Structural and magnetic properties of Sr -,M,IrO (M =Ca Zn Cd Li Na) Nanu Segal," Jaap F. Vente," Timothy S. Bush"*band Peter D. Battle," "Inorganic Chemistry Laboratory University of Oxford South Parks Road Oxford UK OX1 3QR bDavy-Faraday Laboratory The Royal Institution of Great Britain 21 Albemarle Street London UK W1X 4BS The compounds Sr4-,CaxIr06 (x=0.5 1,2,3,4) and Sr,MIrO (M =Li Na Zn Cd) have been synthesized and studied by X-ray powder diffraction and SQUID magnetometry. They all adopt the K,CdCI structure with the cations M and Ca occupying only the trigonal prismatic sites in the [Ool] chains unless x > 1. Those compounds which contain only IrIV order antiferromagnetically at temperatures between 14 and 22 K and those which contain only IrV show temperature-independent paramagnetism.The development of a self-consistent interatomic potential for the IrIV-O interaction is described. The lattice parameters predicted for the K,CdCl,-like phases using this new potential agree with the experimental values to within ca. 2%. Recently the level of interest in ternary and quaternary oxides of the platinum metals has been increasing rapidly with the K,NiF,-like compounds Sr2M04 (M=Ru Rh and Ir) attracting particular attenti011.l~~ Oxides of Ir Ru Rh and Pt which are isostructural with K4CdCl6 are also being studied exten~ively~-~and we describe below the synthesis and charac- terisation of some iridium-containing members of this struc- tural family. Sr,IrO was prepared originally by Randall and Ward,' and subsequently by Rodig and McDaniel and Schneider," but a detailed description of the crystal structure has only been published relatively recently." It is a rhombohedra1 K4CdC1,- like structure (Fig.1) in which IrO octahedra and SrO trigonal prisms share triangular faces to form chains of alternat- ing polyhedra parallel to the three-fold axis. The interchain space is occupied by the remaining alkaline-earth-metal cat- ions. Substitution of Ca for all the Sr leads to the formation of the isomorphous compound Ca,Ir0,.12*13 It has been rep~rted~,'~that the transition-metal cations Ni2+ Cu2+ and Fig. 1 Crystal structure of Sr41r06. IrO octahedra are hatched open circles represent Sr2 ions.+ Zn2+ can be substituted for the strontium cations in trigonal- prismatic coordination.In the case of Sr,CuIrO the structure distorts to give a square-planar coordination around the Cu atom and there is a concomitant lowering of the symmetry to monoclinic (C2/c).14. The magnetic susceptibilities of the com- pounds Sr,MIrO (M=Sr Ni Cu Zn) have been studied in detail. For M =Sr or Zn the compounds are antiferromagnetic below 1215and 20 K5respectively. The success of an alternating Heisenberg chain model in accounting for the magnetic behav- iour of the latter compound has been taken to indicate that a structural distortion as yet undetected in diffraction experi- ments is present in Sr,ZnIrO,. Sr,CuIrO is a weak ferromag- net (T,~40K) with a saturation magnetization of 0.61 pBt per mole while the magnetic susceptibility of Sr,NiIrO shows a complex behaviour with different (anti)ferromagnetic inter- actions dominating in different temperature region^.^ Ca,lrO has been shown to be paramagnetic in the temperature range 77<.T/K<290,l3 but no measurements have been made at lower temperatures. We undertook the present study of cation substitution in Sr,IrO with a number of aims in mind. We hoped to stabilize iridium in a formal oxidation state of five in this structure by substituting one quarter of the strontium with lithium or sodium. The substitution of strontium by calcium zinc and cadmium was carried out to enable us to study the structural and more importantly the magnetic properties of Ir" in a range of isomorphous compounds.Such a study was deemed to be interesting because of the possibility of a transition from antiferromagnetism to ferromagnetism as was observed when Mg was replaced by Zn in La2Mgl-xZn,Ir0,.'5. The final aspect of the work described below is concerned with the use of computer modelling in crystal chemistry. We recently devel- oped' a set of self-consistent potentials for M"+-02-and 02-02-interactions by simultaneously fitting the crystal properties (e.g. unit-cell parameters relative permittivity and elastic constant) of a wide range of binary metal oxides using a single shared O2-O2 -potential. Transition-metal cations with electron configurations other than do high-spin d5 and d" were omitted from that study a deficiency which we began to rectify by developing RuIV-O and RuV-0 pair p0tentia1s.l~ These potentials are valid in relatively complex ternary oxide stru~tures,'~,~~and their value was demonstrated by our recent ab initio determination of the structure of Li,Ru0,.18 We believe it is important to establish the extent to which this type of potential can be used in structural inorganic chemistry and we have therefore developed an IrIV-O potential and t pB (Bohr magneton)=9.27 x J T-I.1.Mater. Chern. 