui Li,"Ruren Xu*" and Yong Yueb "Key Laboratory of Inorganic Hydrothermal Synthesis, Jilin University, Changchun 130023, P.R. China Wuhan Institute of Physics, the Chinese Academy of Science, Wuhan, P.R. China The synthesis of a novel aluminophosphate microporous compound AlP0,-JDF (where J means Jilin University and DF is Davy Faraday Laboratory) containing organic amine in the presence of the template HOCH,CH,NH, from predominantly non- aqueous media is described. Chemical and elemental analyses disclosed its idealized formula as 2AlP04 -HOCH2CH,NH,. AlP0,-JDF has been characterized by X-ray powder diffraction (XRD), IR spectroscopy, scanning electron microscopy (SEM), differential thermal analysis (DTA) and thermogravimetry (TG), and magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy.Scientists at the Union Carbide Corporation have synthesized a series of alumin~phosphates'-~ (designated AlPO,-n with n representing various structures), and the syntheses and struc- tural determinations of aluminophosphates and aluminoarsen- ates, etc., with frameworks as open as those of AlP04-n, have been reported. Some of these compounds are isostructural with known zeolites or other molecular sieves, e.g. AlP04-20 is the analogue of sodalite, A1Po4-24 is the analogue of analcime, A1Po4-37is the analogue of faujasite, A1P04-42 is the analogue of Na-A, CoA1P04-47 is the analogue of chabazite and AlP04- 17 is the analogue of eri~nite.~-" A large number of these compounds have been synthesized from aqueous systems thus far, and only a few of them are synthesized from non-aqueous systems." Here, we report the synthesis of a novel microporous aluminophosphate, AlP0,-JDF, the analogue of AlAs0,-1, from predominently non-aqueous media in the presence of EAN (EAN =ethanolamine) amine templating agents. Experimental AlP0,-JDF was synthesized from a predominantly non-aque- ous system.Aluminium triisopropoxide (Pr'O),Al and phos- phoric acid (85 mass% H3PO4) were used as the aluminium and phosphorus sources, respectively. (PI-'O)~AI was firstly stirred with HOCH,CH,OH, then EAN was added with stirring, and phosphoric acid was lastly added dropwise.The whole mixture was stirred until it became homogeneous. The final gel was transferred to an autoclave and heated under autogenous pressure. The crystalline product was filtered, washed with distilled water, and dried at ambient temperature. The typical gel molar compositions and crystallization con- ditions are shown in Table 1. Elemental analysis was performed on a Perkin-Elmer 240C element analyser. X-Ray powder diffraction data were collected on a Rigaku D/MAX I11 + diffractometer with Ni-filtered Cu- Ka radiation (A= 1.5418 A). The sample was scanned from 4 to 40" (28) at a scan speed of 0.5" min-'. The IR measurements were carried out on a Nicolet 5DX FTIR spectrometer using KBr pellets.A Perkin-Elmer DTA 1700 differential thermal analyser was used to obtain the differential thermal analysis (DTA) data, and a Perkin-Elmer TGA 7 thermogravimetric analyser was used to obtain the thermogravimetry (TG) curves in an air atmosphere. The heating rate was 20 "C min-'. 27Al MAS NMR spectra were recorded on a Varian XL-200 spec- trometer with a magnetic field strength of 4.7 T. The spinning rates were 3 kHz. The single-pulse excitation technique was applied and the spectra were obtained at 52.1 MHz. Other parameters were: pulse width, 0.5 ps; recycle delay, 10 s; number of transients, 1000. The chemical shifts were relative to the external standard A1(N03)3 aqueous solution. 13C and 31P MAS NMR spectra were measured on a Bruker MSL-400 spectrometer (9.4 T).The spinning rates were 5 and 6 kHz, respectively. The cross-polarization technique was applied. The contact time was 5.0ms and the scan number 1000 with a recycle delay of 14 s. The relaxation delays of 27Al, 31P and 13C were sufficiently long that the spectra were quantitatively reliable. The chemical shifts were relative to SiMe, and H3PO4 (85 mass %) respectively. Results and Discussion Formation and composition AlP0,-JDF or AlAsO,-l forms only in the presence of EAN as a template (the former comes from the alcohol media and the latter comes from the water media)." The preferred tem- perature for crystallization of AlP0,-JDF is 180°C. A large amount of AlP0,-tridymite forms and a little AlP0,-JDF is found >2OO"C. When the temperature is below 150"C, only an amorphous phase exists. The alcohol solvent may be ethylene glycol (EG), propane-1,2-diol ( 1,2-PG), propane-1,3- diol ( 1,3-PG), butane-1,4-diol ( 1,4-BG), triethylene glycol (TEG) or hexanol (HexOH).Chemical analysis and EDAX give rise to a P/A1 value of 1.0for the product. Element analysis indicates that the material contains 4.13 mass% N, 8.38 C, and 2.86 H with molar ratio N :C :H =0.42: 1.00:4.09. TG shows that the mass loss is 24.7 mass%. The empirical composition calculated on the basis of the above data is 2.00(AlP04)* 1.03( HOCH,CH,NH,), which can be normalized to 2AlP04 -HOCH,CH,NH,!. This formula corresponds to that of A1As04-1, the formula of which is 2A1AsO4 .HOCH,CH,NH,.X-Ray powder diffraction and SEM analyses The XRD pattern of the as-synthesized AlP0,-JDF is shown in Fig. 1. It is seen that the peak positions are essentially identical with those for the pattern of AlAsO,-l, except for the difference of peak inten~ities.'~ This indicates that AlP0,-JDF is the analogue of A1AsO4-1, which is orthorhombic, and whose space group is Pcab. SEM (Fig. 2) shows that the particles are aggregates of numerous small crystals. The average particle and crystal sizes are ca. 6 pm and 0.5 pm, respectively. The particles are homo- J. Muter. Chem., 1996, 6(7), 1207-1210 1207 Table 1 The typical gel compositions and crystallization conditions for AlP0,-JDF moles P,05" moles EAN moles organic solvent moles H,O ETN t/days ~ 2.4 13.5 60 EG 12 0.790 5 2.4 14.0 44 EG 12 0.790 7 1.8 6.8 34 1,3-PG 9 0.747 7 1.8 7.0 45 1,3-PG 9 0.747 14 1.8 6.8 33 1,2-PG 9 0.722 22 2.4 13.5 30 TEG 12 0.7 13 24 1.8 6.8 28 1,4-BG 9 0.704 25 1.8 6.7 20 HexOH 9 0.559 30 Molar content of A1,0,, 1.0; reaction temperature, 180°C.Expenmental polar constants (ref. 22). geneous and the crystal shape is similar, indicating that the as-synthesized solid is a pure phase. IR analysis The absorption bands of the skeleton of the product are at 1181, 1096, 1025, 709, 681, 632, 562 and 534crn-l, which are similar to those of AlAs0,-1 (Fig. 3). According to the frame- work vibration models of A1P04-n,14 the IR absorption spec- trum of A1P04-JDF can be assigned as follows: 1181, 1096 and 1025 cm-l are attributable to asymmetric stretching vibrations of PO4; 709, 681 and 632 cm-' are associated with symmetric stretching vibrations of PO4; 562 and 534 cm-I are assigned to the P-0 vibration (Table 2).Comparing the two IR spectra for AlP0,-JDF and AlA~0,-1,'~ we find that the 111111 10 20 30 40 Fig. 1 XRD patterns of AIP0,-JDF (a) and AlAs0,-1 (b) 1 I I I Y I 1 4OOO.O 3200.0 uoO.0 1900.0 1500.0 1100.0 m.0 660.0 400.0 wavenumber/cm-1 Fig. 3 IR spectra of A1As04-1 (a) and AlP0,-JDF (b) Table 2 Assignment of the IR bonds of the AlP0,-JDF skeleton crystal phase Y as Ys Y AlPOqJDF 1181 709 562 1096 681 534 1025 632 AlAsO4-I 1089 630 498 962 555 450 920 520Fig.2 Scanning electron micrograph of AlP0,-JDF 1208 J. Mater. Chem., 1996, 6(7), 1207-1210 frequencies of the AlP0,-JDF skeletal vibration bands are higher than those for the A1As04-1 vibration bands by ca. 70-100 cm -'. According to eqn. ( 1); T,, T, =P, As the radius of the As atom is larger than that of P atom and the electronegativity of the As atom is smaller than that of P atom, which results in the As-0 bond being less strong than the P-0 bond i.e.fAs-o<fP-o, and the atomic mass of the As atom is larger than that of the P atom, resulting in pAs-O'>h-Oo, so vP-O'vAs-O* According to ref. 16 and comparing the IR spectrum of AlP0,-JDF with that of AlAsO,-l, the bands of the template EAN are assigned in Table 3.The bands of EAN are similar to those of free EAN.17 The characteristic -(CH,), -in-plane rocking vibration band is at 740 cm-', which is very weak in the TR spectrum of AlP0,-JDF, and it is too weak to be observed in that of AlAs0,-1. *'Al, 31Pand I3CMAS NMR Fig.4 shows the experimental 27Al MAS NMR spectrum of as-synthesized AlP0,-JDF. The peaks centred at 6 ca. 31.9 and ca. -4.9 correspond to the two types of A1 atoms. For open framework aluminophosphates with occluded amines, tetrahedrally coordinated A1 atoms gave MAS NMR signals at chemical shifts between 6 20 and 50 and octahedrally coordinated A1 atoms gave ones at around or below 6 0.'s-20 Therefore, the signal of the as-synthesized AlP0,-JDF at 6 3 1.9 corresponds to tetrahedrally coordinated A1 and the signal at 6 -4.9 to octahedral Al.This assignment corresponds with the 27Al MAS NMR results for A1As04-1 and that of the single-crystal structural analysis.21 The 27Al MAS NMR results also show that the template, EAN, has the same sites in both Table 3 Assignment of the IR bonds of the EAN template AlPO4-JDF AlAsO4-1 mode band intensity' band intensity 'IN -H 3400 vw 3402 W "0-H 3 165 S 3163 S "C ~ H 2953 m 2952 m 4-H (in-plane) 1609 S 1609 S (out-of-plane) 1510 S 1510 S dO-H (in-plane) "C -N 1462 984 m m 1462 980 m m b'c -0 885 W 87 1 W -(CH,P 740 vw 'vw =very weak, w =weak, m =medium, s =strong. Band of charac-teristic -(CH,), -in-plane rocking vibration.31.9 I i n a I I 1 I 100 50 0 -50 -100 6 Fig. 4 "A1 MAS NMR spectrum of AlP0,-JDF AlP0,-JDF and AlAs0,- 1, i.e. the four-coordinated A1 shares four oxygen atoms with four adjacent P atoms and the six- coordinate A1 not only shares four oxygen atoms with four adjacent P atoms but also is double bridged with another equivalent A1 atom by two oxygen atoms or nitrogen atoms of EAN molecules located in two eight-membered ring chan- nels. Because of the different bond strengths of P-0 and As-0, the peaks of the as-synthesized AlP0,-JDF are at lower field than those of AlAsO,-l, nearly 6 ca. 9.5 and ca. 6.6, respectively. The 31PMAS NMR spectrum (Fig. 5) of the as-synthesized AlP0,-JDF exhibits two distinct lines located at 6 ca.-13.8 and ca. -30.6 with an intensity ratio of 1 :1. The structure of the as-synthesized AlP0,-JDF molecular sieve with occluded template is shown in Fig. 6. Owing to the presence of template HOCH,CH,NH,, two different P sites are present in the AlP0,-JDF structure: one is due to P(a) and P(b) linked with two six-coordinate A1 and one four- coordinate A1 (only coordinated A1 in the plane are considered because the out-of-plane A1 is the same for both of the two P sites), the other corresponds to P(c) and P(d) linked to one six-coordinate A1 and two four-coordinate Al. Because the symmetry of P(a) and P(b) is altered more than that of P(c) and P(d) comparing with external H$O, (85 mass%), the peak at 6 -30.6 arises from P(a) and P(b) and the one at 6 -13.8 is assigned to P(c) and P(d).The as-synthesized AlP0,- JDF only gives one 13C MAS NMR signal at 6 ca. 52.9, showing that the template is in the channel. TG and DTA Thermogravimetry (Fig. 7A) indicates that the total mass loss is 24.7 mass% from 281 to 698"C, corresponding to the -13*8 I I -30.6 I I I I 1 100 50 0 -50 -100 6 Fig. 5 31PMAS NMR spectrum of AlP04-JDF Fig. 6 Structure of as-synthesized AlP0,-JDF molecular sieve with occluded template. Two different P sites are present in the AlP04- JDF structure: one is due to P(a) and P(b) joined with two six- coordinate A1 and one four-coordinate A1 (only coordinated A1 atoms in the plane are considered because the out-of-plane A1 is the same for both of the two P sites); the other corresponds to P(c) and P(d) joined to one six-coordinate A1 and two four-coordinate Al. J.Muter. Chern., 1996, 6(7), 1207-1210 1209 50.0 B I endo It 1 1nnl 11 I I 1 V." 30.0 190.0 350.0 510.0 670.0 830.0 TI% Fig.7 A, TG (a) and DTG (b) curves, and B, DTA curve for AlPO4- JDF desorption of template EAN The first step is from 281 to 400 "C, and the progress of mass loss is very fast, corresponding to the destruction of EAN Meanwhile, a small amount of carbon produced by the decomposition of EAN aggregates at the sample The second step is from 400 to 698"C, and the mass loss proceeds more slowly, corresponding to the oxidation of the aggregated carbon DTA (Fig 7B) also shows this progress the endothermic peak at 384°C corresponds to the first process and the broad exothermic peak at 590°C corre-sponds to the second process This result is in good agreement with the XRD result Note that AlP0,-JDF retains its structure after calcination at 300°C It collapses to an amorphous form above 400 "C Conclusion AlP0,-JDF is a new member of the family of aluminophos- phates based on microporous materials, and it has been successfully synthesized from predominantly non-aqueous solutions The results of investigating the crystallization of AlP0,-JDF are summarized and compared with those of AlAsO,-l 31P MAS NMR spectroscopy shows that the P atoms have two types of sites because of the presence of the template HOCH2CH,NH2 one type of P atoms joins with two six-coordinate A1 and one four-coordinate A1 (only in- plane coordinated A1 are considered because the out-of-plane A1 are the same for both the two types of P sites), the other type of P atoms are attached to one six-coordinate and two four-coordinate A1 The most important feature of this study is that the non- aqueous synthesis technique is very important for the synthesis of microporous materials We believe that other known and unknown structural microporous materials will be synthesized by this technique, which will contribute greatly to the under- standing of the nature and chemistry of microproducts We thank the National Natural Science Foundation of China and the Ph D Studentship Foundation of the State Education Commission for financial support References 1 S T Wilson, B M Lok and E M Flanigen, US Pat, 1982, 4310440 2 E M Flanigen, B M Lok, R L Patton and S T Wilson, Pure Appl Chem, 1986,58,1351 3 M E Davis, C Saldarriaga, C Montes, J Garces and C Crowder, Nature (London), 1988,331,698 4 J Felsche, S Luger and C Baerlocher, Zeolites, 1986,6, 367 5 G Ferraris, D W Jones and J Yerkess, Z Kzstallogr, 1972, 135, 240 6 D H Olson, J Phys Chem, 1970,74,2758 7 M L Costenoble, W J Mortier and J B Uytterhoeven, J Chem Soc Faradav Trans I, 1976,72,1877 8 B M Lok, C A Messina, R L Patton, R T Gajek, T R Cannan and E M Flanigen, J Am Chem Soc , 1984,106,6092 9 J J Pluth and J V Smith, J Phys Chem, 1989,93,6516 10 J M Bennett and B K Markus, Proc Int Symp on Innovations in Zeolite Materials Sciences, Belgium, 1987, ed P J Grobet, W J Mortier, E F Vansant and G Schulz-Ekloff, 1988, p 269 11 Q Gao, S Li and R Xu, J Chem Soc Chem Commun, 1994.1465 12 G Yang, L Li, J Chen and R Xu, J Chem Soc Chern Commun, 1989,810 13 Collection of Simulated XRD POM ders Patterns for Zeolites Zeolites, 1990,10,I344S 14 E M Flanigen, H Khatami and H A Szymanski, Adv Chem Ser ACS, 1971,101,201 15 J Chen, Ph D Thesis, Jilin University, Changchun, 1989 16 Qinghan Jin, Instrument Analysis, Jilin University, 1990 17 C J Pouchert, The Aldrich Library of Infrared Spectra, Aldrich Chemical Company, 1981. p 197 18 D Muller, E Jahn, B Fahke, G Ladwig and U Haubenriisstr, Zeolites, 1985, 5, 53 19 I P Appleyard, R K Harris and F R Frich, Zeolrtes, 1986,6,428 20 C S Blackwell and R 1 Pation, J Phys Chem, 1988,92,3965 21 J Chen, Ph D Thesis, Jilin University, Changchun, 1989 22 C Reichardt, Solcents and Solvent Effects in Organic Chemistry, Verlag Chemie, Weinheim, 2nd edn ,ch 2, 1988 Paper 51069985, Received 23rd October, 1995 1210 J Mater Chem, 1996, 6(7), 1207-1210

