Lithium intercalation and copper extraction in spinel sulfides of general formula Cu,MSn,S, (M =Mn, Fe, Co, Ni) Pedro Lavela," JosC Luis Tirado," Julian Morales," Josette Olivier-Fourcadeb and Jean-Claude Jumas*b "Laboratorio de Quimica Inorgcinica, Facultad de Ciencias, Universidud de Cbrdoba, ES-1404 Cbrdoba, Spain bLaboratoire de Physicochimie des Mutkriaux Solides (URA 407 CNRS), Universitk Montpellier II, Place E. Bataillon, 34095 Montpellier Ckdex 5, France Cu,MSn,S, (M =Mn, Fe, Co, Ni) and Cu, -,MSn,S, (M =Fe, Co) spinel compounds have been studied and used as cathode materials in Li/LiClO,( PC)/spinel sulfide cells. The pristine compounds and electrochemically inserted products have been characterized by X-ray powder diffraction (XRPD) and 'I9Sn, 57Fe Mossbauer spectroscopy (MS).Rietveld analysis and 57Fe MS of the copper extracted phases show that copper extraction takes place from the 8a sites of the spinel structure with oxidisation of the M element. The lithiation process involves a loss of crystallinity and a slight reduction of the host materials. Discharge<harge curves of lithium cells using pristine and deintercalated spinels as cathode materials have been compared. A better performance for the Ni- and Co-containing spinels is observed. After extraction of copper, a significant increase in cell voltage and reversibility is obtained for Co. Since the early report of Eisenberg,' chalcogenide spinels have proven adequate host lattices for the intercalation of lithium ions, which show some advantages over other chalcogenide frameworks.The occurrence of a non-negligible number of empty sites is particularly favourable to guest ions. Although a detailed single-crystal X-ray analysis has shown a very weak tetragonal distortion (not distinguishable from the X-ray powder patterns) for compounds with Cu,MSn,S8 (M =Mn, Fe, Co, Ni) stoichiometry,2 these compounds can be considered as belonging to the Fd3m space group. The sulfur atoms are arranged in a cubic close-packing (ccp) arrangement. The M2+ and Sn4+ ions are randomly distributed in 16d octahedral sites and Cu+ ions occupy the 8a tetrahedral sites defined by the anion array. Thus, the 16c octahedral, 8b and 48f tetrahedral interstices are The 'bottleneck' formed by the S atoms is large enough to allow lithium diffusion through the structure.In addition, the cubic structure allows an isotropic lattice expansion on lithium insertion, and a three-dimensional (3D) electronic and ionic conductivity. Another advantage is that the rigidity of the spinel framework prevents the insertion of water and/or solvent molecules from the electrolyte. The diffusion path of lithium ions can be improved by removing certain ions from the spinel structure. Copper extrac- tion by treatment with mild oxidising agents was first reported in CUT~,S,.~*~ Further work has shown that copper can also ionic distribution can be written (Cus)ga [M,Sn,,],,,S32, have been studied. The complementary use of X-ray powder diffrac- tion and Mossbauer spectroscopy allowed us to characterize the pristine sulfides and to follow the evolution of the electro- chemically inserted products in order to establish the mechan- ism of lithiation.In addition, copper extraction experiments have been performed to study the modified spinels and their lithiation products. Experimental Cu,MSn,Sg thiospinels (M =Mn, Fe, Co, Ni) were synthesized as follows. Stoichiometric amounts of the constituent elements were mixed and sealed in evacuated quartz tubes. The mixture was heated at 300 "C for 24 h, after which the temperature was increased to achieve a final constant value of 750 "C for 8 days. The occurrence of impurities for the phase containing manga- nese led us to change the final