1996 6(3) 395-401 395 tested it against the behaviour observed in the system Sr -,Ca,IrO,. Experimental The synthesis of polycrystalline samples of Sr4-xMxIr06 (M = Li Na Ca Zn and Cd) was initially attempted by firing the appropriate stoichiometric mixtures of dry SrCO Ir Li2C03 Na,CO CaCO ZnO and CdO (all Johnson Matthey Chemicals). The reaction mixtures contained in alumina cru- cibles were heated at 800°C for 1 day in order to decompose the carbonates.Subsequently the temperature was increased in a stepwise manner to the following values Sr,-,Ca,IrO (0.5bx62) and Sr,ZnIrO 1000°C (for up to 2 weeks); Sr,CdIrO 950°C (1 day); Sr,LiIrO 800°C (2 weeks); and Sr,NaIrO 900 "C (4 days). The syntheses were interrupted frequently to facilitate re-grinding and pelletizing of the samples. All the preparations were carried out in air. Attempts to prepare Ca,IrO in this way failed. A mixture of Ca -xIr301219 and CaO was produced initially and the compo- nents of this mixture subsequently reacted only very slowly at temperatures up to 1200°C.We were thus unable to produce a single-phase sample of Ca,IrO by this method. However we were able to suppress the formation of Ca5~xIr3012 and prepare a sample of Ca,IrO by firing a stoichiometric mixture of CaCO and Ir in air at 1100°C for 24 h without prefiring at 800°C. The disadvantage of this method is that it can lead to the loss of iridium owing to the formation of volatile iridium oxides.20 Attempts to prepare Cd,IrO and Sr4.-xMxIr06 (M = K Mg Co and Pd) were unsuccessful as were those to synthesize Ba -,M,IrO,. The progress of the reactions was monitored using X-ray powder diffraction (XRD) and they were deemed to be com- plete when the diffraction pattern did not change on heating the sample further.X-Ray data (Cu-K,a) were recorded on the final products of successful reactions using a Siemens D5000 diffractometer operating at room temperature in Bragg-Brentano geometry. These data were collected over the angular range 15< 28/" d 100 using a 28 step size of 0.02". The results of our powder diffraction experiments were analysed by the Rietveld method2' using the GSAS program package.22. The peak shape was described by a pseudo-Voigt function and the background level was fitted with a shifted Chebyshev function. For each diffraction pattern a scale factor a counter zeropoint three to five peak shape parameters four background param- eters and two unit-cell parameters were refined.The number of atomic parameters (fractional coordinates and thermal parameters) refined varied as will be described below. The magnetic susceptibility of each product was measured using a Quantum Design or a Cryogenic Consultants supercon- ducting quantum interference device (SQUID) magnetometer. Samples were loaded at room temperature and measurements were usually taken in the temperature range 2 < T/K d300 after cooling the sample in zero magnetic field (zero-field cooled ZFC) and also after cooling in the measuring field (field cooled FC). However Sr,CaIrO and Sr,LiIr06 were only measured under FC conditions. The applied field was chosen to lie in the range 1 dH/kG b 10. In the Born model the energy associated with the inter- actions between a pair of ions i and j in an ionic solid can be written as qtqJ = #lJ(rLJ) -(1)rV The second term on the right hand side of eqn.(1) is simply the long range Coulomb interaction. The first term represents the short-range interatomic interactions and is often taken to have the form of the Buckingham potential These short-range interacti9ns are cut off for interionic dis- tances r, greater than 12A. The parameters A, ptJ,c,,are then the variables to be refined when a new ion-pair potential is developed.16. The values which best describe the Ir"-O interaction were calculated using the static lattice computer program GULP,23 which is based on the Born model of the ionic lattice. The parameters were derived from a simultaneous consideration of the structural data on Sr41r06" and La2MgIr06.24 We restricted ourselves to the use of these two model compounds because we believe that the reliability of the potential parameters for the interaction between a heavy metal and oxygen is likely to be enhanced if structures deter- mined from neutron diffraction data are used in the construc- tion of the potential.A detailed description of the fitting methodology has