ISSN:0959-9428    

DOI:10.1039/JM9960601207

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Intercalation reactions of UTeO and USeO Peter G. Dickens Ewan P. Stradling Christopher A. Bearchell and Ian D. Fawcett Inorganic Chemistry Laboratory South Parks Road Oxford UK OX1 3QR New intercalation compounds of the pseudo-layered oxides Uv'Te'v05 and UV'Se'V05 have been prepared and characterised At ambient temperatures products of the type AxUM05 (x< 1 A =Li Na H NH,) are formed by both host oxides with minimal structural change Magnetic susceptibility measurements confirm that Uv species are formed on intercalation Several metal oxides MO where M is a transition metal (or uranium) in a high oxidation state have tunnelled or layered structures which undergo insertion or intercalation reactions at ambient temperatures. These result in the formation of products AxMO in which a electropositive element A is incorporated interstitially into the host oxide with minimal structural rearrangement (at least for small values of x).The intercalation process is invariably accompanied by a redox reaction A =A +e M with incorporation of the guest species as a cation and the formation of a mixed valence compound A necessary condition for the facile incorporation of a guest species into an oxide host is that the host structure should possess interconnected voids sufficiently large both to accom- modate the guest without significant structural change and to enable ion transport between alternative sites to occur An important electronic requirement is that the metal M should be in a readily reducible oxidation state Most of the transition- metal oxide hosts so far investigated' have framework rather than layered structures and are capable of incorporating only smaller cations such as those of Li Mg and Zn without undergoing irreversible structural change or amorphisation Uranium oxides and mixed oxides such as UO U308 and UTiO behave similarly but provide cavities large enough to incorporate the larger Na ion2 without significant structural change For the most part however the rigid tunnelled or pillared-layer structures of the framework oxides do not pro- vide suitable pathways for rapid migration of guest species and they behave as much less versatile hosts than do the well known layered metal dichalcogenides which have the structural flexibility to expand reversibly in a direction perpendicular to the layers and thus accommodate a much wider range of guest species.The only simple binary oxide which has a true layer structure is MOO In the representation of Its structure in Plate 1 full lines are drawn between Mo-0 linkages to show conventional chemical bonds which have been defined oper- ationally by Altermatt and Brown4 for these and for the other M-0 bonds shown in Plate 1 as ones with bond valences s30 1 (here s is calculated from the measured M-0 bond length r and from tabulated parameters ro and B through the expression s =exp {Jro-r)/B}). The shortest interlayer Mo-0 distance is 3 55 A (s=O 012) and the neutral layers are judged to be separated by a van der Waals' gap In its intercalation chemistry MOO behaves similarly to the metal dichalcogenides and can incorporate and ion-exchang? a wide range of larger cations causing changes of several A in the interlayer separation to occur but with simultaneous retention of the intralayer structure V,O has a structure superficially very similar to that of MOO (Plate 1) but in this ctse weak interlayer bonds between V and 0 of length 2 79 A corre- sponding to a larger bond valence value s =0 07 are presumed to be present (shown by broken lines) which cause the oxide to behave as a framework structure in its intercalation chemis- try Intercalation of Li Mg Zn or H causes only very small changes in lattice parameters (for x< 1) in the formation of AxV205 at ambient temperatures However attempts at the insertion of larger cations such as those of K or NH lead to an irreversible transformation into a new lamellar phase of different structure and stoichiometry A +xV308 In the present work we seek to extend knowledge of the intercalation chemistry of the oxides and mixed oxides of uranium by investigating the reactions undergone by two layered or pseudo-layered compounds UTeO and USeO which meet both structural and electronic requirements to act as host materials.These are well characterised oxides of Uv' whose structures have been determined by X-ray and neutron diffraction,67 they are shown (viewed along [OOl]) in Plate 1 In UTeO edge-sharing chains of U07 bipyramids are joined into neutral buckled layers by TeO units as shown sche- matically in plan (viewed along [loo]) in Fig 1. The local arrangement around uranium consists of five equatorial oxygens in the form of a distorted pentagon and two axial oxygens situated above and below the plane constituting a discrete uranyl group characteristic of Uv'. The tellurium atom is surrounded by four oxygen atoms in a distorted trigonal bipyramidal arrangement with the empty equatorial site occu- pied by a lone pair of electrons.This stereochemistry is common for Te" and is present eg in ct-Te02. The layers are stacked in registry to complete the structure shown in Plate 1. The shortest interlayer Te-0 distances are at 3 11 and 3 18 A with bond strengths s<OO5 and thus lie outside the range of normal chemical bonds Accordingly the structure has been drawn as a layer structure USeO has a closely related structure but the local arrangements about uranium and selenium differ slightly from those for UTeO In USeO edge-sharing U08 hexagonal bipyramids form parallel chains which are held together by trigonal pyramidal SeO units to form buckled layers of composition USeO,.This is shown in schematic form in plan in Fig 1 Again in the complete structure the buckled layers are stacked in regisJry (Plate 1) The shortest interlayer Se-0 distance is 3 06 A which also falls outside the range of conventional chemical bonds with s<O 05 Both UTeO and USeO are electronic insulators non-magnetic and pale yellow in colour which is compatible with their electronic formulation as UV'M'V05 in chemical terms Uv' is readily reducible in the solid state whereas Te" and SeIV are not In the work described below UTeO and USeO are shown to act as hosts for intercalation reactions at ambient tempera- tures with Li Na H and NH as guests Products have been characterised by chemical and structural analysis and associ- ated changes in electronic properties monitored by measure- ments of magnetic susceptibilities Experimental Preparations UTeO and USeO,.UTeO was made by two methods In the first following Kh~dadad,~ UTeO was formed as a J Muter Chem 1996 6(7) 1211-1217 1211 Plate 1 Structures of layered and pseudo-layered oxides. (a) MOO,; (b)V,O,; (c) UTeO,; (d) USeO,.precipitate from aqueous solution by adding a saturated solution of uranyl acetate dihydrate to a cooled wet precipitate of tellurous acid? (U:Te =2 1) and by subsequent stirring of the resulting suspension for 72 h. After careful washing the product was dried slowly at 100"C and finally heated at 400 "C to give a polycrystalline yellow powder. In the second method UTe05 was made by a direct solid-state reaction between stoichiometric proportions of TeO and amorphous UO in a sealed evacuated silica tube the final heating being at 700°C for 24 h. Amorphous U03 was made by heating U04.2H,0 lo in air at 350 "C for 1 h.. The products obtained by both methods were the same though that made by the direct route was the more crystalline..The powder X-ray diffraction patterns were indexed on an orthorhombic cell7 and refined lattice param- eters are given in Table 1. USeO was made from a mixture containing SeO and amorphous U03 (U :Se=1:1.1)by heat- ing in a sealed tube at 360°C for 12 h. Excess SeO was removed on further heating of the product by sublimation into the cool empty part of the tube.. The product which hydrates readily was handled in an argon-filled glovebox and had a powder X-ray diffraction pattern which could be indexed on a monoclinic unit cell;7 refined lattice parameters are given in Table 1. t. The 'tellurous acid precipitate was made beforehand by dissolving a known mass of TeO in the minimum amount of hot concentrated hydrochloric acid to give a viscous liquid from which hydrated TeO was subsequently formed as a suspension by addition of a large excess Fig.1 Plan views of (a) USeO and (b)UTeO of ice-cold water. 1212 J. Muter. Chem. 1996 6(7) 1211-1217 Table 1 Lattice parameters of UTeO USeO and intercalation compounds x iY) ,/A b/A CIA Pidegrees ~~ ~ UTeO 5.356(2) Li,UTeO 0.39 5.37(3) 0.85 5.35(2) 1.02 5.37(1) 1.1" 5.34(1) Na,UTeO 0.83 5.36( 1) H,UTeO 0.80 5.32(1) 1.1 5.35(2) (NH,),H,-,.UTeO 0.71 (0.30) 5.36(1) USeO 5.409( 2) Li,USeO 0.35 5.40( 1) 0.58 5.40( 5) 0.68 5.41( 1) Na,USeO 0.16 5.41 (1) H,USeO 0.61 5.40( 1 ) (NH,),H ,USeO 0.80 (0.19) 5.40(1)~ ~ "Electrochemically formed product. Intercalation compounds. Chemical intercalation reactions of UTeO and USeO involving lithium and sodium were carried out at ambient temperature under nitrogen on a Schlenk line using Bu"Li in hexane and sodium benzophenone in tetra- hydrofuran respectively as reagents..The procedures used followed those described previously for the preparation of intercalates of other oxides of Products were filtered and washed with the dried solvents in situ and finally vacuum-dried. In the case of the sodium-containing products unreacted sodium was first removed by washing with propan- 2-01.. The dark products of targetted compositions A,UMO with 0 < .Y < 1 were treated as air- and moisture-sensitive and stored in an argon-filled drybox prior to characterisation. Hydrogen intercalation compounds were prepared on a vacuum line by reaction of the parent oxide impregnated with finely dispersed platinum as catalyst with hydrogen gas at ambient temperature and atmospheric pressure.12 Quantitative absorption of gas by the parent oxide was determined mano- metrically.For the preparation of H,UMO (x z 1; M =Te Se) oxide samples containing CU. 2% Pt by mass were used. Most of the reaction with hydrogen was completed in a few hours but samples were then allowed to equilibrate and reach their final targeted hydrogen contents over a further period of 1 or 2 days before any surplus gas was removed rapidly and the sample tubes sealed and transferred to a glove box. For the preparation of ammonia-containing intercalation compounds (NH3),,H,UM05 the freshly prepared hydrogen-containing samples H,UMO were retained in the vacuum system and then exposed to ammonia gas from a connected gas burette.The subsequent reaction at ambient temperature and atmos- pheric pressure was again monitored manometrically to com- pletion. No reaction was observed to take place when the parent oxides UTeO and USeO were exposed to ammonia gas under the same conditions. All products were chemically analysed and their powder X-ray patterns recorded.. The X-ray diffraction patterns revealed the products to be still crystalline except in the case of one high sodium content sample Na,UTeO (x E 1.7) which was amorphous to X-rays. All the other samples gave patterns which could be indexed readily through comparison with those of the parent oxides and refined to give new lattice parameters which changed very little either with the extent of intercalation or with the nature of the inserted species (Table 1).Li and Na contents were determined by AA (or ICP) spectroscopy and also by redox titration using acidified ceric sulfate solution as oxidant. Agreement between Li contents (x values) determined by the two methods was within 5%; for Na where larger discrepancies occurred the redox value was chosen as being the more accurate.. The hydrogen contents in H,UMO 10.108( 3) 10.10(2) l0.05(2) 9.98(2) 10.1 l(2) 10.1 1( 2) 10.09(1) 7.844( 2) 7.83( 1) 7.84( 1) 7.87( 1) 7.84(2) 7.85(2) 7.84( 1 ) 10.1 l(5) 10.09(1) 7.84(4) 7.83(1) 9.284( 3) 9.27( 1) 9.27( 1) 9.29( 1) 9.27(1) 9.27 ( 1) 9.27( 1) 4.25 1(2) 4.25( 1) 4.26( 1 ) 4.25( 1 ) 4.26( 1 ) 4.25( 1) 4.25( 1) 93.46(5) 93.46(8) 93.48(10) 93.46(8) 93.47( 10) 93.45( 12) 93.47( 9) samples determined by the measured volumetric uptake of gaseous hydrogen were confirmed by redox titration.For the ammonia-containing samples ammonia uptake was recorded volumetrically and the total hydrogen and nitrogen contents in the products confirmed using a Carlo Erba 1106 CHN analyser.. These data established the empirical formulae for the products (NH,),H,UMO shown in Table 1. Further redox titrations with ceric solution confirmed that uranium oxidation states in the ammoniated products and the original starting materials H,UMO were unchanged.Errors in x and y are estimated at a 0.05. IR spectra of all compounds were recorded in the range 400-4000 cm-using a Fourier transform spectrometer (Perkin-Elmer 1710 or Galaxy 6020); samples were ground with excess dried CsI or KBr in a glove box and pressed into discs. Electrochemical measurements Simple two-electrode non-aqueous cells with propylene carbon- ate (PC) as solvent of the type Li(s)(O.Smol dm-3 LiClO PCILi,UMO,(s) were housed in an argon-filled glovebox and discharged galvanostatically under computer control enabling several cells to be monitored simultaneously.. The construction and usage of such cells have been described previously." Open-circuit measurements were made at coulometrically determined values of x by interrupting the current and allowing the cell potential to relax to a constant value..The open-circuit values so determined for the cell Li(Li+ IUTeO are shown in Fig. 2. 3.4 I 13-2k3 2.4 .. 2.2-I 0 0.2 0.4 0.6 0.8 1 X Fig.2 Cell emf us. x for LilLiC10,,PC)Li,UTe05. - measured emf for xLi+UTeO,=Li,UTeO,; -- calculated emf for Li + UTeO = LiU03+ Te02. J. Muter. Chern. 1996 6(7) 1211-1217 1213 Magnetic measurements Magnetic susceptibilities of several intercalation compounds and of the parent oxides UTeO and USeO were measured over the temperature range 5-300 K and at field strengths of 0 1-1 T using a model S600C SQUID susceptometer (Cryogenic Ltd) Raw values were corrected for atomic diamag- netic contributions and the data converted to molar susceptibil- ities xrn(T)Results are presented in Figs 3-5 and in Table 4 (later) Results and Discussion Intercalation reactions of UTeO and USeO The ambient-temperature intercalation reactions undergone by the host oxides leading to incorporation of Li Na H and 0 50 100 150 200 250 300 14 I 1 0-c I 0 50 100 150 200 250 300 TIK Fig.3 (a)xrn us T for Li,USeO x =O (M) 0 35 (0),0 58 (A),0 68 (0)(b)xm-' us T for Li ,,USeO A Measured - calculated 90 1 1 60'Ot 1 0 50 100 150 200 250 300 14 12 m 2 06 02 0 50 100 150 200 250 300 TIK Fig. 4 (a) xm us T for Na ,,UTeO (b)xrn-' us T for Na 83UTe05 0,measured - calculated 1214 J Muter Chem 1996 6(7) 1211-1217 7.30 lx (a) O+ 0 50 100 150 200 250 300 0 50 100 150 200 250 300 TIK Fig. 5 (a) xrn us T for H ,UTeO (b) zrn us T for H ,UTeO x ,measured - calculated LiJJMO BunLit Fig. 6 Intercalation reactions of UMO (M =Te Se) NH4 are summarised in Fig 6 At the levels of insertion x reported below all samples were polycrystalline and as far as could be judged by close scrutiny of the measured X-ray powder patterns all appeared to be single-phase products although loss of intensity and broadening of peaks at higher x values made it difficult to detect small structural changes reliably Refined lattice parameters of the new compounds which have been made and characterised are given in Table 1 The topotactic intercalation reactions undergone by UTeO and USeO with lithium hydrogen and sodium are similar to those observed previously' for UO U308 UTiO and V205 (though ambient-temperature sodium intercalation has not been reported for V205). The reaction of H,UMOS with gaseous ammonia results in the intercalation of an ammonium ion subsequently identified by vibrational spectroscopy (see below) and the reformulation of the product as (NH4)yHx-,UTeOS ( y z 0 4) In this respect H,UTe05 and H,USeO resemble H,MoO l3 and other layered protonic acids in reacting with bases such as ammonia and amines by proton transfer and intercalation of the corresponding cation However the most striking structural feature provided by these data for intercalation is that the changes in lattice parameters presented in Table 1 for both UTeO and USeO as hosts are insignificantly small even for intercalation of the larger cations of sodium and ammonium.This suggests strongly that both USeO and UTeO have inflexible framework struc- tures and their intercalatlon behaviour for the same guest species contrasts with