temperature to 670°C to improve the purity of the spinel phase.The products obtained were reground and kept under an Ar atmosphere in a dry box. In order to extract Cu+ ions from the structure, the thiospinels were suspended in an excess of 0.1 mol dm-3 1,-CH3CN solution at 50 "C with magnetic stirring for different periods of time (one, two and three weeks). The electrochemical lithium insertion was carried out in Oxidative extraction of copper was carried out in CuZr2S4 by using strong oxidising agents." Also, treatment of CuZr,S4 with a large excess of concentrated butyllithium resulted in the expulsion of almost all the Cu ions as metallic copper, and subsequent treatment with iodine in acetonitrile allowed the preparation of the defect thiospinel The electro- chemical behaviour of the resulting solids in lithium cells was found to improve significantly.",'2 This process has also been carried out with success in CuMS, (M=Cr and Ti) where Cu' ions have been removed by oxidation with an I,-CH,CN s01ution.l~ This method makes evident the presence of the cation M"+ whose oxidation state can be increased in order to equilibrate the net charge into the stoichiometric compound.In all cases, the electrochemical results show a considerable improvement. In the present work, a group of thiospinels of general stoichiometry Cu2MSn,Sg (M =Mn, Fe, Co, Ni), in which the be easily extracted from Chevrel phases such as CU,MO,S~.~-~~ two-electrode cells. The cathode pellets were prepared by pressing at 200 MPa a mixture containing 90% sample and 10% PTFE to improve the mechanical properties of the electrode.The anode consisted of a metallic lithium disk. The electrolyte solution ( 1 mol dmP3 LiClO,/PC; PC =poly-propylenecarbonate) was supported by porous glass-paper disks. The purity and structure of the pristine and modified phases and their corresponding lithiated products were checked by X-ray powder diffractometry (XRPD) with a Siemens D500 diffractometer furnished with Cu-Ka radiation and a graphite monochromator. Rietveld refinements were carried out with the aid of the computer program DBWS-9006 developed by Saktivel and Young.', Mossbauer spectra of the polycrystalline samples were meas- ured with an ELSCINT AME 40 constant acceleration spec- trometer. The y-ray sources were 119mSn in a BaSnO, matrix and 57C0 in a Rh matrix, used at room temperature.The velocity scale was calibrated by using a 57C0 source and a J. Muter. Chem., 1996, 6( l), 41-47 metallic iron foil as absorber. The spectra were fitted with Lorentzian profiles by a least-squares method. The goodness of fit was controlled by chi-squared and misfit tests. All isomer shifts reported here are given with respect to the centre of a BaSnO, and an a-Fe spectrum, respectively, for each source. Results and Discussion Cu2MSn3S8 and LixCU2MSn3s8 (M =Mn, Fe, Co, Ni) The indexation of XRD patterns of the pristine compounds in the Fd3m space group of a cubic spinel shows the purity of the phases obtained by direct synthesis.The "'Sn Mossbauer spectra are characteristic of Sn4+ in a slightly distorted octa- hedral environment. Thus, XRPD and "'Sn Mossbauer data are in good agreement with previous results." Intermediate lithiated phases with nominal stoichiometry Li,Cu,MSn,S, were obtained by discharging a lithium cell at a constant current of 100 pA cm-, for different periods of time. I 1 Cu2MnSn3S8 1 4 F mol-'b-*r 3 1 F mol-' AA . 