been given previously.16 Potentials describing the two-body interactions between other ion pairs (cg. Sr-0) were taken from Bush et al..',. The newly determined Ir'"-O potential was used to predict the lattice parameters of Sr,-,Ca,IrO (x= 1,4) and Sr,IrO,.,. The number of composi- tions that could be considered for the former system was limited by our inability to deal with compositions having a disordered cation array.Results We were able to prepare samples with the general formula Sr4-,MXIrO6 for M =Ca (x =0.5 1 2 3 and 4) and for M = Zn Cd Li or Na (x=l). The samples of composition Sr,,Ca,,IrO and Sr,Ca,IrO were used only to provide additional data for our study of the composition dependence of the unit-cell parameters; they were not investigated by magnetometry. The refinements of the structures of the majority of the compounds under investigation proceeded smoothly in the hexagonal setting of the space group R3c. However in the case of Ca,IrO we detected the presence of a minor (ca. 1%) face-centred cubic impurity phase having a ~4.8A.We believe this to be CaO and we assume that the enforced omission of the 800 "C firing resulted in a small loss of Ir as II-O. The lattice parameters determined by the profile analyses are presented in Table 1 along with the agreement indices; the lattice parameters determined previously" for Sr41r0 are included for comparison purposes. The fact that the DW-d values2 are smaller than the lower limit of the 90% confidence level indicates that the estimated standard deviations associ- ated with the refined parameters are likely to be underestimated owing to serial correlation effects. It became clear that the dopant cations (M=Li Na Ca Zn and Cd) occupy the 6a (O,O,a)Sr( 1) site in the [OOl] chains rather than the larger 18e (x,O,$) Sr(2) site in the interchain space.In the Ca-doped samples having 1<x <4 the 6a sites are occupied only by calcium and the 18e sites are occupied by a disordered arrangement of calcium and the remaining strontium. The values of the variable coordinates of the latter site and the 36f (x,y z) oxygen site are given in Table 2 along with those of the thermal parameters U,,,. The resulting bond lengths are given in Table 3. The low X-ray scattering factors of Li Na and 0 resulted in poor definition of their structural parameters and in order to ensure physically reasonable values it was sometimes necessary to constrain U, to be the same for all the ions in the structure or to fix the value for a light atom to be equal to the overall temperature factor determined in preliminary refinements.Furthermore in some refinements it was necessary to fix the fractional coordinates of the oxide ions to ensure that chemically reasonable bond lengths were maintained. Such instances are indicated in Table 2. The observed and calculated diffraction patterns for two representa- tive structure determinations Ca41rO6 and Sr,ZnIrO are given in Fig. 2 and 3. All the Ir" compounds studied show a susceptibility maxi- mum in the temperature range 12< T/K<22. The results of our measurements on Ca41r06 are shown in Fig. 4. Qualitatively similar results were obtained for all compositions in the series Sr4-,Ca,Ir06. The data collected on Sr,CdIrO are plotted in Fig. 5. They show a paramagnetic tail at low Fig.3 Observed (dots) calculated (full line) and difference X-ray powder diffraction profiles of Sr,ZnIrO,. Reflection positions are marked. temperatures suggesting that the sample contains a very small amount of an impurity phase. The results of our experiments on Sr,ZnIrO are different (Fig. 6) in that the ZFC and FC susceptibilities do not overlie below ca. 90 K. The data in the temperature range 70 <T/K <296 have been modelled for all these compounds using the Curie-Weiss Law with an additional term a,to allow for a temperature-independent paramagnetic (TIP) contribution to the molar magnetic suscep- tibility (T-6) (3) The predicted value of C for a low-spin d5 ion on a site of cubic symmetry is 0 375 emu K mol-'.The TIP contribution is inversely proportional to the spin-orbit coupling constant [ of the cation which can be evaluated for Ir" as 26 (4) The resulting parameters are listed in Table 4 along with the temperature of the susceptibility maximum It can be seen that the latter changes little in the series Sr4-,Ca,Ir06 but increases in the Zn and Cd compounds. The data on the IrV compound Sr,LiIrO do not show a susceptibility maximum (Fig 7) and eqn (3) was used to fit them over the full measured temperature range. The magnetic susceptibility of a low-spin d4 ion (J =0) is predicted to be temperature-independent and the value of C refined by fitting to eqn (3) can