that shown by layered host materials such as MOO or the metal dichalcogenides for which intercal- ation is invariably accompanied by changes in interlayer spacings In the host UTeO structure shown in Plate 1 (space group Pbcrn 2=4) the empty special sites 4b (1/2 1/2 1/2) and 4c (x,1/4 1/2) areFach surrounded by six oxygen atoms at distances of 2 2-2 7 A and provide large enough cavities to accommodate Li or Na ions without interlayer expansion A similar situation is found for USeO However tte bottlenecks connecting such sites are not more than ca 4A in diameter which is too small to allow facile ionic transport within a rigid framework except in the case of lithium An attempt to ion exchange (NH,),H -,UTe05 with potassium ions through prolonged boiling with a concentrated KC1 solution was unsuccessful implying that ion mobility for larger ions was frustrated by the absence of a flexible layer structure in the host material All the intercalation reactions investigated pro- duced a noticeable colour change from the starting pale yellow of the host oxides to the dark brown or black of the products The corresponding magnetic changes and the evidence they provide for electron transfer on intercalation are discussed below Vibrational spectroscopy The IR absorption spectra of UTeO and USeO are shown in Fig 7 To assist assignment the fundamental vibration I I 1000 I 900 I 800 I 700 I 600 1 500 n 1 I I I I I I' 3500 3000 2500 2000 1500 1000 500 v/cm-l Fig.7 (a) IR spectrum of USeO (b) IR spectrum of UTeO (c) IR spectra of (i) HI lUTeO and (it) (NH,) 36HO 67UTe0 frequencies and atomic displacement coordinates were simu- lated for USeO within the simple valence force field (SVFF) approximation using the Sotonvib program14 with force con- stants chosen from literature data for U'I-0 and Se"-O bonds.The good agreement between calculated and meas- ured frequencies observed in the IR spectrum of USeO is shown in Table 2 Vibrational modes are labelled by both the factor group symbols for the extended solid (C,,) and also by symbols for the discrete ions (U02)2+and (Se0,)'- located at sites with associated symmetries C and C respectively The very similar spectrum of UTeO was assigned by analogy without further detailed calculation In both spectra the two highest observed frequencies in the range 1000-850 cm-' were unambiguously assigned to the antisymmetric (vj) and symmetric (vl) bond stretches of the UO (uranyl) group the appearance of both vibrations as IR active modes implies that U is not at a strictly centrosymmetric site either in UTeO or USeO (as the reported space group of the latter requires) The numerical simulation enabled the vibrational spectrum of the parent oxide to be related directly to identifiable structural features and thus allowed the effects of intercalation on the U-0 or Se-0 bond strengths to be monitored spectroscopi- cally. This is illustrated by consideration of the spectra observed for H,UTeO and (NH4),H,-,UTe05 (Fig 7) For H,UTeO (xz 1) the uranyl stretching frequencies are unchanged at 940 and 880cm-' which implies that the O=U= 0 (uranyl) bonds remain intact and an oxy-hydroxide U02Te02(OH) is the likely product with H attaching as -OH to an 'in-plane' 0 (Fig 1) Oxy-hydroxides of this type are the normal products of hydrogen insertion into transition-metal and actinide oxides at ambient temperatures for low levels of insertion l8 For example the analogous reaction between atomic hydrogen and Moo3 leads to the formation of MOO,-,(OH) for xd 1,with hydrogen attached to a bridging oxygen leaving the molybdenyl group and its characteristic vibrational frequency unaffected l3.The 6-UOH bending fre- quency occurs in UO,(OH) at 902 cm-',I9 and the presence of a frequency of a similar magnitude in UO,TeO,(OH) would be obscured by the strong uranyl stretching absorptions found in this region (the small peak recorded at 1625 cm-l is common to other spectroscopy samples and is ascribed to adsorbed water). The Te-0 stretching frequencies remain almost unshifted from those of the parent oxide. The corresponding spectrum of (NH,),H,-,UTeO however shows a new and distinctive feature.This IS the appearance of a strong band at 1409 cm -l characteristic of the fundamental deformation fre- quency (v4) of the ammonium ion in solids. The smaller peaks at 3152 and 3031 cm-l are assigned to the v3 and v1 stretching modes of the same ion the latter being IR active only for a site symmetry lower than Td Frequencies below ca 400 cm ' which were not readily observable with the IR instruments used were measured in a parallel inelastic neutron scattering (INS)experiment on the same compound2' and a strong peak at 380 cm-' was observed by analogy with the analysis other ammonium salts this is ascribed to the fundamental of the ammonium ion librational mode (v6). The corresponding librational mode in NH,Br falls at 350cm-1,21 and in the layered intercalate NH4U02P043H20 at 420 cm-' 22.These data all support the conclusion that H,UTeO acts as a Brransted acid towards NH3 leading to the formation and incorporation of an interstitial ammonium ion in the host structure there are no spectroscopic or analytical data to support the incorporation of free molecular ammonia in the intercalation compound As in the case of the formation of H,UTeOS the unchanged stretching frequencies in the range 1000-500cm-1 confirm that no significant changes in the intralayer U-0 or Te-0 bond arrangements take place on intercalation Similarly the alkali-metal intercalation reactions with both UTeO and USeO produced no significant shifts in the main U-0 and M-0 stretching frequencies although J Muter Chern 1996 6(7),1211-1217 1215 Table 2 Vibrational analysis of USeOj (space group P2,/m 2=2) symmetry labels 973 970 918 910 824 823 709 742 (broad) 710 487 492 471 472 stretching force constants used ducJ/A 172 klmdyni 7 81 O=U=O O=U=O antisymmetric stretch symmetric stretch Se-0 stretch Se-0 stretch Se-0 stretch Se-0 deformation U-0 in-plane stretch Se-0 deformation U-0 in-plane stretch 2 39 2 46 2 64 dSA 168 173 0 71 0 55 0 10 4 15 4 10 All angle-deformation force constants were taken as 0 20 mdyn A rad-2 (mdyn =aJ A I) peaks tended to broaden and lose intensity.This result has the implication that electron transfer to the host which accompanies intercalation is to non-bonding rather than to M-0 antibonding orbitals a conclusion which is supported by the magnetic data provided below Energetics of intercalation The smooth fall in the open circuit E us x plot (O<x,<O5) for the cell LiILi' IUTeOs shown in Fig 2 is consistent with solid-solution formation and the retention of a single phase in the product Li,UTeO,.The plateau in E at higher x values suggests the subsequent co-existence of two phases in the product but as stated previously any changes in lattice parameters associated with the formation of a new phase were too small to be detected by powder X-ray diffraction A similar shaped E us x curve was obtained for the cell Li(Li' IUSeO under galvanostatic conditions at low current densities but the extremely slow intercalation rates encountered made relaxation to open-circuit values unreliable and thermodynamic data were not collected for this system Numerical integration of the E us x data in the solid-solution regime shown in Fig 2 enabled the corresponding integral free energy of intercalation AGx to be evaluated for the reaction xLi(s)+UTeO,(s)+Li,UTeO,(s) (x GO 5) The value obtained for the energy of insertion per mol of lithium at low x value is compared in Table 3 with values reported previously for lithium intercalation into other uran- ium oxides and mixed oxides as hosts at 298 K ' These favourable energy changes provide the thermo-dynamic driving force for the intercalation of lithium into the host oxides but at elevated temperatures where kinetic con- straints to major structural change are removed conversion to new products of lower energy may occur.The cell potential for one such alternative reaction Li +UTeO -+LiUO +TeO was calculated from available standard thermodynamic data at 298 K by making the assumption that Ez -AH/F for reactions between solid phases. This value is plotted in Fig 2 and lies below the open-circuit voltage for the monitored cell Table3 Energies of insertion at low x for lithium intercalation into uranium oxides and mixed oxides oxide (AG,/x)/kJ mol-' (k5) UTeOs -270 UTiO -290 Y-UO -310 r-U,O -298 reaction at low values of x implying that the intercalation compound Li,UTeO is thermodynamically stable with respect to the proposed decomposition products in this range However the two potentials are seen to converge at higher x values suggesting that the intercalation compound at higher lithium contents becomes increasingly susceptible to decompo- sition to these or other neighbouring phases It was observed that heating a sample of Li,UTeO (x z1 ) in a sealed tube at 500°C led to its decomposition and the appearance in the products of a component readily identified by powder X-my diffraction as an fcc phase with lattice parameter a =1070 A other products in the mixture caul< not be identified. The cubic phase was not UO (a= 547 A) but its powder X-ray pattern matched exactly that of a known U0,-related phase of approximate composition L12U4O11 which is formed by the removal of Li,O from LiUO on heating2,.This would be compatible with the following possible decomposition path at elevated temperatures 4LiUTe0 .+[4LiU0 +4TeO,] -+ Li2U4O1+Li,Te03 +3Te02 In contrast the cathodic product of ambient-temperature electrochemical intercalation when removed from the cell (at Y z 1) was shown by powder X-ray diffraction to have retained the original UTeO structure (Table 1) Electronic structures and magnetism The molar magnetic susceptibilities xm,of a range of chemically prepared intercalation compounds of UTeO and USeO of known compositions were measured in the temperature range 5< T/K <300 and typical xm 1;s Tplots are shown in Fig 3-5 Analysis of these data shows that over the higher temperature range (200<T/K <300) the Langevin-Debye relationship j = (C/T)+A(where C is the Curie constant and A a temperature- independent paramagnetic term) is followed for all the intercal- ation compounds.This behaviour is predicted theoretically by the Van Vleck equation for a magnetically dilute assembly of paramagnetic centres with degenerate ground states which are well separated in energy from their nearest excited states. The values of the constants C and A for the compounds studied are given in Table 4 If the assumption is now made that the number of paramagnetic centres (presumed to be Uv) present in the formula unit A,UMO is equal to Y the effective magnetic moment per Uv is given by peff/pB= (3kC/N,,~~~p~x)~~~,where C and numerical values of the stan- dard physical constants are expressed in SI units.The values thus determined are included in Table4. The fully oxidised parent oxides UTeOs and USeO are effectively non-magnetic showing only weak temperature-independent paramagnetism over the whole range studied. The paramagnetic intercalation 1216 J Muter Chem,1996 6(7) 1211-1217 Table 4 Magnetic data for A,UMO compound TIK C/lO-' m3 mol-' K A/lO-'l m3 mol-' PeffIPB ~~~ ~~ ~~~ UTeO 50-300 0 Li,,,,UTeO,Lie.95UTeO Li .ozUTe05 250-300 250-300 250-300 431 605 646 H1.01UTe0s 250-300 559 Na0.83UTe05 250-300 387 USeO 50-300 0 Li,~,,USeO 250-300 464 Li,,,,USeO 250-300 524 Lio.,,USe05 250-300 770 Lio,68USeOs 250-300 919 compounds show small deviations from the simple Langevin- Debye law at lower temperatures as is made clear in the plots of l/xm us.T shown in Fig. 3-5 where experimental and calculated data are compared over the whole temperature range using the C and A values determined from the high- temperature region.. This behaviour is consistent with the occurrence of some small degree of antiferromagnetic coupling between paramagnetic Uv centres an effect which is signifi- cantly less than that found previously for the more magnetically concentrated alkali-metal Uv uranatesZ4 and for UVO .". The presumption that intercalation is accompanied by a pro-portionate electron transfer A =Ai' +e' is justified by the narrow ranges of peffvalues found for each host oxide irrespec- tive of the nature and amount of the intercalating species (Table 4).Values of peffin the range 0.6-1.5 pB are found for other Uv-containing oxides which have local pentagonal or hexagonal bipyramidal coordination such as e.g. U308,26 USb05,25 UNb,Olo 27 and Li,UTiOS.25 A Uv (f') species in an idealised cylindrical environment experiencing typical actinyl ion ligand-field and spin-orbit coupling perturbations and occupying an anticipated ground state would have a calculated value of peffM 1 pB. The measured moments in Table 4 approach this theoretical limit; they fall well below the free Uv ion (2F)value of 2.5 pB since the effects of ligand-field and spin-orbit coupling inter- actions are of comparable significance in the Uv-containing oxides.26. This interpretation of the observed magnetic moments implies that electron transfer on intercalation is to U 5f orbitals of rnl = +_ 2 parentage.. These are orbitals directed between axial and equatorial ligands and are non-bonding in character; this could be an important factor in accounting for the small changes in intralayer lattice parameters found on intercalation.Conclusions New intercalation compounds of the mixed uranium oxides UTeO and USeO have been prepared and characterised.. The host oxides which have pseudo-layered structures readily undergo ambient-temperature intercalation reactions to form products A,UMO (A=Li H Na; O<x<l).. The hydrogen insertion compounds H,UMO (x \< l) behave as Brernsted acids towards NH and form ammonium intercalation com- pounds of the type (NH4),H,-,MOS..The reactions all result in only minimal changes in lattice parameters implying that the intercalation behaviour of both oxides is of the framework type,o despite interlayer shortest M-0 contact distances of >3 A.. The intercalation chemistry of UTeO and USeO thus resembles that of Vz05 rather than that of a true lamellar compound such as MOO,.. The intercalation compounds are paramagnetic with isolated Uv species formed quantitatively 142 384 440 389 224 590 150 289 657 537 388 0 0.57 0.63 0.64 0.60 0.55 pav=0.6 & 0.04 0 0.92 0.82 0.93 0.93 pa"=0.90&0.05 according to the electron transfer reaction A =Ai' +e' which accompanies intercalation.References 1 A. M. Chippindale P. G. Dickens and A. V. Powell Prog. Solid State Chem. 1991,21 133. 2 P. G. Dickens A. V. Powell and A. M. Chippindale Solid State ionics 1988,28-30 1123. 3 A. J. Jacobson in Solid State Chemistry Compounds ed. A. K. Cheetham and P. Day Oxford University Press New York 1992. 4 D. Altermatt and I. D. Brown Acta Crystallogr. Sect. B 1985 41 240. 5 P. G. Dickens A. M. Chippindale and S. J. Hibble Solid State ionics 1989,34 79. 6 G. Meunier and J. Galy Acta Crystallogr. Sect. B 1973,29 1251. 7 B. 0.Loopstra and N. P. Brandenburg Acta Crystallogr. Sect. B 1978,34,1335. 8 0.Lindqvist Acta Chem. Scand. 1968,22,977. 9 P. Khodadad C.R. Seanc. Acad.Sci. Ser. B 1962,255,1617. 10 G. W. Watt S. L. Achorn and J. L. Marley J.Am. Chem. SOC,1950 72 3341. 11 P. G. Dickens D. J. Penny and M. T. Weller Solid State ionics 1986,18-19,778. 12 P. A. Sermon and G. C. Bond J. Chem. SOC. Faraday Trans. I 1976,72,730. 13 P. G. Dickens S. J. Hibble and G. S. James Solid State ionics 1986,20,213. 14 I. R. Beattie N. Cheetham M. Gardner and D. E. Rogers J. Chem. SOC.A 1971,2240. 15 J. R. Bartlett and R. P. Cooney J.Mol. Struct. 1989,193,295. 16 A. V. Powell D. Phil.. Thesis Oxford 1990. 17 H. Siebert Z. Anorg. Allg. Chem. 1955,275,225. 18 P. G. Dickens and A. M. Chippindale in Proton Conductors ed. P. Colomban Cambridge University Press Cambridge New York 1992,p. 101. 19 A. M. Deare J. inorg. Nucl. Chem. 1961,21,238. 20 P. G. Dickens and A. M. Chippindale Rutherford Appleton Lab. Rep. 1995 A291. 21 P. S. Goyal J. Penfold and J. Tompkinson Chem. Phys. Lett. 1986 127,5. 22 G. J. Kearley A. N. Fitch and B. E. F. Fender J.Mol. Struct. 1987 160,91. 23 S. Kemmler-Sack and W. Riidorff 2. Anorg. Allg. Chem. 1967 354 255. 24 B. Kanellakopulos E. Henrich C. Keller F. Baumgartner,E. Konig and V. P. Desai Chem. Phys. 1980,53,197. 25 P. G. Dickens G. P. Stuttard R. E. Dueber M. J. Woodall and S. Patat Solid State ionics 1993,63-65,417. 26 S. Kemmler-Sack E. Stumpp W. Rudorff and H. Erfurth Z. Anorg. Allg. Chem. 1967,354,287. 27 C. Miyake S. Ohana and S. Imoto inorg. Chim. Acta 1987 140 133. Paper 6/01580H; Received 5th March 1996 J. Mater. Chem. 1996,6(7) 1211-1217 1217