2 F mol-' 3 F mol-' 4 F mol-' 7 After cutting off the current, the cell voltage was allowed to subside for several days in order to achieve a constant potential and, consequently, a high homogeneity of the lithiated samples. The XRPD patterns of the lithiated spinels [Fig.l(u)-(d)] show a gradual decrease of intensity of the diffraction lines as a consequence of loss of long-range ordering. This fact is less marked for the spinel containing Mn [Fig. 1 (a)].A high degree of crystallinity remains in this sulfide by comparison with the other highly lithiated compounds. In addition, a weaker modi- fication of the a parameter (Table 1) has also been observed in the Mn sulfides with respect to the amount of inserted lithium. For the other compounds, the lithiation process gives way to an increase of the initial u parameter (Table 1).For Co and Ni compounds [Fig. l(c) and (41,these changes are more marked and, additionally, three new effects are clearly observed. First, a new group of wide and weak signals (*) appears when the lithium concentration increases.The low resolution does not allow a reliable indexation of the diffraction planes, but their positions to the left of the main lines seem to indicate 4 F mol-' 28/degrees Fig. 1 XRD patterns of pristine and electrochemically lithiated samples at different depths of discharge: (a) Cu,MnSn,S,, (b) Cu2FeSn3S8, (c) Cu,CoSn,S, and (d) Cu,NiSn,S, Table 1 Cubic cell parameters of the pristine compounds Cu,MSn,S, and after electrochemical lithiation M Mn Fe co Ni pristine compound 10.416( 7) 10.312(5) 10.272(5) 10.276(5) electrochemical lithiation: 2 F mol-' 10.405( 9) 10.322( 7) 10.295(6) 10.284( 6) 3 F mol-' 10.324(7) 10.274(5) 4 F mol-' 10.41 (1) 10.323(5) amorphous 10.293( 6) 6 F mol-' 10.41 (1) amorphous amorphous amorphous 42 J.Muter. Chem., 1996, 6(l), 41-47 that lithium insertion originates in a quasi-amorphous phase with a large spacing which corresponds to highly lithiated domains. Secondly, a very weak signal at 28=43.16" was observed. This peak can be indexed as belonging to Cu metal and is due to de-intercalation of reduced Cu0.l6 Thus, the general stoichiometry can be expressed as Li,Cu2 -,MSn,S8 (O<y<x). The removal of the Cu' ion would give rise to an important distortion of the structure and a reduction of the rigidity of the framework, thus explaining the origin of the low crystallinity phases. Finally, two additional small peaks (@) often occur at 28 = 18.28" and 20.20" for lithiated samples.These lines can be ascribed to the occurrence of a tetragonal superstructure as a consequence of the ordering of the remain- ing Cu atoms in the 8a sites. .-1.24 91 .-1.20 1.16 1.35 1.33 1.30 2.48 2.46 2.44 2.42 2.30 2.28 2.25 -6-4-2 0 2 4 6 '19Sn Mossbauer spectroscopy was demonstrated to be an important tool for studying the behaviour of tin atoms during the redox reaction,17 even where the XRPD patterns show an important loss of long-range ordering at high concentrations of lithium in the structure. The gradual appearance of a new signal at ca. 3.00 mm s-l, corresponding to Sn2+ ions, signals the reduction of tin atoms during the process [Fig. 2(a)-(d)]. However, the evolution was not homogeneous for every com- pound (Table2).Thus, the Sn atoms in Co and Ni sulfides [Fig. 2(c) and (d)] show a higher tendency to be reduced by lithium insertion. The behaviour of the isomer shift, 6, us. depth of discharge is displayed in Fig. 3, where small changes and a non-linear evolution are observed for all compounds. Nevertheless, a 1.24 1.20 1.16 1.12 I .05 I .01 1.04 1.02 I.65 I .64 I.62 2.79 2.75 VJ2.70 E 35 W .z 1.50 $ 1.76 2 5 I .72 I .68 1.50 1.48 I.46 1 .A4 2.34 2.31 2.28 2.25 2.22 1.72 1.70 3' (4 1.68V 4-4-2 0 2 4 t velocity/mm s-1 Fig. 2 l19Sn Mossbauer spectra of pristine and electrochemically lithiated samples at different depths of discharge: (a) Cu,MnSn,S,, (b)Cu,FeSn,S,, (c)Cu2CoSn,S, and (d) Cu,NiSn,S, J.Mater. Chem., 1996, 6(l), 41-47 Table 2 Il9Sn Mossbauer parametersa of the pristine compounds Cu2MSn,S8, and after electrochemical lithiation M Mn Fe co Ni pristine 