be used to estimate the IrIV content of the sample and hence in the case of a monophasic 26sample the vacancy concentration on the anion sublattice.The derived value of C (Table4) leads to a composition Sr,LiIrOS 99 for our sample Electron repulsion effects render the relationship between the spin-orbit coupling constant of IrV and the TIP27more complex than in the case of Ir" and [ cannot be evaluated analytically in the former case.The results of a preliminary experiment to measure the magnetic susceptibility of Sr,NaIr06 in the temperature range 80<T/K<300 are shown in Fig 8(a). The data are qualitat- ively similar to those measured for Sr,LiIrO and the resulting parameters are included in Table 4 However when the sample was remeasured after being stored in air for some months the results were somewhat different [Fig 8(b)] with the suscepti- bility showing a small but sharp increase on cooling below ca 250 K We take this to indicate that this compound is unstable in air and that ferromagnetic Sr21r0 is one of the decompo- sition products.This phase was undetectable by XRD and the magnitude of the susceptibility change suggests that it is present in a concentration of only ca 500 ppm The potential parameters AZJ,pv and C tor the Ir"-O Jon pair refined to the values 1772 83 eV 0 3039 A and 1 98 eV AP6 Table 4 Magnetic susceptibility parameters for Sr4-xMxIr06 Fig. 7 Inverse molar magnetic susceptibility of Sr,LiIr06 measured in a field of 5 kG. 0,field cooled; - Curie-Weiss with TIP. respectively. Energy minimisation calculations using these par- ameters predicted the unit-cell parameters listed in Table 5.In addition to the model compounds the table includes the compositions Sr,CaIrO and Ca,IrO which are central to this work and as an additional test compound Sr21r04. The experimental values are also tabulated and for ease of compari- son the observed and calculated unit-cell parameters of the compounds Sr -,Ca,IrO are plotted as a function of composi- tion in Fig. 9. The calculated cell dimensions agree with the experimentally determined values to within 2%. Discussion The results described above show that the Sr cations occupying the trigonal prismatic 6a sites in Sr,IrO can be replaced by a T/K Fig. 8 Inverse molar magnetic susceptibility of Sr,NaIrO measured in a field of 1 kG (a)initially and (b)after ca.2 months. A zero-field cooled; 0,field cooled; - Curie-Weiss with TIP. Table 5 Predicted and observed unit-cell parameters for some iridium oxides predicted experimental difference compound parameter value value (Yo) wide range of divalent or monovalent dopants without causing any major disruption to the crystal structure. In the case of a monovalent dopant charge neutrality could be preserved either by the introduction of anion vacancies or by the oxidation of the transition-metal cations. Our magnetic and structural data both suggest that the latter occurs although we have been unable to find a reliable method of chemical analysis by which we can determine the oxygen content of our samples directly.The relatively straightforward synthesis of two more IrV com- pounds Sr,LiIrO and Sr,NaIrO suggests that this can no longer be considered as an unusual oxidation number for Ir in the solid state although the subsequent decomposition of the Na-containing sample must be noted. The fact that the dopant cations all of which are smaller than Sr2+ occupy the 6a sites in preference to the 18e sites is consistent with the lower coordination number and hence smaller size of the former Although we were able to replace more than 25% of the Sr2+ cations with Ca2+ and thus dope the 18e site we were unable to dope beyond 25% with the smaller Cd2+ cation. The unit-cell volume (Table 1 Fig 9) is a linear function of composition in the series Sr4-,Ca,Ir06 but the unit-cell parameters a and c are not.The volume reduction required by the introduction of Ca is initially (x d 1) achieved by a contraction along the c axis with a remaining essentially constant both a and c decrease as the Ca concentration increases beyond x =1. The net effect is that the rhombohedra1 distortion of the unit cell parameterized by c/a (=J1 5 for pseudo-cubic symmetry) is largest when x= 10. The aniso- tropic nature of the dilation enables the Ir cations to retain their optimum coordination geometry over a wide composition range although the Ir-0 bond length (Table 3) is unusually short in the fully substituted compound Ca,Ir06 For all compositions in the series Sr -,Ca,IrO other than Sr41r06 the IrO octahedra are elongated along the three-fold axis despite the overall compression of the unit cell (c/a<Jl 5) The elongation of the octahedra