ISSN:0959-9428    

DOI:10.1039/JM9960601211

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Synthesis and structure of Ba,InO,X (X = F, C1, Br) and Ba,ScO,F; oxide/ halide ordering in K,NiF,-type structures Richard L. Needs,' Mark T. Weller,*' Ulrich Schelerb and Robin K. Harrisb 'Department of Chemistry, University of Southampton, Southampton, UK SO1 7 1BJ bDepartment of Chemistry, University of Durham, Durham, UK DH1 3LE The structures of three indium and one scandium complex layered oxide halides have been determined from time-of-flight powder neutron diffraction data. The indium phases Ba,InO,F, Ba,InO,Cl and Ba,InO,Br all crystallize in the space group P4/nrnm and exhibit complete oxide halide segregation, producing alternating sections of halide [ BaF] and oxide [ BaO] ions separating infinite InO, layers. However, Ba,ScO,F shows only partial oxide/fluoride ordering with the fluoride ions occupying the two apical sites on scandium equally with oxide ions, in the space group I4/mrnm.19FMAS NMR data have been collected from Ba,InO,F and Ba,Sc03F, and confirm the structural analysis in terms of a single type of fluoride ion site in each material. The structural chemistry of complex oxide halides has become the subject of renewed interest following the recent reports of high-temperature superconductivity in various layered oxide halide cuprates. There appear to be at least three routes to these p-type oxide halide superconductors: interstitial halogen doping, cation substitution and anion substitution. Interstitial fluorine doping of Sr,CuO, affords the 46 K superconductor Sr,CuO,F,+,,' in which the fluoride ions occupy solely the apical sites about the copper, giving effective CuO, square planes and an interstitial site in the new SrF rocksalt layer.Cation substitution of calcium by sodium in both C~,CUO,C~,~ and C~,CU,O,C~,~,~ at high pressure results in the formation of ( Ca,Na),CuOzClZ5 and (C~,N~),C~CU,O,C~,~ with T,values of 26 and 49 K, respectively. Anion substitution of chloride by oxide in (Sr,Ca)3Cu204C12 at high pressure gives the 80 K further 48 h. The resultant ceramics were grey-green in colour and were checked for phase purity by powder X-ray diffraction. The indates were of low crystallinity but were pyre, and wer? indexed on a primitive tettagonal cell of a = 4.22 A, c = 15.03 A and a = 4.24 A, c= 15.48 A for the oxide chloride and oxide bromide respectively.The scandate Ba,ScO,F was indexe4 on a body-centred tetragonal cell of a=4.15 A and c= 13.54 A. The end member of this series is the oxide iodide Ba,InO,I, and the synthesis of this phase was attempted; owing to the hygroscopic nature of BaI,, as far as possible all procedures were carried out under argon, but after transfers to and from the glove box and powder X-ray analysis under Mylar film some hydrolysis occurred, resulting in the evolution of iodine vapour upon reheating. Reactions at all temperatures in the range 700-1050 "C produced complex barium indium oxides. superconductor Sr2.3Cao.7Cuz04+ ,C1, .7 ,, -These examples The reaction was hampered further by the volatile nature demonstrate clearly how the design of new superconductors with high transition temperatures can be achieved uia the use of oxide halides to achieve both layering and segregation.Apart from copper chemistry, there are relatively few struc- turally characterised layered oxide halides. K2Nb0,Fs and Sr,FeO,Fg have disordered apical fluoride oxide ions. Sr,Fe,05Cl,,10 Sr3Fe205Br2,11 their barium indium analogues Ba,In,05C1212 and Ba31n205Br2,13 and S~,CO,O,~~C~~'~ crys-tallize as Ruddlesden-Popper phases and as such are structur- ally similar to the La, -xSr,Cu206+ al' superconducting phases. As indium and scandium in complex oxides often form struc- tural analogues of cuprates, a study of their oxide halides was initiated and B~,IIIO,F~~ and Ba,In,05F,17 were reported.Both contain fully ordered segregated oxide and fluoride ions, with Ba,InO,F exhibiting a new K,NiF, superstructure, similar to that of the T* phases of cuprate superconductors.lS In this paper we report the synthesis and structure, as determined from analysis of time-of-flight powder neutron diffraction data, of the indates Ba,InO,Cl and Ba,InO,Br and the scandate Ba,ScO,F, and confirmation of the structure of Ba,InO,F as reported previously. Experimental The synthesis of Ba,InO,F has been reported previously.16 The other barium indates and scandates were all synthesised by high-temperature sintering of well ground mixtures of appropriate molar ratios of BaCO, (99.95%), BaF, (99%), BaCl, (99.9%), BaBr, .2H20 (99% pre-dried at 500 "C), In,O, (99.9%) and Sc203 (99.5%).The powders were fired at 850 "C for 24 h, reground and fired in the range 1025-1075 "C for a of BaI,. The NMR experiments were performed on a Chemagnetics CMX-200 spectrometer operating at 188.288 MHz for fluorine nuclei. A Chemagnetics double-resonance high-speed magic- angle spinning (MAS) probe was used, with spin rates of 12.5 and 16.0kHz. The MAS technique was applied in order to average to zero the chemical shift anisotropy of the fluorine nuclei. Homonuclear (F,F) dipolar coupling also can be aver- aged by MAS, provided that high spin-rates are used. In order to reduce the influence of the deadtime of the probe, which would show up in distortions of the baseline of the spectra, a spin-echo pulse sequence with detection at the echo maximum was applied.To avoid phase distortions generated by the overlap of rotor and spin echoes, the latter were synchronised with the MAS period. The echo times were 160 and 125 ps at the 12.5 and 16.0 kHz, respectively. The other experimental parameters were: fluorine 90" pulse duration, 3.5 ps; spectral width, 100 kHz. The chemical shifts were referenced by replace- ment to the signal of CFC1,. Structure Determination Powder X-ray data were collected using a SiemensoD5000 diffractometer employing Cu-Ka, radiation (1.5406 A) over the 20 range 20-120". Powder neutron diffraction data were collected on the POLARIS medium-resolution diffractometer at the Rutherford-Appleton Laboratory over the time-of-flight range 3-19 ms.Data from the high-resolution back-scattering bank were used for the refinement. The data were first normal- ised to the vanadium standard, a background pattern was deducted and the data were then analysed using the GSAS1' J. Mater. Chern., 1996, 6(7), 1219-1224 1219 Table 1 Final refined atomic coordinates for Ba21n03F at 298 K, estimated standard deviations (e s d s) are gven in parentheses anisotropic temperature factors/A2 Jw1) 2c 0 25 0 25 0 38075( 10) 0 97(4) 0 97(4) 093(5) Ba(2) 2c 0 25 0 25 0 10282(9) 0 95(4) 0 95(4) 0 85(5) In 2c 0 75 0 75 0 23245( 10) 0 M(2) 0 44( 2) 0 70( 6) O(1) 4f 0 75 0 25 0 25701(8) 1 07( 3) 048(2) 190( 4) O(2) 2c 0 75 0 75 0 08239(10) 193(4) 193(4) 0 58(4)F 2c 0 75 0 75 0 42745( 15) 3 02(6) 3 02(6) 1 97( 7) x2=209, R,= 147%, Rp=2 74% Space group P4/nmm a=4 1640(2) A,c= 13 9439(8) A Table 2 Final refined atomic coordinates for Ba21n0,C1 at 298 K, e s d s are given in parentheses anisotropic temperature factors/A2 -atom site X Y Z B11 B22 B33 Wl) 2c 0 25 0 25 0 35253(8) 0 72( 2) 0 72( 2) 0 73(5) Ba(2) 2c 0 25 0 25 0 09469(8) 0 93(3) 093(3) 0 96(5)In 2c 0 75 0 75 0 21405( 10) 049(2) 0 49( 2) 0 61(6) O(1) 4f 0 75 0 25 0 23777( 6) 108(3) 0 59(2) 132(4) O(2) 2c 0 75 0 75 0 07545( 10) 2 06(3) 2 06(3) 0 48( 5) Cl 2c 0 75 0 75 0 42591(5) 1 ll(1) 1 ll(1) 156(4) x2=3060, RWp=141%, Rp=287% Space group P4/nrnm a=42208(2)A, c=150308(2)A Table 3 Final refined atomic coordinates for Ba,InO,Br at 298 K, e s d s are gven in parentheses anisotropic temperature factors/A2 atom site X Y Z Bll B22 B33 Ba(1) 2c 0 25 0 25 0 34054( 12) 0 71(5) 0 71(5) 121(9) W2) 2c 0 25 0 25 0 09274( 12) 1 30(6) 1 30( 6) 0 67(8) In 2c 0 75 0 75 020681(14) 0 72(5) 0 72(5) 0 19(I) (31) 4f 0 75 0 25 0 22980( 7) 1 37( 5) 0 40(5) 161(7) O(2) 2c 0 75 0 75 0 07263( 12) 203(5) 203(5) 0 82(8) Br 2c 0 75 0 75 0 42450( 10) 124(4) 124(4) 2 13(9) x2=331, RWp=127%, R,=235% Space group P4/nmm a=42365(3)& c=154805(1)A Table 4 Final refined atomic coordinates for Ba2Sc03F at 298 K, e s d s are gven in parentheses anisotropic temperature factors/A2 atom site X Y z Bl1 B22 B33 ~ ~~ Ba sc 4e 2a 0 0 0 0 0 36140(9) 0 0 88(2) 0 52( 1) 0 88(2) 0 52(1) 116(5) 4 33(6) O(1) 0(2)/F 4c 4e 0 0 05 0 0 0 16383(8) 0 88(3) 197(3) 0 25(2) 197( 3) 2 10( 6) 1 22( 5) x2=227, R,=l 97%, Rp=3 55% Space group I4/mmm a=4 1480(2)& c=13 5441(8)w package with neutron scattenng lengths and absorption cross- peak shape, zero point and lattice parameters Further param- sections taken from Koster and Yelon '' eters included were both anion and cation positions, isotropic The quality of the powder X-ray data collected for Rietveld thermal parameters followed by an absorption correction, to analysis of both Ba,InO,Cl and Ba,InO,Br was poor, owing allow for the high neutron-absorption cross-section of indium, mainly to their hygroscopic nature The collection of time- and anisotropic thermal parameters Close inspection of the of-flight powder neutron diffraction data on the high-flux profile fits revealed a small amount of impunty in all three medium-resolution diffractometer POLARIS at RAL in indates This impurity level increased from oxide fluoride to vanadium cans produced good quality data Rietveld quality bromide and, by inspection of d spacings, was found to be the powder X-ray diffraction data were collected for Ba,ScO,F Rudlesden-Popper-type series Ba,In,O,X, The impurity level and no evidence of any primitive peak was observed, indicative in Ba,InO,Br was high enough, cu 5% Ba3In2O5Br,, to be of oxide halide disorder The initial model for refinement of included as a second phase using the crystal data reported this phase was the coordinate set of Sr2Fe0,21 with all anion previously l3 The neutron diffraction data from Ba,ScO,F positions refined as oxide Refined parameters included the were refined using the X-ray coordinates as an initial model background, lattice parameters, peak shapes, atomic coordi- and proceeding as for the indates, with no evidence for a nates and thermal parameters lowering of the symmetry from 14/mmm to P4/nmm observed The initial model for powder neutron data refinement of all The anion distribution was determined by bond-valence calcu- three indates was that of Ba,In03F, as derived from refinement lations using the refined neutron data anion positions There of powder X-ray data and published earlier The refinements is no quoted value for the bond-valence parameter,, r, for an proceeded smoothly mth the initial inclusion of background, Sc-F system so it was calculated, using ten scandium fluorides 1220 J Muter Chem, 1996, 6(7),1219-1224 from ICSD, as 1.756.The model selected was that of K2Nb03F and Sr,FeO,F with the indium apical anion positions occupied equally by oxide and fluoride. This structure gave the best results for the valence of both metal ions, compared with both fully and equatorially disordered models. The results of all powder neutron diffraction data refinements are in Tables 1-4 with important bond lengths and angles in Tables 5 and 6 and the bond-valence calculations of Ba,ScO,F in Table 7. Final profile fits for Ba,In03F, Ba,ScO,F and the I I I I I II-'-I 'I:8l6-I f4l2 two-phase refinement of Ba,InO,Br are shown in Fig.1-3, respectively. Discussion The phase Ba,InO,F has been reported previously,16 and its anion environment has been derived from bond-valence con- siderations owing to the similar X-ray scattering powers of oxide and fluoride. This problem is not alleviated by using neutron diffraction as the scattering amplitudes of oxide and fluoride are very similar, but the actual positions of the anions, surrounded by the powerful X-ray scatterers barium and indium, are determined more easily by neutron diffraction data analysis. As shown in Table 1 and Fig. 1, there is no evidence for any anion disorder as the profile fit is very good, the positions are stable with low e.s.d.s and the thermal parameters mainly are satisfactory, confirming the structure determined from X-ray studies.16 However, owing to the slightly high thermal parameters of the fluoride ion site partial oxide/ fluoride disorder was considered. 19FMAS NMR studies show only one resonance at 6, -60.5 f.1 (Fig.4) consistent with Table 5 Important derived bond lengths and bond angles for Ba,In03F, Ba,In03C1 and Ba,InO,Br distance/& angle/degrees bond Ba,InO,F Ba,InO,CI Ba,InO,Br Ba( 1)-O( 1) x 4 Ba( 1)-X x 4 Ba( 1)-X x 1 Ba(2)-0( 1) x 4 Ba(2)-O(2) x 4 Ba(2)-O( 2) x 1 In-O( 1) x 4 In-O(2) x 1 In-X x 1 O(1)-In-O( 1) 2.704( 1) 3.0156( 6) 2.674( 2) 2.9928( 2) 2.9582( 2) 2.582( 2) 2.1100(3) 2.092 1 (2) 2.719( 2) 161.32 2.726( 1) 3.18 13 (5) 3.329(2) 3.01 22 (9) 2.9986( 2) 2.557( 2) 2.1406( 4) 2.0817(9) 3.186(2) 160.70 2.725( 1) 3.2657( 9) 3.638( 3) 2.998( 1) 3.01 17( 3) 2.559( 3) 2.1479( 4) 2.077( 2) 3.380( 3) 160.9(2) Table 6 Important derived bond lengths for Ba,ScO,F bond dis t ance/A Ba-O(2)/F x 4 2.9 529 (3) Ba-O( 2)/F x 1 2.676(1) Ba-O( 1) x 4 2.7974( 8) sc-O( 1) x 4 2.0740( 1) SC-O( 2)/F x 2 2.219( 1) I I 1 c.s23456789fO 0 I1 I 1 I I I I I I I1 \ 2 10 .