6 1.102(2) 1.246( 1) 1.202( 1) 1.136( 1) compound d 0.31( 1) 0.321(6) 0.318(6) 0.312(8) r 0.88(1) 0.863(6) 0.863(6) 0.863(8) electrochemical lithiation: 2F mol-' 6 1.104(1) 1.243(4 1.220( 3) 1.163(4) d 0.26(2) 0.29(3) 0.35( 1) 0.28(2) r 0.8qi) 0.92(2) 0.95( 1) 0.85( 2) c 100 100 94( 1) 96( 1) 3.1(1) 0.24(4) A LO( 1) 0 r 1.9(4) 0.8(2) 4F mol-' 6 1.121(3) 1.245( 2 1.205( 4) 1.179( 2) A 0.29(2) 0.31(2) 0.36( 2) 0.303(8 r 0.86(2) 0.79(2) 0.90(1) 0.863(4 c 100 81(1) 86( 1) 84(1)6 2.9( 1) 3.16(5) 2.81 (2) A 1.1(1) 0.6(2) 0.2(2) r 1.6(3) 1.3(2) 1.2( 1) 6 F mol-' 6 1.113(5) 1.224(2 1.214(4) 1.162(4 A 0.32(1) 0.30( 1) 0.36( 1) 0.35 (2) r oq2) 0.85(2) 0.85 (2) 0.84(2) c 81(1) 87(1) 77(1) 76(1)6 2.57(9) 2.97(5) 3.OO (2) 2.76(2) A 0.8(1) 0.87(7) 0.70(3) 0.46(8) r i.6(2) 1.2( 1) 1.01(7) 1.1(1) a 6=isomer shift (mm s-l) relative to BaSnO,; A =quadrupole splitting (mms-I); r=full width at half maximum (mms-I); C= contribution (YO).o CupMnSnsS8 Cu,FeSn3S8 v Cu,CoSn,S, 'I Cu2NiSnSSe1.28 1.24 1.16 'T 1.12 u) E 0 Cu,MnSn,S, 3.6 CulFeSn3S, v Cu,CoSn,S, 3.4 3.2 ' Pi3.0 2.8 ' 2.6 -0 I 1 I I 1 I Fig. 3 Variation of isomer shift (6) versus concentration (x) for Sn4+ and Sn2+ signals 44 J.Mater. Chem., 1996, 6(l), 41-47 small tendency to increase the 6(Sn4+) and decrease the 6(Sn2+) values seems apparent. This fact can be interpreted by the increase of the covalent character of the bonding. The sharing of electrons increases the s electron population around the Sn4+ and gives rise to the opposite effect for Sn2+ ions. cU2-,MSn3S8 and Li,cU, -,MSn3S8 (M =Fe, CO) New phases with a low copper content (CUl.8FeSn& and CU,.~COS~~S~)[Fig. 4(u) and (b)] have been identified by XRPD from the reaction of the pristine spinels with Fe or Co in their compositions with a 0.1 mol dm-3 12-CH3CN solution. A diminution of the cell parameter and the intensity of re- flection lines such as (220) and (422), which was proportional to the Cu concentration, was observed.These peaks originate from the tetrahedral Cu sites in a perfect spinel. A detailed Rietveld analysis (Fig. 5) of the diffraction data shows that copper extraction effectively takes place from the 8a sites without significant redistribution of the metal ions in octa- hedral coordination (Table 3). Moreover, the site occupancy values obtained by profile fitting agree well with the values obtained by energy dispersive X-ray microanalysis (EDXA; Table 3). In addition, the higher width of the above-mentioned peaks demonstrates that the copper extraction process causes a decrease in crystallinity. This preferential broadening may be taken as indicative of incomplete displacement of copper ions from 8a sites.In order to achieve the Cu reduction, an increase of the transition-metal oxidation state is necessary to maintain elec- troneutrality, and this was examined by 57Fe Mossbauer spectroscopy. The spectrum obtained (Fig. 6)shows two signals 1 AIk 1\ A A. S roekiI ..-A &.. A A-LI1 1 10 20 30 40 50 60 28 /degrees Fig. 4 XRPD patterns of pristine samples and those after several periods of treatment with 1,-CH3CN solution: (a) Cu2 -,FeSn,S, (x =0, 0.2) and (b) Cu2-,CoSn3S8 (x =0, 0.9) Table 3 Results of the Rietveld analysis of XRPD data of Cu,FeSn,S, ,Cu,-,FeSn,S, ,Cu,CoSn,S, and Cu,-,CoSn,S," 8a site time of occupancy I, treatment/ Y y in weeks (EDXA) a/W CU, -,MSn,S, ZS s=R,/Rex, Cu,FeSn,S, 10.303(5) 2.00 0.255(3) 2.41 Cu, -,FeSn,S, 2 1.84(5) 10.303(5) 1.91 0.254(3) 2.38 Cu,CoSn,S, 10.266(5) 2.00 0.257(3) 2.20 Cu, -,CoSn,SB 1 1.11(3) 10.150(5) 1.43 0.249(3) 2.79 Cu, -,CoSn,S, 2 1.16(3) 10.150(3) 0.99 0.257(3) 2.54 "Space group Fdjm; Cu site 8a: 1/8, 1/8, 1/8; Fe/Co site 16d: 1/2, 1/2, 1/2; S site 32e: z, z, z.which can be considered as due to the overlapping of a pair 1.204(1) mm