compresses the trigonal prisms and hence facilitates the accommodation of the smaller Ca2+ cations.The composition dependence of the bond lengths M,,-0 and MlSe-0 in Sr3ZnIr06 and Sr,CdIrO as well as in Sr4-xCaxIr06 reflects the fact that the dopant cation does not occupy the 18e site unless x> 1 Our refinement of the structure of Sr,ZnIrO resulted in a lower R-factor than that reported by Nguyen and zur Loye,' and we have no reason to believe as they did that we have overlooked a structural distortion. The unit-cell parameters reported for the two different samples are in good agreement but it is possible that their sample reacted with the Pt used to line the alumina 400 J Muter Chem 1996,6(3) 395-401 crucible,28 thus modifying the composition slightly.The con- straints imposed during the structural refinement of Sr3NaIr06 and Sr,LiIrO make it impossible to discuss the resulting atomic coordinates in any depth but we note that the rhombo- hedral distortion of the unit cell increases as the size difference between the dopant cation and Sr2+ becomes greater. The same effect can be seen by comparing Sr xCaxIrO (x 6 1) and Sr,ZnIrO although in the case of Sr,CdIrO the distor- tion is not as large as might be expected The magnetic susceptibilities of all the Ir" oxides considered in this study were well modelled by eqn (3) in the high- temperature region In each case (Table 4) 0 is small and negative thus indicating that weak antiferromagnetic inter- actions are always present. The data suggest that these inter- actions are stronger in the Zn- and Cd-containing compounds The relatively low values of C (<O 375 emu K mol-') are presumably a consequence of the low symmetry of the crystal field at the Ir (6b) site.The mean value of the calculated spin- orbit coupling constants 2 2 x lo4crn-l is comparable to the values which can be deduced from some previous but higher than those reported elsewhere 26. The low-tempera- ture susceptibility data on Sr -,Ca,IrO suggest that all compositions in this series order antiferromagnetically at low temperatures.The composition-dependence of the Neel tem- perature is consistent with a strengthening of the magnetic interactions as the unit-cell volume decreases and the super- exchange pathways get shorter. The temperature of the suscep- tibility maximum observed for Sr,ZnIrO is very similar to that reported previously,' but the value is somewhat higher than would have been expected on the basis of cell-volume arguments However the most striking feature of our data is the previously unobserved low-temperature hysteresis in the susceptibility. The presence of this effect which was reproduc- ible in Sr,ZnIrO but unobserved in the other compounds suggests that blocked spin clusters form at relatively high temperatures (ca 90 K) in the Zn compound alone It is not clear why this should be the case.The Neel temperature of Sr,CdIrO is the highest observed in these compounds as would have been expected from the magnitude of the Weiss constant 8 Again the value is enhanced to a greater extent than can be explained using simple size arguments and it seems likely that the electronic structure of the Zn2+ and Cd2+ cations which both have a relatively polarizable d10 core is an important factor In contrast to Nguyen and zur Loye' we have not attempted to model our compounds as one-dimen- sional magnets Although it is convenient and usual to describe this crystal structure in terms of [OOl] chains we have shown previously' that the intrachain Ir-Ir distance is not significantly shorter than the interchain distance and the structure should be considered as being three dimensional Darriet and Subramanian3' have recently presented a less misleading description of the structure. The temperature dependence of the magnetic susceptibility of Sr,LiIrO is typical of an IrV oxide.The compound is essentially non-magnetic with a weak Curie tail caused by the presence of a small concentration (ca 1 5%) of reduced Ir" cations. This impurity level is comparable to that observed in other IrV oxides". The limited data available on Sr,NaIr06 show a similar behaviour Our attempts to model the structures of complex Ir oxides have met with limited success. The refined potential parameters model La,MgIrO very well with the observed and calculated unit-cell parameters agreeing to better than 1%.The unit-cell volume is also predicted accurately However the agreement is less satisfactory in the case of the compounds Sr,-,Ca,IrO Although each cell parameter is accurate to better than 2% and their non-linear composition dependence is well repro- duced the calculated values are all systematically greater than the experimental