-c@I ii ILl I_J ! --L f0 11 12 13 14 15 16 f7 18 19 time of flight/ms Fig. 1 Final profile fit for the refinement of powder neutron diffraction data of Ba,InO,F. Upper solid line, calculated pattern; crosses, observed pattern; tick marks, allowed reflections; lower continuous line, difference plot.the proposed anion ordered model. Any disorder over these anion positions would produce two quite different fluorine sites which should be detectable as two resonances owing to the large spectral range of "F; however, the linewidth is substantial (ca. 10 ppm). The compounds Ba21n03C1 and BazIn03Br have been syn- thesised and structurally characterised by the analysis of powder neutron diffraction data. Both are isostructural with Ba,InO,F and as such contain infinite layers of In02 sheets separated by alternating layers of halide and oxide rocksalt layers. The structure and indium coordination polyhedra of all three indates are shown in Fig. 5 and 6. In the Ba,InO,X series the lattice parameters increase linear5 with halide ion radius; the increa2e in a is small (0.07 A), while that in c is much larger (1.54 A).These a5e very similar to the increases in Ba,In,O,X, C0.07 and 3.03 A (two In-0,X layers) for a and c, respectively], from X=F to X= Br in each case.12,13*17 The coordination geometry of the InO, square pyramid remains nearly constant in all three phases, with the large expansion in c arising from the increasing size of the Ba-X layer (Fig. 4 and 5). Again, these observations are similar to those seen in the series Ba31n205X2 (X =F, C1, Br) and Sr3Fe20SX2 (X =C1, Br). A structural consequence of these trends is that both the In-X and Ba-X bonds increase as a Table 7 Final bond-valence calculations for Ba,ScO,F apical anion disorder equatorial anion disorder full anion disorder cation valence X(l)=O X(2)=O/F X(l)=O/F X(2)=0 X( 1)=O/F X( 2) =O/F 2.177 0.649 2.826 sc 1.920 0.736 2.656 2.044 0.69 1 2.735 1.001 0.882 1.881 Ba 0.877 1.010 1.887 0.946 0.942 1.888 X( 1) and X(2) refer to the anion sites O(1) (4c)and 0(2)/F (4e) as given in the table of atomic coordinates (Table 4).J. Mater. Chem., 1996, 6(7), 1219-1224 1221 I I I I I I I I I 345678970 I I I I I I I I I] 5 I-h01 7i L-l--I I I 12.LU-10 11 12 13 14 15 16 17 18 19 time of flightlms Fig. 2 Final profile fit for the refinement of powder neutron diffraction data of Ba,ScO,F Upper solid line, calculated pattern, crosses, observed pattern, tick marks, allowed reflections, lower continuous line, difference plot function of halide size, along with the level of structural distortion from the body-centred coordinates of the ideal K,NiF, structure (Tables 1-3) As shown in Fig 5, the In-Oaplcal bond length decreases slightly from Ba,InO,F to Ba21n03C1 and very slightly to Ba,In03Br There are three possible reasons for this First, there may be a small residual In-X interaction, decreasing from fluoride to bromide, thus requiring a greater bonding interaction between the indium and apical oxygen Secondly, the ordering of the two apical sites is not complete, leading to the observation of two average sites, one mainly fluoride and the other mainly oxide, although the I9F MAS NMR data described above suggests otherwise Finally, the In- Oequatorlal bond increases from oxide fluoride to oxide bromide owing to the small lattice expansion in a, thus requiring a greater In-Oaplal interaction to satisfy the indium valence A similar trend is observed in the B?31n20,X2 system, where In-Oaplcal bonds decrease from 2 07 A in the oxide fluoride to 2 05 A for both oxide chloride and bromide In all three in4ates there is one rather short Ba(2)-0(2) bond pf ca 2 56 A, this is not unprecedented, with a value of 2 53 A seen in Ba31n20, 23 A companson of the indium coordination environments between Ba,InO,X and Ba31n,05X, shows the In-X interactions to be longer in the Ba,InO,X series The scandate Ba,ScO,F is not isostructural with Ba,InO,F, it crystallises in the normal K2NiF4 space group with dis- ordered apical anions forming a distorted octahedron, as in K,NbO,F The 19FMAS NMR spectrum of Ba,ScO,F shows only one resonance at 6, -790+1 (Fig 7), consistent with the proposed structure, although as for Ba21n03F the linewidth is substantial In both spectra there are some very minor peaks, in addition to the spinning sidebands suggestive of small amounts of impurity Anion ordering is to be expected in the indium oxide chloride and bromide where the two halide anions have very different sizes compared with oxide, and this ordering is clear from refinement of the powder neutron diffraction data In the case of the oxide fluondes either 1222 J Muter Chem, 1996, 6(7), 1219-1224 IuL L U A L - 1U 11 12 13 14 15 16 17 18 19 time of flight/ms Fig. 3 Final profile fit for the two-phase refinement of powder neutron diffraction data of Ba21n03Br Upper solid line, calculated pattern, crosses, observed pattern, upper tick marks, Ba,In,O,Br, allowed reflections, lower tick marks, Ba,InO,Br allowed reflections, lower continuous line, difference plot -605 I d,d100 0 100 6 200 -300 Fig.4 "F MAS NMR spectrum of Ba21n0,F collected at a spin rate of 16 kHz Some very weak spinning sidebands are visible 129 transients were accumulated using a recycle delay of 4s complete ordering (Ba,In03F) or apical anion disorder (Ba,ScO,F) is possible The difference between the K2NiF4 structure and Ba,InO,F superstructure is shown in Fig 8 The change in structure between Ba,InO,F and Ba,Sc03F is probably due to the change in size and coordination requirements between In3+ and Sc3+ The ionic racii of In3+ and Sc3+ in six-fold coordination are 0 80 and 0 745 A, respect-ively In general, the larger the metal ion the greater the coordination number, therefore the expected coordination number of indium is either similar to or greater than that of scandium However, as has been shown above (Fig 8), the coordination of indium is effectively square pyramidal and that of scandium is distorted octahedral in these oxide fluorides A major difference between the two ions is their electronegativ- ity Scandium is significantly less electronegative than indium, the Pauling electronegativity values for indium and scandium Fig.5 Structures of the layered segregated indium oxide halides Ba,InO,X (X=F, C1 or Br) showing the InO, sheets separated by Ba-0 and Ba-X rocksalt slabs ""(I! -& Fig. 6 The In0,X coordination environment of (a) Ba,InO,F, (b)Ba,InO,Cl and (c) Ba,InO,Br r ii Fig. 7 19FMAS NMR spectrum of Ba,ScO,F collected at a spin rate of 12 kHz. The echo technique was not used in this case, which accounts for the baseline distortion. Some weak spinning side bands are visible. 256 transients were accumulated, using a recycle delay of 4s. being 1.78 and 1.36, respectively; this has a noticeable effect on their bonding requirements. In crystal structures, scandium is often found in six-fold c~ordination~~with some examples of seven-, eight- and nine- coordination kno~n.,~?,~ Examples of lower coordination are limited to a few cases with very bulky ligands, whereas indium is often found in less than six-fold coordination.In general, electropositive elements form ionic compounds with many, longer, non-directional bonds, i.e. Na and Mg form ionic bonds with relatively large coordination numbers, rarely less than six. Similarly, less electropositive species form fewer, more covalent interactions, which are shorter and directional in nature; tetrahedral environments are common and octahedral geometry is the largest coordination environment found com- monly. Hence, in crystal chemistry scandium, which is the more electropositive, will form preferably coordination spheres K2NiF4 Ba21 no3F Fig.8 Comparison of the body-centred structure of K,NiF4 and the primitive superstructure of Ba,InO,F. K,NiF, structure: large light spheres K; small black spheres, Ni; medium grey spheres, F. Ba,InO,F structure: large grey spheres, Ba; small black spheres, In; medium grey spheres, 0;medium light spheres, F. which are more ionic in nature than those formed by indium. A comparison of the structures of the triiodides, InI, and ScI,, illustrates this difference in bonding character. 11-11, is a low- melting-point dimer consisting of pairs of distorted InI, tetra- hedra.27 ScI,, a high-melting-point ionic solid, crystallises in the Bi1328 structure, alternate-edge-sharing octahedra forming layers linked by van der Waals bonds.In the Ba2M03F compounds (M=Sc, In) this difference is manifested by the adoption of a distorted octahedral environment for scandium and a square-pyramidal geometry for indium. This large differ- ence in electronegativity can be attributed to the different electronic configurations as the indium d-electron subshells shield the outer electrons poorly. In conclusion, we have synthesised and structurally charac- terised a series of complex barium indium oxide halides, whose structures are similar to the T* cuprate superconductors, which contain infinite sheets of InO, layers separated by alternating rocksalt slabs of Ba-0 and Ba-X. The structure of the equivalent scandium oxide fluoride has been determined in the normal K,NiF4 space group with disordered apical anion sites containing an equal mix of oxide and fluoride ions.This relatively unusual difference between the crystal chemistry of a complex layered indate and the corresponding scandate has been explained by consideration of their respective electronega- tivities and the consequent degree of covalency exhibited in their bonding. This wide area of relatively unexplored chemis- try has yielded already interesting structural modifications of known structure types. Therefore, the investigation of indi- um and scandium oxide halides has been widened to include both other metals, such as iron, copper, titanium and tin, and related structure types, e.g. perovskite and Sr,Ti,Ol,-type Ruddlesden-Popper phases.We wish to thank the EPSRC for a grant in support of this work and the Department of Chemistry at Southampton for partial studentship funding for R. L. N. References 1 M. Al-Mamouri, P. P. Edwards, C. Greaves and M. Slaski, Nature (London), 1994,369,382. 2 Von B. Grande and H. K. Muller-Buschbaum, 2. Anorg. Allg. Chem., 1977,429,88. J. Mater. Chem., 1996, 6(7), 1219-1224 1223 8 9 10 11 12 13 14 15 16 T Sowa, M Hiratani and K Miyauchi, J Solid State Chem , 1990, 84,178 J Huang, R D Hoffmann and A W Sleight, Mater Res Bull, 1990,25,1085 Z Hiroi, N Kobayaashl and M Takano, Nature (London), 1994, 371,139 Y Zenitani, K Inan, S Sahoda, M Uehara, J Akimitsu, N Kubota and M Ayabe, Physica C, 1995,248,167 C Q Jin, X J Wu, P Laffez, T Tatsuki, T Tamura, S Adachi, H Yamauchi, N Kosbzuka and S Tanaka, Nature (London), 1995,375,301 F Galasso and W Darby, J Phys Chem , 1962,66,1318 F Galasso and W Darby, J Phys Chem, 1962,67,1451 W Leib and H K Muller-Buschbaum, 2 Anorg Allg Chem, 1984,518,115 W Gutau and H K Muller-Buschbaum, 2 Anorg Allg Chem, 1990,584,125 M Abed and H K Muller-Buschbaum, J AIloys Compd, 1992, 183,24 M Abed and H K Muller-Buschbaum, 2 Anorg Allg Chem, 1991,592,73 J Boje and H K Muller-Buschbaum, Z Anorg Allg Chem ,1991, 592,73 N Nguyen, L Er-Rakho, C Michel, J Choisnet and B Raveau, Mater Res Bull, 1980,15,891 R L Needs and M T Weller, J Chem SOC Chem Commun, 1994,353 17 R L Needs and M T Weller, J Chem SOC Dalton Trans, 1995, 3015 18 J Akimitsu, S Suzuki, M Wantanabe and H Sawa, Jpn J Appl Phys, 1988,27, L1859 19 A C Larson and R B Von Dreele, GSAS General Structure Analysis System, MS-H805, Los Alamos, NM, 1990 20 L Koster and W B Yelon, Neutron Diffraction Newsletter, 1982 21 S E Dann, D B Curne and M T Weller, J Solid State Chem, 1991,92,237 22 I D Brown and D Altermatt, Acta Crystallogr Sect B, 1985, 41,244 23 K Mader and H K Muller-Buschbaum, Z Anorg Allg Chem, 1988,559,89 24 G W Melson and R W Stotz, Coord Chem Rev, 1971,7,133 25 I Kubach, Gmelins Handbuch der anorganischen Chemie Scandium 39, Verlag Chemie GMBH Weinheim/Bergstrasse, 1973 26 G Wilkinson, R D Gillard and J A McCleverty, Comprehensiue Coordination Chemistry, Pergamon Press, Oxford, 1987, vol 3, p 1060 27 J D Forrester, A Zalkin and D H Templeton, Inorg Chem ,1964, 3,63 28 D Brown, Halides of the Lanthanides and Actinides, Wiley-Interscience, New York, 1968, p 218 Paper 6/00211K, Recewed 10th January, 1996 1224 J Muter Chem, 1996, 6(7), 1219-1224