s-l for Cu1.,CoSn3S8] that can be interpreted as of quadrupole split signals. At room temperature the spectrum due to a small increase in the covalent character of the of Cu,FeSn3S8 corresponds to a hyperfine interaction which is Sn-S bonding. a population-weighted average of the low-spin (LS) and high- Electrochemically lithiated Li,Cul.,FeSn3S8 and spin (HS) states.18 The study of the Mossbauer parameters for Li,Cu1~,CoSn3S8 products have been obtained.In both cases, Cu1.8FeSn& allows us to estimate the decrease of the isomer the lower Li' concentration gives rise to the occurrence of shift values (Fig. 6). It can be explained by a slight reduction double lines in XRPD patterns, more conspicuously for the of the screening effect caused by the loss of d electrons. It Co spinel (Fig. 7). After considering the u p?rameters corre-favours an increase of the interaction of 4s electrons with the sponding to these domains (10.16 and 10.26 A), these phases atomic nucleus. In addition, a slight decrease of the quadrupo- can be ascribed to the heterogeneity of the lithiation reaction. lar splitting values has been ascribed to the presence of a d5 At a discharge depth of 6 F mol-l, complete loss of crystallinity electronic configuration which homogenizes the electronic dis- was observed.tribution around the Fe atoms. The decrease of 6 and d values Once again, "'Sn Mossbauer spectroscopy shows a very expresses a partial oxidisation of Fe" to Fe"'. The treatment slight modification of the parameters (Table 4), except for of experimental data by the superposition of two doublets is L~,CU~~~COS~,S,(6 F mol-l) where a non-negligible decrease a simplifying calculation. In fact, the spectrum for CUl.8FeSn3S8 of the isomer shift and the presence of SnO, were observed, is probably a population-weighted average of Fell, FeI'I, LS probably caused by the highly amorphous nature of the and HS states.A study of spectra recorded at different tempera- compound. Thus, a similar behaviour of the Sn atoms in these tures would be necessary to distinguish these different states. compounds to that of Sn in Li,Cu2MSn3S8 can be expected. In the same way, these conclusions can be applied to Co atoms. Discharge<harge curves of lithium cells using pristine and ll'Sn Mossbauer spectra show a very slight increase of the Cu deintercalated spinels as cathode materials are shown in isomer shift [S= 1.272(2)mm s-l for Cul.8FeSn& and 6 = Fig. 8. For the pristine samples, the following reaction is 8 60 I I II L. I r~ r I Ir ....' ... ..-L..'... 10.00 2o:oo 30:00r60:00 7 w 2 8 /degrees Fig. 5 Selected results of the Rietveld refinement of XRPD data: (a)Cu,FeSn,S,, (b)Cu,-,FeSn,S,, (c) Cu,CoSn,S8 and (d)Cu2-,CoSn,S8 J.Muter. Chem., 1996, 6(l),41-47 45 Table 4 l19Sn Mossbauer parameters' of pristine and electrochemically lithiated Cu,.,FeSn,S, and Cu,.,CoSn3S, pristine compound 6 1.275( 1) 1.204( 2) A 0.32 1 (9) 0.34( 1) r 0.924(8) 0.95( 1) 2 F mol-' 6 1.25 1 (3) 1.192( 3) A 0.27(2) 0.33(2) r 0.94(2) 0.94(2) C 100 100 4 F mol-' 6 1.244(3) 1.191(3 A 0.31(1) 0.32( 3) r 0.830( 6) 0.97(2) C 92( 1) 95( 1) 6 3.09(7) 3.28 (9) A 0 0.6(3) r 1.5(6) 1.1(4) 6 F mol-' 1.250(6) 1.178( 3 0.34( 1) 0.32(2) 0.80(2) 0.97(2) 80( 1) 85(1)2.7( 1) 3.04(4) LO( 1) 0.72( 7) 1.5(2) 1.0(1) ~ '6=isomer shift (mm s-l) relative to BaSnO,; A=quadrupole splitting (mm s-'); T=full width at half maximum (mm s-'); C= contribution (%).0.o 2.44 0.2 2.43 0.4 0.6 2.42 -s v E0.8 Y YL 1.290g0.0 .-0 n v)m 9 .,. 