values and the calculated unit-cell volume is thus only accurate to ca 4% In the case of Sr21r0 a layer structure which was used only as a test compound the unit- cell volume was accurately reproduced but through the cancel- lation of errors in the individual cell parameters rather than through a truly accurate calculation We conclude that the parameters derived in this study for the Ir4+-02- potential have limited transferability.The reasons for this might include the different environment of the Ir cations in the various 3 4 5 M K Crawford M A Subramanian R L Harlow J A Fernandez-Baca Z R Wang and D C Johnson Phys Rev B 1994,49,9198. T N Nguyen D M Giaquinta and H C zur Loye Chem Mater 1994,6,1642. T N Nguyen and H C zur Loye J Solid State Chem 1995 117,300 compounds. The IrO octahedra in La,MgIrO are linked to neighbouring MgO octahedra through one common vertex whereas those in Sr,-,Ca,IrO share a face with the neighbour- ing trigonal prismatic group thus markedly reducing the Ir-M2+ distance Our previous study of complex Ru oxides17 also showed that different potentials were needed to describe 6 7 8 9 10 S Frenzen and H Muller-Buschbaum 2 Naturforsch.Tell B 1995,50,581 J F Vente J K Lear and P D Battle J Mater Chem 1995 5,1785 J R Randall and R Ward J Am Chem Soc ,1959,81,2629 F Rodi,. Thesis Universitat. Tubingen 1963 C L McDaniel and S J Schneider J Res Nat Bur Stand A face-and corner-sharing situations Furthermore the Sr2+-02 potential derived previously may not be appropriate when the cation occupies a trigonal-prismatic site. The unsatis- factory unit-cell parameters calculated for SrJrO may reflect the inability of our method to deal with anisotropic cation 11 12 13 1971,75,185 A V Powell P D Battle and J G Gore Acta Crystallogr Sect C 1993,49,189 C L McDaniel and S J Schneider J Solid State Chem 1972 4,275 R F Sarkozy C W Moeller and B L Chamberland J Solid environments but it has been suggested32 that the oxide sublattice in this compound is disordered in which case we would not expect our calculations to be accurate Finally note that the new IrIV-O potential described above is defined by a relatively small number of parameters and that many pub- lished potentials include extra terms which model the polariz- 14 15 16 State Chem ,1974,9,242 M Neubacher and H Muller-Buschbaum 2 Anorg A& Chem 1992,607,124 A V Powell J G Gore and P D Battle J Alloys Comp 1993 201,13.T S Bush J D Gale C R A Catlow and P D Battle J Muter Chem 1994,4,831 ability of the cation It may prove possible to improve the transferability of our potential by the use of these additional parameters but the fit to the model compounds used in this study did not improve significantly when they were included 17 18 19 P D Battle,.T S Bush and C R A Catlow J Am Chem Soc 1995,117,6292. T S Bush,C R A Catlow andP D Battle,J Muter Chem 1995 5,1269 F J J Dijksma J F Vente E Frikkee and D J W IJdo Muter Res Bull 1993,28 1145 Conclusion 20 21 J H Carpenter J Less-Common Met 1989,152,35 H M Rietveld J Appl Crystallogr ,1969,2,65 We have shown that Sr41r0 can tolerate a wide variety of cation substitutions on the Sr sublattices Although all the Ir" compounds order magnetically at low temperatures none of the compounds described above shows the spontaneous mag- 22 23 A C Larson and R B von-Dreele General Structure Analysis System Los Alamos National Laboratories Report LAUR 86- 748,1990 J D Gale General Utility Lattice Program (GULP) Royal Institution of Great Britain London 1994 netisation that IS relatively common in complex oxides of Ir l5 More high-quality structural data are needed if reliable Ir-0 potentials are to be developed and used for structure prediction 24 25 26 J G Gore and P D Battle manuscript in preparation R J Hill and H D Flack J Appl Crystallogr ,1987,20,356 K Hayashi G Demazeau M Pouchard and P Hagenmuller Muter Res Bull 1980 16 1013 We are grateful to EPSRC for financial support to C R A 27 J Darriet G Demazeau and M Pouchard Muter Res Bull 1981 16,1013 Catlow for useful discussions and to S G Carling for exper- imental assistance 28 S J Schneider J L Waring and R E.Tressler J Res Nut Bur Stand A 1965,69,245 29 J F Vente Thesis University of Leiden 1994. 30 R C Byrne and C W Moller J Solid State Chem 1970,2,228 References 31 32 J Darriet and M A Subramanian J Muter Chem ,1995,5,543 Q Huang L Soubeyroux 0 Chmaissem I Natali-Sora 1 Y Maeno H Hashimoto K Yoshida S Nishizaki T Fujita A Santoro R J Cava J J Krajewski and W F Peck Jr J Solid. J G Bednorz and F Lichtenberg Nature 1994,372,532 State Chem 1994,112,355 2 T Shimura M Itoh and T Nakamura J Solid State Chem 1992 98,198 Paper 5/06721I Received 10th October 1995