ISSN:0959-9428    

DOI:10.1039/JM9960601219

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Volatile products formed by carboreduction and nitridation of clay mixtures with silica and elemental silicon Thommy C. Ekstrom, Kenneth J. D. MacKenzie," G. Vaughan White, Ian W. M. Brown and Glen C. Barris New Zealand Institute for Industrial Research and Development, P.O. Box 31 -31 0, Lower Hutt, New Zealand Simultaneous carboreduction and nitridation (CRN) of clays provides a useful route for the preparation of low-cost p-sialon powders. During the course of the CRN reaction at 1400-1450 "C, mass losses of cu. 10% in excess of the theoretical amounts are typically found. This discrepancy is mainly due to transport of gaseous SiO from the reactants into the N2-CO gas stream, followed by condensation in the cooler parts of the furnace system. In this work, the condensed material lost from the reacting system was collected under controlled conditions, and its constitution studied by 27Al and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) and complementary X-ray powder diffraction (XRD).Predictions from thermodynamic calculations of the various possible gas-phase reactions are compared with the observed results. Silicon nitride- based ceramics have good potential as very-high- temperature structural materials with extraordinary chemical and mechanical properties. Among the nitrides, the sialon ceramics are of special interest as they are more readily densified and many of their properties can be tailored by selection from a number of different sialon phases and by control of their mi~rostructure.'-~ One low-cost route for the preparation of sialon powder is the simultaneous carbothermal reduction and nitridation (CRN) of silicate or aluminosilicate minerals such as clays.The CRN technique has proved very successful for preparing p'-sialon powders from ka~linite~~~ and halloysite.lO.ll New Zealand halloysite clay is a particularly good starting material for p'-sialon powders, because of its high purity and fine particle size, which gives p'-sialons (Si6-ZA1,0zN8-z, z = 2.3-2.6) with excellent sintering proper tie^.^*'"-'^ The CRN process involves a complex sequence of reactions which depend on the nature of the reactant clays and the process parameters ~sed.~*~*~~,~~,~~ It is generally accepted that gas-phase transport plays an important role in the reactions within the powder bed.Transport of N2 to the reaction site and of CO away from the reaction site is of crucial importance. The reaction of C or CO with silica or silicates also produces a significant partial pressure of gaseous SiO within the powder bed at 31400"C.15 Previous studies of Si,N4 formation by carboreduction-nitridation of Si02 have reported significant mass losses which have been attributed to the loss of gaseous SiO from the reaction zone.6,16-21The reaction of silica or silicates with carbon or CO to form SiO gas becomes significant at ca. 1400"C.15 The observed mass losses generally increase with both temperature and time. Volatilisation from the sample in the hot zone of the furnace is confirmed by the deposition in a cooler zone of a product often resembling wool, which has been variously reported as being either X-ray amorphous" or silicon nitride whisker^.'^ When SO2-clay mixtures are used, the chemistry becomes even more complex, and the consequences for the transported material have not been studied in detail.However, for better process control it is necessary to understand the nature of the loss of material from the reaction zone during CRN reactions in these more complex reactant mixtures. This study addresses the loss of material and the nature of the resulting products deposited from the vapour phase in two sialon-forming CRN systems: (i) mixtures of Si02 with halloy- site clay, batched to give p'-sialon with z values of 0.25-2.0 at 1400-1450 "C, and (ii) mixtures of elemental Si with halloysite clay, with compositions giving p'-sialons in the same z-value range as in (i).Some of the samples studied here also contained small amounts (<3 mass%) Y203, which has been found to have a beneficial influence on these CRN reactions.22 The reaction products were analysed by 27Al and 29Si magic- angle spinning nuclear magnetic resonance (MAS NMR) and X-ray powder diffraction (XRD), and their constitution com- pared with predictions made on the basis of thermodynamic calculations. Experimental The starting materials were New Zealand halloysite clay (NZ China Clays Ltd), carbon (Degussa Lampblack), Si02 (Silica Superfine, dS0=1.45 pm, Commercial Minerals Ltd) and elemental Si (Sicomill, grade 4D, d50 =4.6 pm, KemaNord Industrikemi, Sweden). The reaction mixtures contained 10% more carbon than the theoretical amount for complete CRN reaction, and additions of 0.5-3.0 mass% Y203 were also made to some mixtures.The samples were heated in alumina crucibles placed in an alumina boat in a horizontal tube furnace (40 mm diameter) in a stream of purified N2 (flow rate 100ml min-l) at temperatures of 1400-1450°C and times of 2-96 h. Careful weighings were made to establish the mass loss during the reaction, and the mass and composition of the transported materials condensed downstream of the samples were monitored by collecting them on a series of alumina tubes and plates placed at varying distances downstream, the temperatures at the various collection points being calibrated by an independent thermocouple.To ensure the collection of all condensibles, a water-cooled glass finger was placed at the furnace tube outlet; negligible amounts of condensate were, however, collected on the cold finger. 27Al and 29Si MAS NMR spectra were obtained at 11.7 T using a Varian Unity 500 spectrometer and a Doty MAS probe spinning at 10-12 kHz. In view of the long 29Si relaxation times known to occur in some sialon-forming systems,' 29Si delay times of up to 1000s were used. The reaction products were also examined by X-ray powder diffraction (XRD) using a Philips PW1700 diffractometer with APD1700 software. A Cambridge scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) was used to study the morphology and composition of the condensed materials.Results and Discussion Mass losses of reaction mixtures The nominal z values of the fully reacted sialon compositions studied here were 0.25, 0.5, 0.7, 1.0 and 2.0. The completeness J. Muter. Chem., 1996, 6(7),1225-1230 1225 of the CRN reactions in mixtures containing both SiO, and elemental Si were found to be sensitive to both heating time and temperature, and also to the presence of Y203, which facilitated the reaction 22 The reaction rates also depended on the nominal z value of the reactant mixture, with lower z-value mixtures, containing less clay, always requiring longer reaction times to fully react to pure p'-sialon At 1400"C, Si0,-clay mixtures in the absence of Y203, especially where z< 1, showed XRD evidence of incomplete reaction at short heating times, with the presence of residual cnstobalite and some unreacted mullite (from the clay) Heating for >48 h gave a product containing only nitrides (p'-sialon and some a-Si3N4) even at z=O 25 Only ca 8 h heating at 1450°C was required for full reaction Similar results were found for mixtures containing elemental Si-clay, in which the reactions in the low-z materials are also time-dependent At short heating times, XRD showed the presence of 0'-sialon ( S1,- xAlxO1 + xN2-x) and some Sic in addition to p'-sialon The measured unit-cell parameters of the 0'-sialon indicated a composition of Si, ,Alo ,01,N1 8, i e the A1 substitution is at its maximum Complete reaction to p'-sialon occurred in these mixtures after heating for 100 and 25 h at 1400 and 1450"C, respectively Additions of Y,O, do not appear to change the reaction products or their formation route, but they do enhance significantly the rate of product formation Fig l(a) shows the observed mass changes for a series of Si0,-clay mixtures with a range of nominal z values, but without Y203, reacted for 8 h at 1450°C XRD indicated that all these samples contained only crystalline p'-sialon Fig 1(a) also includes, for comparison, the theoretical mass losses calculated on the assumption of complete reaction to p'-sialon All the observed mass losses are greater than the theoretical -30 -40 -501 0 0 50-(b)3 E 25 -0---25 -50 ' ." ' a * * ' * " ' " ' ' " ' Fig. 1 Observed (0)and theoretical (-) mass changes in CRN reactions of (a) Si0,-clay, (b) Si-clay mixtures The materials were reacted without Y,O, at 1450°C for 8 h 1226 J Mater Chem, 1996,6(7), 1225-1230 values by ca lo%, regardless of the nominal z value of the starting mixture The same results were obtained for mixtures containing Y203 Similar mass losses of ca 10% greater than the theoretical losses were reported by Cho and Charles14 for the formation of p'-sialon (z= 3) from kaolinite at 1450 "C using an optimal N2 flow rate Thompson and Hendr~,~ also reported mass losses of ca 12% above the theoretical values in the preparation of z = 1 p'-sialon powder Thus, the SiO, clay ratio appears not to affect the mass loss or the deposition in the cooler zones of the wool-like material, which is found by XRD to be different from the whisker-like that is often observed on the surface of the sample in the hot zone Fig l(b) shows the mass changes in mixtures of elemental Si with clay, heated without Y203 at 1450°C The different behaviour of these mixtures reflects the mass gain due to nitridation of Si, the theoretical mass change for the overall reaction, which depends on the Si clay ratio of the mixture, is also shown in Fig l(b), in which the calculations were made on the basis of full reaction to p'-sialon The observed mass changes show a similar trend to the theoretical changes, but are lower by an almost constant amount, irrespective of z value This lower than predicted mass gain can again be understood in terms of mass losses due to volatilization from the sample Similar results were obtained with samples contain- ing y203 Table 1 shows the results of a number of repeated experi- ments on an elemental Si-clay mixture of composition z=O 5, without the addition of Y203 The theoretical mass gain of this sample composition is 24 1%, according to reaction (1) A1,03 2 4Si0, -2 2H20 + 19 6Si + 5 8C + 1 5N2 +4s15 5Alo 500 sN7 5+ 5 8C07+2 2H207 (1) The mean of the observed mass gains, including the condensed material, is 177%, and can be accounted for in terms of the presence of a small amount of Sic and a trace of AlN, according to reaction (2) A1,03*24Si02*22H20+19 6Si+8 2C+13 6N2 +3 6Si5 SAlO500 5N75 + 2 2SiC + 0 2A1N + 6COT+ 2 2H20T( 2) (91 3%) (79%) (07%) The presence of Sic and A1N in reacted mixtures of z=O 5 composition has been demonstrated by 29S1 and 27Al MAS NMR spectroscopy (Fig 2) The 29S1 NMR spectrum [Fig 2(a)] shows that reaction for 24 h at 1450 "C produces Si resonances corresponding to p'-sialon (6 -48 4), Sic (6 -18 9) and amorphous or crystalline S102, evidenced by the broad band at 6 -110 The partition- ing of the Si over these three types of environment were estimated, by integration of the 29S1 NMR spectrum, to be 59% as sialon, 18% as Sic and 23% as SiO, The 27Al NMR spectra [Fig 2(b), (c)] show a sharp resonance at 6 115 corresponding to AlN,' together with a broad tetrahedral A1-0 peak at 6 61 The extent of heat treatment influences the nature of the A1-N bonds present, in some samples [Fig 2(c)] there is evidence of the presence of both A1N (6 115) and the A1-N units of p'-sialon, which appear at 6 ca 105' Constitutionof condensed material Most of the evaporated material was deposited in a 20-30 mm band in the temperature zone of ca 1000-12OO0C, with an occasional small amount of material found at ca 1300-1350 "C, z e ca 100 "C below the reaction temperature The total amount of collected material was typically ca 3-5% of the original sample mass, ze about half the additional mass loss unac- counted for in the systematic discrepancy between the observed and theoretical mass losses (Fig 1) Table 1 Amounts of material condensed from five replicated expenments with an Si-clay mixture z =0.5 p'-sialon composition, fired at 1450"C for 8 h sample mass/g mass gain in sample total mass gain before firing after firing in g YO mass of condensed material/g In g % 5.794 6.479 0.685 11.8 0.300 0.985 17.0 6.001 6.802 0.801 13.3 0.242 1.043 17.4 6.323 7.168 0.845 13.4 0.230 1.075 17.0 7.899 9.168 1.269 16.8 0.200 1.417 17.9 20.023 23.374 3.351 16.7 0.366 3.717 18.6 I l l , 0 -40 -80 -120 s(29Si) A Fig. 2 Typical 11.7 T MAS NMR spectra of an Si-clay mixture of composition z=0.5, containing 3.0 mass% Y,O, reacted at 1450"C for 8 h.(a), ,'Si spectrum (ref. Me&); (b), (c) "A1 spectra [ref. Al( H,o),~ 1.+ The deposited material was collected on a number of 20 mm square flat, thin alumina plates placed in a row on the bottom of the furnace tube downstream of the reaction zone, and analysed by XRD and SEM. A typical series of plates contain- ing the condensed material are shown in Fig. 3, which also includes the relative amounts collected on each plate and its temperature zone. Most of the deposited material was X-ray amorphous, evidenced by a typical baseline hump, but a few very weak and broad peaks were also detected; these were identified as Sic, but reflections from a small amount of a disordered sialon polytypoid (2H6) may also be present (Fig.4). The small amount of material which condensed on the alumina plates closest to the reaction zone could not be separated from the alumina plates for separate measurements, but displayed XRD reflections peaks similar to Si2N20 or the related Al-substituted 0'-sialon (Fig. 5). Addition+ reflections in these diffractograms at d =4.43, 3.41,3.33,2.69 A, etc., which Fig.3 A typical series of alumina plates after condensation of the vapour-transported phases indicating the temperature zone of each plate and the amount of material deposited I 20.0 40.0 60.0 80.0 100.0 2 Bldegrees (Co-Ka) Fig. 4 XRD powder diffractogram of predominantly amorphous mate- rial transported from an Si-clay CRN reaction mixture (composition z=0.5) and condensed in the furnace temperature zone 1000-1200 "C.S=Sic (JCPDS reference pattern 29-1131), P=sialon 2H6 polytype. partly overlap the Si2N20 peaks, are probably due to a new low-temperature form of silicon oxynitride (to be published). Scanning electron micrographs of the condensed material (Fig. 6) reveal several morphologies, depending on the tem- perature zone in which condensation occurred. In the cooler zones, where most of the wool-like material is found, the fibres occur in close association with fine particles and droplets of solidified melt [Fig. 6(4. EDX analysis of the bulk wool (Table 2) indicates the presence of variable but small amounts of A1 in addition to Si and the known impurities in the starting materials (K from the clay and Ca,Fe from the elemental Si).No Y was found in any of the condensed materials originating from compositions containing Y,O,; this element is thus not transported in the vapour phase. The material which condenses in the hotter zone closer to the reaction mixture [Fig. 6(b)] contains droplets which are sufficiently large (1-4 pm) for EDS spot analyses. The major constituent was Si, with significant amounts of A1 and K from the clay, and occasional traces of Fe or Ca, but no Y. Typical analysed elemental ratios were J. Mater. Chem., 1996, 6(7), 1225-1230 1227 lA A A ' 0. + 0' 0' &dII...lll....+ 2o.a 25.0 30.0 35.0 40.0 45.0 50.0 2 Bldegrees (Co-Ka) Fig.5 XRD powder diffractogram of the sparse deposit from an Si-clay CRN reaction mixture (composition z =OS), condensed on a flat alumina plate at 13OO-135O0C, just downstream of the reaction zone.A= alumina substrate (JCPDS reference pattern 10-173), 0'= 0'-sialon (JCPDS reference pattern 33-1162), O* =previously unre-ported modification of Si,N,O. Table 2 EDS analyses of overall composition of condensed matenal at different temperature zones, from silica-clay and silicon-clay start- ing mixtures, composition corresponding to z =0.5 p'-sialon elemental con tent (atom Yo) starting mixture T/"C Si A1 K Ca Fe Si0,-Clay lo00 32.5 1.7 0.7 --Si0,-Clay 1200 32.8 0.9 0.3 --Si-Clay 1000 31.5 1.5 0.9 0.4 0.3 Si-Cla y 1200 32.8 0.7 0.3 02 0.2 Si-Clay 1350 31.0 1.8 0.8 0.4 0.4 A1 :Si =0.15-0.20 :1 and K :Si =0.06-0.09 :1.Since the A1 :Si ratio is 0.09 :1 in the clay starting material, A1 enrichment has occurred in the melted droplets, which have an overall composi- tion, on an oxide basis, corresponding to the silica-rich feldspar region of the K20-A1203-Si02 system. Eutectics exist in this system at temperatures even below 1000°C. The SEM images [Fig. 6(b)] indicate clearly the growth of whisker-like crystals from the liquid drops by well known vapour-liquid-solid (VLS) processes. Fig. 7 shows solid-state 27Al and "Si MAS NMR spectra of the condensed material from z =0.5 sialon mixtures, collected downstream of the reaction zone. All the 29Si spectra show two major resonances, corresponding to uncombined amorph- ous SiOz (6 -108) and to Sic (6 -18)' In addition, the sample derived from Si-clay +3 mass% Y203 [Fig.7(d)] con- tained a major resonance at 6 -79, corresponding to elemental Si; by contrast, this feature was absent from the same material reacted without Y203 [Fig. 7(c)]. A small but possibly signifi- cant region of intensity was noted at 6 ca. -62 in the 29Si -109.9 -18.9 Fig.6 SEM images of material deposited from the vapour phase during CRN reaction of an Si-clay mixture containing 3.0 mass% Y,03 at 1450°C for 8 h. (a) Wool-like matenal from cooler collection zone (ca. 1ooO-1200°C); (b) sparse deposit from hotter collection zone (CU. 1300-1350 "C). 1228 J. Muter. Chem., 1996, 6(7), 1225-1230 Fig.7 Typical 11.7 T solid-state 29S1 and "A1 MAS NMR spectra of wool-like material [refs. Me& and Al(H,0),3 , respectively]+ deposited dunng CRN reactions in Si-clay and Si0,-clay mixtures at 1450°C for 8 h. Spectra (a) and (e), Si0,-clay; spectra (b) and (f), Si0,-clay with 3.0 mass% Y203;spectra (c) and (g), Si-clay; spectra (d) and (h), Si-clay with 3.0 mass% Y203. NMR spectrum of material from an SO2-clay+3 mass% Y203 mixture [Fig. 7(b)]; this spectral region corresponds to Si2N20,5 consistent with the XRD indication in this sample of a previously unreported form of silicon ~xynitride.,~ The 27Al NMR spectra of all these condensed materials contain only one resonance, at 6 53-63, indicative of A1 in tetrahedral coordination with oxygen [Fig.7(e)-(h)]. The 27Al NMR spectra of these materials are consistent with the 11.7 T spectra reported for the potassium alkali-metal feldspars mic- rocline and sanidine (6 54-57 and 6 58, re~pectively).~' Thermodynamic calculations in this CRN system Thermodynamic calculations were made on this system in an attempt to understand aspects of the complex vapour transport which is occurring. Although these calculations refer to equilib- rium conditions and the present experiments were all conduc- ted under dynamic gas flows, the thermodynamic predictions can provide an insight into the possible formation of various possible gaseous species generated under various conditions of temperature and gas composition, and thus indicate the relative likelihood of the various condensable phases forming. A com-puter program26 was used to calculate, from tabulated thermo- dynamic data, the composition of the product assemblage as a function of gas concentration and reaction temperature.As a guide to the choice of the initial parameters, the vapour- phase composition was assumed to be similar to the atomic ratios experimentally observed in the deposited material (Table 2) and the calculation was made for a range of nitrogen concentrations and reaction temperatures. These calculations indicate that the predicted phases depend strongly on the composition of the gas atmosphere, i.e. whether it is assumed to be rich in CO or N2, as follows. 12 I(a) si02l4 4 sic2t r I I I.I -600 800 1 000 1200 1400 1600 potassium silicate \ Om4 0.3 1 600 800 lo00 1200 1400 1600 Reaction in CO-rich atmospheres. The thermodynamic prob- abilities of the various possible phases formed under typically CO-rich conditions (10 moles each of initial CO and N,) are plotted as a function of temperature in Fig. S(a). The calcu- lations indicate that in the presence of sufficient CO, Sic formation is predicted above ca. 1450°C, at the expense of SO2. Under these conditions, the formation of Si2N,0 is also favoured. Over the complete temperature range, all the avail- able A1 is associated with K and Si in the feldspar sanidine [not shown in Fig. S(a)]; the additional K in excess of the requirements for sanidine formation is incorporated in various potassium silicates at temperatures <1400 "C, but above this temperature a significant amount of K vapour is predicted to be present, together with SiO and liquid Si [Fig.S(a)]. Reaction in CO-poor atmospheres. The thermodynamic prob- abilities of the various phases formed under these conditions (calculated on the basis of 90 moles of initial N2 and 10 moles of initial CO) are plotted as a function of temperature in Fig. 8(b).Under these conditions, the formation of significantly more Si2N,0 is predicted, by comparison with CO-rich atmos- pheres, at the expense of Sic which is not thermodynamically favoured. Sanidine [not shown in Fig. 8(b)] is again stable over the whole temperature range and takes up the total A1 content.The additional K is present as potassium silicates, which become more Si-rich at ca. 1250°C, reflected by the step in the Si02 curve at this temperature [Fig. 8(b)].The formation at higher temperatures of K vapour and SiO is significantly more favoured than under CO-rich conditions. Liquid Si is not predicted under CO-poor conditions. Thus, the presence of Sic in all the present wool-like samples is consistent with a high CO concentration in the reaction atmosphere, under which conditions the presence of Si liquid 4l2 0 I 600 800 1000 1200 1400 1600 1 .oo /c 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 600 800 1000 1200 1400 1 600 TPC Fig.8 Concentrations of the various species predicted from thermodynamic calculations as a function of temperature for the present CRN reactant composition. (a)Assuming typically CO-rich conditions; (b)assuming typically CO-poor conditions. J. Muter. Chem., 1996, 6(7), 1225-1230 1229 is also accounted for However, the thermodynamic calcu- lations provide no obvious explanation why the presence of Y,O, in the reactant mixture should promote the transport of elemental Si into the condensed phase from compositions containing Si initially, but not from mixtures initially contain- ing Si02, which give me only to SiO, and Si,N,O in the deposited product Conclusions During CRN reactions in Si-clay and Si0,-clay mixtures, mass losses of ca 10% above the theoretical values are recorded These discrepancies can be understood in terms of loss of reactant in the vapour phase which is transported in the dynamic nitrogen atmosphere and deposited as a wool- like material in the cooler parts of the furnace system Another source of discrepancy between the observed and theoretical mass changes is the formation of small amounts of Sic and AlN in addition to the desired p’-sialon Both these products can be identified by 29S1 and 27Al MAS NMR in the material remaining in the reaction zone The material deposited from the vapour phase contains, in addition to Si, some A1 and K from the reactant clay When elemental Si is present in the reactant mixture, its associated impurities Ca and Fe also appear in the deposited material 27Al solid-state MAS NMR spectroscopy indicates the location of the A1 in tetrahedral sites, consistent with the potassic feldspar sanidine which is the thermodynamically favourable phase over the complete temperature range of this study The 29S1 MAS NMR spectra indicate the presence of SiO, and Sic in all samples, with some samples also containing Si2N20 or elemental Si Thermodynamic calculations suggest that the present reac- tion atmosphere is CO-rich, under which conditions the appearance of all the condensed phases can be accounted for, in terms of vapour-phase transport of gaseous SiO and A1 and K as the monatomic elemental vapour T E is grateful for a senior research fellowship from Industrial Research Ltd References 1 K H Jack, J Muter Sci, 1976,11, 1135 2 T Ekstrom and M Nygren, J Am Cerum SOC, 1992,75,259 3 T Ekstrom, in Tailoring of Mechanical Properties of Si,N, Ceramics, ed M J Hoffmann and G Petzow, Kluwer Academic, Dordrecht, 1994, p 149 4 J G Lee and I B Cutler, Am Cerum SOC Bull, 1979,58,869 5 K J D MacKenzie,R H Meinhold,G V White,C M Sheppard and B L Shernff, J Muter Scz ,1994,29,2611 6 I Higgms and A Hendry, Br Ceram Trans J, 1986,85,161 7 E Kokmeijer, C Scholte, F Blomer and R Metselaar, J Muter Scz ,1990,25,1261 8 H-L Lee, H-J Lim, S Kim and H-B Lee, J Am Ceram SOC, 1989,72,1458 9 A D Mazzoni, E F Aglietti and E Pereira, AppE Clay Scz , 1993, 7,407 10 D S Perera, J Aust Ceram SOC , 1987,23,11 11 M E Bowden,K J D MacKenzieandJ H Johnston,Mater Sci Forum, 1988,34-36,599 12 I W M Brown, Proc Austceram 92, ed J Bannister, Australian Ceramic Society, Melbourne, 1992, p 494 13 I Higgins and A Hendry, Proc Br Ceram SOC,1986,39,163 14 Y W Cho and J A Charles, Muter Sci Technol, 1991,7,399 15 M Ekelund and B Forslund, J Am Cerum Soc ,1992,75,532 16 D S Perera, J Muter Sci ,1987,22,2411 17 S Bandyopadhyay and M Mukerji, Ceram Int ,1991,17,171 18 H Yoshimatsu, H Kawasaki, Y Miura and A Osaka, J Muter Scz ,1989,24,3280 19 S-C Zhang and W R Cannon, J Am Ceram SOC,1984,67,691 20 M Mori, H Inoue and T Ochiai, in Progress in Nitrogen Ceramics, ed F L Riley, Martin Nijhoff, The Hague, 1983, p 149 21 A D Mazzoni, E F Aglietti and E Pereira, J Am Ceram SOC, 1993,76,2337 22 G V White, T C Ekstrom, I W M Brown, G C Barris and C M Sheppard, to be presented at PacRim 2, the 2nd International Meeting of Pacific Rim Ceramic Societies, Cairns, Australia, July 1996 23 A Thompson and A Hendry, Br Cerum Proc ,1991,47,95 24 G C Barris, M E Bowden,T C Eckstrom, C M Sheppardand G V White, to be presented at PacRim 2, the 2nd International Meeting of Pacific Rim Ceramic Societies, Cairns, Australia, July 1996 25 R J Kirkpatrick, R A Kinsey, K A Smith, D M Henderson and E Oldfield, Am Mineral, 1985,70, 106 26 A G Turnbull and M W Wadsley, CSIRO Thermochemistry System, Version 5 1, 1988 Paper 6/005976, Received 25th January, 1996 1230 J Muter Chem, 1996,6(7), 1225-1230