1.zee.g a21 Fe 1I CU,,~F~S~~S~ 1.286 (1 week) I -1.284 -1.282 -1.280 V I ~~~ ~ -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 velocity/mm s-1 Fig. 6 57Fe Mossbauer spectra of (a) Cu,FeSn,S, [low spin: 6 = 0.810(4), A=O.96(2), r=0.38( l), C=69.5%; high spin: 6=0.850(6), A= 1.35(2), r=0.28(3), C=30.5%] and (b) Cu,,,FeSn,S, [low spin: 6=0.742(8), d=O.604(2), r=0.43(3), C=64.1%; high spin: 6= 0.82(1), A= i.i7(2), r=o.35(3), c=35.9%1 J. Mater. Chern., 1996, 6(l), 41-47 4 P mo1-' I 10 20 30 40 50 60 2O/degrees Fig.7 XRPD patterns of pristine and electrochemically lithiated samples at different depths of discharge: (a) Cu, -,FeSn,S8 (x =0.2) and (h)Cu,-,CoSn,S, (x=0.9) possible during the first steps of cell discharge: xLi +Cu2MSn3S8+Li,Cu2 -.MSn3S8 +xCu (x d 1, E/V us. Cu'/Cu) The extent of this reaction for each transition metal, M, under the dynamic conditions of the discharge experiments affects the reversibility of the process for discharge depths below 1F mol-'. This is higher for Co and Ni compounds than for Fe and Mn. Copper is not reabsorbed into the spinel on cell charging as evidenced by the presence of Cuo reflections in the XRPD patterns of the recharge product. For larger discharge depths: yLi +Li,Cu2 -,MSn3S8 +Li,+,Cu,-,MSn,S8 (EIVus.Sn4+/Sn2+) This latter reduction plus lithium-ion insertion process takes place with a progressive amorphization of the reaction product. The tin reduction process was evidenced by the Mossbauer data of samples obtained at discharge depths of ca. 2 F mol-' and is accompanied by the amorphization of the solid as evidenced by the XRPD patterns of the samples. For those samples submitted to chemical copper extraction, according to: CU2MSn3S8+(X/2)12+cU2 -,MSn3S8 +XCUI the first part of the discharge curve of the lithium cells is: xLi +Cu2_,MSn3S8 +Li,Cu2-,MSn3S8 (Epus. M3+/M2+) This reaction implies that larger cell voltages are expected under open circuit voltage (OCV) conditions, as a result of the 5.0 -(a) Cu,MnSn,S, Cu,Fe Sn,S, 4.0 -3.0 -\ 1.o 1.0 tkt 5.0 -CuzFeSn,S, 5.0 1 Cu,,,FeSn,S, 4.0 r 1 .o 5.0 -Cu,CoSn,S, 1.0 11 1.0 \I{ Cu,NiSn,S, 5.0 1 CU,,~C~S~,S~ 2 .o 1.o 1.0' I I I I I 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 depth of discharge/F mol Fig.8 Galvanostatic discharge-charge experiments of lithium cells using (a)pristine and (b)Cu-deintercalated compounds as cathodic materials reduction pair involved (E/V us. M3+/M2+),as compared with pristine samples. This is shown by a difference of 2.2 us. 1.8 V in the plateau observed under continuous galvanostatic dis- charge. Thus the strategy presented here allows control of the cell voltage. Moreover, the better diffusivity of Li+ ions into the copper-extracted products decreases the polarization of the cells during discharge.Further lithiation will result in a similar reaction to the pristine solid and with similar progressive amorphization: yLi +Li,Cu, -.MSn,S8 4Li, +,Cu2 -,MSn3S8 Thus, this behaviour makes copper extraction an interesting procedure to increase the performance of lithium cells using cathodic materials with a spinel structure, and to control the cell voltage. The authors acknowledge the financial support of EC (Contract JOU2-CT93-0326) and the Ministries of Education (Spain) and Foreign Office (France) (Picasso Program). References 1 M. Eisenberg, J. Electrochem. SOC., 1980,127,2382. 2 J. C. Jumas, E. Philippot and M. Maurin, Acta Crystullogr., Sect. B, 1980,36, 1993. 3 P.de la Mora and J. B. Goodenough, J. Solid State Chem., 1987, 70, 121. 4 A. C. W. James, J. B. Goodenough and N. J. Clayden, J.Solid State Chem., 1988,77,356. 5 J. Morales, J. L. 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Atlung, Electrochim. Acta, 1989,34, 1473. 17 M. L. Elidrissi Moubtassim, J. Olivier-Fourcade, J. Senegas and J. C. Jumas, Muter. Res. Bull., 1993, 28, 1083. 18 M. Womes, J. C. Jumas, J. Olivier-Fourcade, F. Aubertin and U. Gonser, Chem. Phys. Lett., 1993,201, 555. Paper 5/03947I; Received 19th June, 1995 J. Mater. Chew., 1996, 6(l),41-47 47