ISSN:0959-9428    

DOI:10.1039/JM9960601225

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

MATERIALS CHEMISTRY COMMUNICATIONS Distinct ferroelectric smectic liquid crystals consisting of banana shaped achiral molecules T. Niori,"T. Sekine,'J. Watanabe,*.a T. Furukawab and H. Takezoeb 'Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan bDepartment of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan The synthesis of a banana-shaped molecule is reported and it is found that the smectic phase which it forms is biaxial with the molecules packed in the best direction into a layer. Because of this characteristic packing, spontaneous polarization appears parallel to the layer and switches on reversal of an applied electric field. This is the first obvious example of ferroelectricity in an achiral smectic phase and is ascribed to the CZVsymmetry of the molecular packing.Ferroelectric liquid crystals are of considerable theoretical and technological interest' and it has been recognized that a tilted smectic phase made up of chiral molecules can be ferroelectric. Chirality of the molecules and their tilted association into a smectic layer, reducing the overall symmetry of the liquid crystals, are essential for conventional ferroelectric liquid Since the essential requirement is the introduction of polar symmetry in the system, ferroelectric liquid crystals need not necessarily be chiral. For this reason, a great deal of attention has been directed, theoretically and experimentally, towards non-chiral ferroelectric system^.^-'^ Tournilhac et u1.' reported the first experimental example to our knowledge.They have synthesized polyphilic molecules comprising three or more chemically different subunits, for example, a fluoroalkyl group on one edge and a fluoromethyl group on another edge, and found for the first time that these achiral molecules can form ferroelectric smectic liquid crystals as a result of the segregation of their units into homogeneous microdomains. Watanabe et uL9 reported that a ferroelectric smectic phase with CZV symmetry7 may be formed from some main-chain types of liquid crystal polymers, if two different aliphatic spacers with odd numbers of carbons are incorporated into the backbone in a regularly alternating fashion and they segregate into different microdomains.Further, we have also found that a novel nematic liquid crystal with polar ordering is formed from a rod-like aromatic polyester with polarity along its chain axis". The polarity in this nematic phase appears along the nematic director as determined from the measurement of the second harmonic generation;"," the strong dipole-dipole interaction between the polar molecules is considered to be the origin of the polarity.6 In this report, we describe a distinct ferroelectric smectic liquid crystal which is formed from a banana shaped achiral molecule. The origin of the ferroelectricity is ascribed to the polar CZv symmetry which is obtained from the efficient packing of the banana-shaped molecules into a smectic layer.Matsunaga and co-~orkers~~~~~have synthesized some banana-shaped materials and found that they can form smectic liquid crystals. However, no detailed analyses of their structure and properties have been reported. These materials, which form a smectic phase, are particularly interesting since, because of their characteristic shape, they form a peculiar smectic phase in which the banana-shaped molecules are closely packed and are all aligned in the direction of bending [see Fig 1 (a)].This type of structure is not unknown in the smectic field since it has already been encountered in main-chain polymers and twin dimers.l4.'' For a quantitative description of this structure, we need to consider the space group of the layered structure.As illustrated in Fig. l(b), each layer has biaxiality, that is in- layer anisotropy exists and the refractive indices are different in the bent direction (y axis) and in the direction normal to the y axis direction (x axis). The space group is analogous to the crystallographic CZV1gro~p.~,~,'~There is a two-fold axis along the y axis and there are mirror planes perpendicular to the x and z axes. Since there is no mirror plane perpendicular to the two-fold axis, spontaneous polarization can be expected to arise along the y axis (bent direction). btYEl*si Fig. 1 (a) Possible smectic structure which may be formed by the banana shaped molecules and (b)its space-group symmetry J. Muter.Chem., 1996, 6(7), 1231-1233 1231 In order to obtain this type of ferroelectric smectic liquid crystals, we prepared compound 1. 1 The banana shape of the molecule comes from linking two benzylideneaniline groups to 1,3-dihydroxybenzene by an ester linkage. The phases observed on heating this material are crystal, S2, S1 and isotropic, as shown in eqn. (1). 91 7 "C 156 4 "C crystal S, --12 2 "C 139 9 "C (4.90 kcal mol -') (2.86 kcal mol -') 161 4 "C S1 isotropic (1) 158 1"C (5.30 kcal mol-l) The smectic phase that draws immediate attention is the S1 phase. Its X-ray diffraction p?ttern shows an inner sharp reflection with a spacing of 37.4 A and an outer broad reflection with a spacing of ca. 4.6A. The spacing between layers is approximately the length of the molecule in the conformation illustrated above, which is a strong indication that the mol- ecules in each layer are in the bent form.The outer broad reflection indicates the disordered lateral packing of the mol- ecules within each layer as seen in conventional SA and Sc phases.17 The viscosity of the S1 phase, similar to the SA and Sc phases, is low and a fan-like texture is observed on cooling from the isotropic melt. The most notable observation from microscopy is that the homeotropic texture, which can be seen by shearing a thin specimen between glass plates, shows distinct birefringence (see Fig. 2). This means that a c-director exists in the layers, and hence that the bent molecules are packed with the molecules aligned in the direction of bending and parallel to the layers.Furthermore, it is a requirement that the long-range orientation correlation of the c-director is main- tained from one layer to another. The X-ray and optical microscopic observations, thus, lead to the layer structure proposed in Fig. 1. In ferroelectric materials, the macroscopic polarization should change its sign on reversal of the applied electric field. To confirm this, the switching current was examined in a pseudo-planar cell, by the triangular wave method,'* where Fig. 2 Optical microscopic texture observed in the homeotropically ahgned S1phase which was prepared by sheanng between glasses. The dark areas are due to air bubbles. 1232 J.Muter. Chem., 1996, 6(7), 1231-1233 55s 9 Q)c 0g -00) Q) -0 -5 2 Pn ([I -10 0.0 0.5 1.o 1.5 tls Fig. 3 Switching current curve obtained by applying a triangular voltage wave (k8.9 V pm-', 1 Hz) at 150 "C.The spontaneous polanz- ation was estimated to be about 50 nC cm-'. the layer normal lies parallel to the glass surface but is randomly oriented between domains. Fig. 3 shows the switch- ing current curve obtained by applying a triangular voltage wave (k8.9 V pm-l, 1 Hz). When the polarity of the electric field changes, a switching current peak is clearly observed. The spontaneous polarization was estimated to be ca. 50 nC cm-2. It should be further noted that the two states under positive and negative electric fields cannot be distinguished by a polarizing optical microscope.Thus, we can conclude that the S, phase of this material is ferroelectric with the tip of the bent molecule orienting to the electric field and reversing its orien- tation on reversal of the polarity of the field. Relative permittivity measurements can also support the existence of spontaneous polarization. Fig. 4 shows the tem- perature dependence of the relative permittivity measured at 1 kHz. In the S1 phase, the relative permittivity is much larger than that in the other phases, namely, crystal, S2 and isotropic. The large relative permittivity is attributed to the so-called Goldstone mode, in which the spontaneous polarization har- moniously responds to the field.From the dispersion of the relative permittivity, its relaxation frequency was determined to be below 100 Hz, though it could not be determined exactly. In conclusion, this system consisting of banana-shaped mol- ecules forms a biaxial S1phase as shown by X-ray and optical microscopic observations. The polarization and relative permit- tivity measurements indicate that the s, phase is ferroelectric. A spontaneous polarization exists parallel to the layers and switches on field reversal. This is the first obvious example of ferroelectricity exhibited by an achiral smectic phase, which is ascribed to the CZvsymmetry of the packing of the banana- shaped molecules into a layer. 10 10 8 8 2 2 n.. 0 40 60 80 100 120 140 160 TPC Fig. 4 The temperature dependence of the real and imagmary parts of the relative permittivity measured at 1 kHz on cooling from the isotropic melt References 1 J.W. Goodby, Ferroelectric Liquid Crystals, Gordon and Breach, Philadelphia, 1991 R. B. Mayer, Mol. Cryst. Liq. Cryst., 1977,40, 33. R. G. Petschek and K. M. Wiefling, Phys. Rev. Lett., 1988,59,343. R. H. Tredgold, J. Phys. D: Appl. Phys., 1990,23, 119. F. Biscarini, C. Zannoni, C. Chiccoli and P. Pasini, Mol. Phys., 1991, 73,439. 6 J. Lee and S.-D. Lee, Mol. Cryst. Liq. Cryst., 1994,254, 395. 7 P. E. Cladis and H. R. Brand, Liq. Cryst., 1993, 14, 1327. 8 F.Tournilhac, L. M. Blinov, J. Simon and S. V. Yablonsky, Nature, 1992,359,621. 9 J. Watanabe, Y. Nakata and K. Shimizu, J.Phys. II (France), 1994,4, 581. 10 T. Watanabe, S. Miyata, T. Furukawa, H. Takezoe, T. Nishi, M. Sone, A. Migita and J. Watanabe, Jpn. J. Appl. Phys., in the press. 11 T. Furukawa, K. Ishikawa, H. Takezoe, A. Fukuda, T. Watanabe, S. Miyata, T. Nishi, M. Sone and J. Watanabe, Nonlinear Opt., 1996,15, 167. 12 Y. Matsunaga and S. Miyamoto, Mol. Cryst. Liq. Cryst., 1993, 237, 311. 13 T. Akutagawa, Y. Matsunaga and K. Yashuhara, Liq. Cryst., 1994, 17, 659. 14 J. Watanabe and S. Kinoshita, J. Phys. II (France), 1992,2,1237. 15 J. Watanabe, H. Komura and T. Niori, Liq. Cryst., 1993,13,455. 16 International Tables for X-ray Crystallography. Birmingham, Kynoch Press, 1959, vol. 1, p. 208. 17 G. W. Gray and J. W. Goodby, Smectic Liquid Crystals, Leonald Hill, Glasgow and London, 1984. 18 K. Miyasato, S. Abe, H. Takezoe, A. Fukuda and E. Kuze, Jpn. J. Appl. Phys., 1983,22, L661. Paper 6/01489E; Received 1st March 1996 J. Muter. Chem., 1996,6(7), 1231-1233 1233

ISSN:0959-9428    

DOI:10.1039/JM9960601231

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

New preparation method for surface-modified inorganic layered compounds Hideyuki Tagaya, * Sumikazu Ogata, Hiroyuki Morioka, Jun-ichi Kadokawa, Masa Karasu and Koji Chiba Department of Materials Science and Engineering, Yamagata University, Yonezawa, Yamagata 992, Japan New surface-modified inorganic layered compounds have been prepared by the reaction of Zn(OH), with organic oxychlorides in which 71-98% of the OH groups reacted. Their layered structures are similar to those of the layered double hydroxides (LDHs). Although LDHs have cationic charges, the reaction products are uncharged. The interlayer spacings of the reaction products were 8.3-14.8 A for the products of Zn(OH), with dioxychlorides, and 11.9-16.7 A for the reaction products of Zn(OH), with monooxychlorides, depending on the lengths of the organic compounds. The incorporation of a molecule into a crystalline inorganic host lattice to form an intercalation compound can lead to ordered materials.',2 It has been well reported that metal phosphonates are useful for organizing molecules into lamellar structure^.^ They are very similar to those formed by Langmuir-Blodgett (LB) techniques, but have better thermal stabilities than LB films.4 Recently we have prepared surface- modified inorganic layered compounds in which the surface of a Zn/Al layered double hydroxide (LDH) was modified by reaction with organic o~ychloride.~ The resulting compound is a well organized inorganic-organic hybrid.LDHs are inor- ganic layered compounds6 and many organic intercalates into LDHs are known.'-'' LDH layers are positively charged, therefore LDHs are anion-exchangeable clays.Surface-modi- fied LDHs also have anionic compounds between the layer^.^.'^ In this communication, we have prepared new surface-modified inorganic layered compounds in which a neutral amorphous compound, Zn(OH),, was reacted with an organic oxychloride as shown in Fig. 1. Both dioxychlorides and monooxychlorides reacted with Zn(OH), giving surface-modified layered com-pounds. The products differed from the surface-modified Zn/Al LDHs because they do not need to include any anionic compound between the layers. No clear peaks were observed in the XRD patterns of Zn(OH), as received, as shown in Fig.2(a). By the reaction of a small excess of amorphous Zn(OH), with sebacoyl chloride Fig. 1 Reaction of Zn(OH), with organic dioxychlorides to give inorganic-organic hybrid layered compounds 10 20 2Bldegrees Fig.2 XRD patterns of (a) Zn(OH),, and the reaction products of Zn(OH), with (b)adipoyl chloride, (c) suberoyl chloride, (d) sebacoyl chloride and (e)hexanoyl chloride [ClCO(CH,),COCl] in acetonitrile, a crystalline product was obtained. The XRD peaks of the reaction product were different from those of sebacoyl chloride and sebacic acid, and were rather similar to those of the surface-modified Zn/Al LDH after reaction with sebacoyl chloride. Similar XRD peaks to those of the surface-modified Zn/Al LDHs were observed in the reaction products of Zn(OH), with various kind of oxychlo- rides.SEM images indicate that clear plate crystals were obtained by the reaction of Zn(OH), with suberoyl chloride as shown in Fig. 3. Similar plate-like crystals were obtained for all the reaction products of Zn(OH), with various kinds of organic oxychlorides. The plate crystals were quite similar to those of the LDHs.l3,I4 The OH absorption at ca. 3500 cm-' in the IR spectrum of Zn(OH), was decreased markedly by the reaction with sebacoyl chloride as shown in Fig. 4(c). Peaks at ca. 1540 and 1465 cm-' indicate the formation of COO-Zn bonds. These results indicate that Zn(OH), reacted with organic oxychlorides giving surface-modified layered compounds. An excess of organic oxychloride was reacted with Zn(OH), in acetonitrile solution.Upon evaporation of the acetonitrile, a powder was obtained. The IR spectrum of the powder suggests the presence of surface-modified Zn(OH), and car- boxylic acid. However, clear XRD peaks of surface-modified J. Mater. Chem., 1996, 6(7), 1235-1237 1235 Fig. 3 $EM images (16000 x magnification) of (a)Zn(OH), and (b)the reaction product of Zn(OH), with suberoyl chlonde layered compounds were not obtained. It was considered that the first step of the reaction involving monooxychlorides (RCOCl) was a dehydration reaction between the OH groups of Zn(OH), and RCOC1, giving RCOO-Zn-OCOR. The cross-sectional Frea of one OH -Zn-OH unit15 was calcu- lated to be 9.6 A2. The layered structure was considered to be assembled by gathering RCOO-Zn-OCOR units.The svr- face area of an RCOO-Zn-OCOR unit is larger than 9.6 A2; therefore, an excess amount of RCOO-Zn- OCOR might interfere with the assembly of the layered structure owing to steric repulsion. As shown in Table 1, about 75% of the OH groups of Zn(OH), reacted with sebacoyl chloride to assemble a layered structure, while 71-74% of the OH groups reacted with dioxychlorides and monooxychlorides, except for products from reactions with small oxychlorides. In the cases of adipoyl chloride and n-butanoyl chloride, 98 and 85% of the OH groups reacted, respectively, probably because of the relatively small steric repulsion. The interlayer spacings (d) of the reaction products from dioxychlorides, 1236 J.Muter. Chem., 1996, 6(7), 1235-1237 4000 3000 2000 1000 wavenumber/cm-l Fig. 4 IR spectra of (a)Zn(OH),, (b)the reaction product of Mg(OH), with sebacoyl chloride, and the reaction products of Zn(OH), with (c) sebacoyl chlonde and (d)hexanoyl chloride Zn(OH),(O -G -0),, increased with increasing methylene chain length, and the same trend was observed in the case of the reaction products from monooxychloride, Zn(OH),(O-G),. However, the interlayer spacings of Zn(OH),(O-G), were larger than those of Zn(OH),(O-G-0),, considering their sizes, which indicates the bilayer structure of the reaction products from monooxy- chlorides. The interlayer spacings and the large y values in Zn(OH),(O-G-0), suggest the presence of bridging struc- tures as shown in Fig.1. We have already reported that water-treated Zn/Al LDHs react with organic oxychloride to give a surface-modified LDHs.' They were different from those of intercalation com- pounds of organic carboxylate anion^.'^,'^ By the reaction of sebacic acid wit! calcined LDH, the interlayer spacing increased to 18.8 A ts shown in Table 1, which is larger than the spacing of 12.8 A for the surface-modified LDH and the reaction product obtained in this study. However, we could not obtain water-treated Mg/A1 LDH and could not achieve surface modification of the Mg/A1 LDH. In this study, we also reacted Al(OH), and Mg(OH)2 with organic oxychlorides. Al(OH), did not react with organic oxychloride, while in the case of Mg(OH)2, the IR spectrum indicated that some of the Mg(OH)2 reacted with organic oxychloride.However, the decrease of the 0-H absorption of Mg(OH)2 was fairly small and clear peaks in XRD patterns were not observed. These results correspond to the facts that surface-modified Zn/Al LDH was obtained but surface-modified Mg/Al LDH was not obtained. The interlayer spacings of the surface-modified Zn(OH), obtained in this study were similar to those of the surface- modified LDHs with organic oxychlorides, probably because the size of aluminium metal is similar to that of zinc metal. We have already shown that LDHs have the ability to recognize the nuclear isomers of naphthalenecarboxylate ions.' Chemical surface modification of inorganic compounds have been studied extensively to change their chemical and/or physical properties in a controlled wa~.'~''~ The present study offers a new preparation method of surface-modified inorganic Table 1 Preparation of surface-modified inorganic layered compounds, Zn(OH),(O-G-0), or Zn(OH),(O-G), by the reaction of Zn(OH), with organic oxychlorides acid chloride n size/A d in XRoD OH-LDH LDH Zn (OH )z X Y Z of acid/A ClCO (CH,),COCI 4 6.4 6.8 7.8 14.8 8.3 0.04 0.98 -(C, 34.10; H, 3.83%) -6 8.9 8.9 10.7 10.8 0.54 0.73 (C, 34.89; H, 4.29%) 8 11.4 11.2 12.8 18.8 12.8 0.50 0.75 -(C, 40.50; H, 5.41%) 10 14.0 13.8 15.2 14.8 0.52 0.74 -(C, 43.82; H, 6.04%) CH,(CH,),COCI 2 4.7 -11.8 11.9 0.31 -1.69 (C, 32.94; H, 4.80%) 4 7.2 -16.1 16.7 0.58 -1.42 (C, 42.80 H, 6.49%) layered compounds which have potential as shape-selective sorbents and catalysts.The method is also useful for controlling the organization of organic molecules in the solid state, leading to new photofunctional materials in which various photo- responsive compounds are bonded or included between the layers. 8 9 10 11 H. Tagaya, S. Sato, H. Morioka, J. Kadokawa, M. Karasu and K. Chiba, Chem. Muter., 1993,5, 1431. T. Kuwahara, H. Tagaya and K. Chiba, Microporous Muter., 1995, 4,247. H. Tagaya, T. Kuwahara, S. Sato, J. Kadokawa, M. Karasu and K. Chiba, J. Muter. Chem., 1993,3, 317. H. Tagaya, A. Ogata, T. Kuwahara, S. Ogata, M. Karasu, J. Kadokawa and K. Chiba, Microporous Mater., in press.12 H. Tagaya, S. Ogata, S. Nakano, J. Kadokawa, M. Karasu and K. Chiba, J. Muter. Chem., submitted for publication. References 13 14 S. Kannan and C. S. Swamy, J. Muter. Sci. Lett., 1992, 11, 1585. J. M. Fernandez, C. Barriga, M. Ulibarri, F. Labajos and V. Rives, 1 D. OHare, in Inorganic Materials, ed. D. W. Bruce and D. OHare, Wiley, Chichester, 1992, p. 165. 2 A. Clearfield, in Progress in Intercalation Research, ed. W. M. Warmuth and R. SchBllhorn, Kluwer, Dordrecht, 1994, p. 223. 3 H. E. Katz, Chem. Muter., 1994,6,2227. 4 M. E. Thompson, Chem. Muter., 1994,6,1168. 5 H. Morioka, H. Tagaya, M. Karasu, J. Kadokawa and K. Chiba, J. Solid State Chem., 1995, 117, 337. 6 F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 1991,11,173. 15 16 17 18 19 J. Muter. Chem., 1994,4, 1117. R. Allmamm, Acta. Crystallogr., Sect. B, 1968,24,972. M. Meyn, K. Beneke and G. Lagaly, Inorg. Chem., 1990,29,5201. S. Miyata and T. Kumura, Chem. Lett., 1973,843. Hybrid Organic-Inorganic Composites, ed. J. E. Mark, Y. Lee and P. A. Bianconi, ACS Symp. Ser. 585, American Chemical Society, Washington DC, 1995. Characterization and Chemical Mod$cation of the Silicu Surface, ed. E. F. Vansant, P. Voort and K. C. Vancken, Elsevier, New York, 1995. 7 H. Tagaya, S. Sato, T. Kuwahara, J. Kadokawa, M. Karasu and K. Chiba, J. Muter. Chem., 1994,4, 1907. Communication 6/01609J; Received 6th March, 1996 J. Mater. Chem., 1996,6(7), 1235-1237 1237

ISSN:0959-9428    

DOI:10.1039/JM9960601235

出版商:RSC    

年代:1996

数据来源: RSC