J. CHEM. SOC. FARADAY TRANS., 1994, 90(8), 1171-1175 Mossbauer Study of Oxygen-deficient Zn"=bearing Ferrites (Zn,Fe,-,O,-~ 0 Ix 5 1) and their Reactivity toward CO, Decomposition to Carbon Masahiro Tabata, Kazuhiro Akanuma, Takayuki Togawa, Masamichi Tsuji and Yutaka Tamaura" Department of Chemistry, Research Center for Carbon Recycling & Utilization, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152,Japan Oxygen-deficient Zn'l-bearing ferrites (Zn,Fe,-,O,-, , 0 <x < 1, 6 > 0) have been synthesized and studied for their reactivity in the decomposition of CO, to carbon at 300 "C. They were prepared by reducing 2n"-bearing ferrites with H, gas at 300°C.The oxygen-deficient 2n"-bearing ferrites consisted of a single phase of a spinel-type structure which was oxygen-deficient compared with stoichiometric composition.Their lattice con- stants were larger than those of the corresponding stoichiometric spinel. Decomposition of CO, to carbon was accompanied by an oxidation of the oxygen-deficient 2n"-bearing ferrite. The amount of carbon deposited on the solid decreased when the Zn content in the 2n"-bearing ferrite increased. The decrease in decomposition rate is due to changes in the electron conductivity according to the Zn content in the Znl'-bearing ferrite. These changes may contribute to its reactivity for decomposition of the CO, to carbon. We have reported the synthesis of an oxygen-deficient mag- netite and its reactivity in the decomposition of CO, to carbon at 300°C.' The oxygen-deficient magnetite is a non- stoichiometric magnetite with spinel structure.It can be obtained by reducing magnetite with H, at 300 "C. Generally, magnetite exhibits deviations from stoichiometry with an excess of oxygen. However, the present magnetite is non- stoichiometric due to oxygen deficiency. This non-equilibrium oxygen-deficient magnetite was found to decompose CO, to carbon at around 300°C. The reaction was carried out by means of a batch system. In this reaction, a decrease in the amount of CO, injected was accompanied by an evolution of CO. No CO, or CO was evolved after completion of the reaction. No other gases were observed during the reaction. The reduction of CO, was due to the incorporation of the oxygen of CO, into the oxygen-deficient site of magnetite during the deposition of carbon. More recently, Mn"-bearing ferrites with the same spinel structure were studied in order to determine the effect of sub- stitution of divalent metal ions into magnetite on its reacti- vity towards decomposition at CO,.,p3 The oxygen-deficient Mn"-bearing ferrite could be synthesized by H, reduction of Mn"-bearing ferrites at 300°C.These ferrites were found to cause decomposition of CO,. The amount of deposited carbon was small compared with that obtained by the reac- tion with oxygen-deficient magnetite. An increase in the Mn content in Mn"-bearing ferrite resulted in a decrease in the amount of deposited carbon. This could be explained by the difference in electron conductivity between Mn"-bearing ferrite and magnetite.In the present paper, we have studied the synthesis of oxygen-deficient Zn"-bearing ferrites (ZnxFe3-xO, -d, 0 < x < l), their Mossbauer spectra and their reactivities for C02 decomposition. The effects of the level of substitution of metal ions and the difference in reactivity for CO, decompo-sition are also discussed. Experimental Materials All of the chemicals employed were of analytical grade, and distilled water was used for preparation of the solution. FeSO, -7H,O, ZnSO, 7H2O and NaOH were supplied by 1 Wako Chemical Industries, Ltd. Preparation of Magnetite and Zn"-bearing Ferrite Magnetite and Zn"-bearing ferrites (Zn,Fe3 -x04-d, 0 < x < 1) were synthesized by oxidation in air of aqueous suspensions of Fe" and Zn" mixed hydr~xide.~ The requisite quantities of FeSO, .7H,O and ZnSO, 7H20 were dis- solved in C0,-and 0,-free water (4 dm3) prepared by passing nitrogen gas through distilled water for a few h. The solution was adjusted to pH 10 by adding 3 mol dm-3 NaOH solution. Then, air was passed through the alkaline suspension for 6 h at 65°C. The reaction pH was kept con- stant at pH 10 by adding NaOH solution. The product was collected by decantation, washed with distilled water and acetone successively, and then dried in UQCUO at 65°C. The dried samples were placed in a quartz tube and heated in an N, stream for 1-3 h at 300"C. The products were identified by means of X-ray diffracto- metry with Cu-Ka radiation (Rigaku model RNT-2000 diffractometer) and Mossbauer spectroscopy. The lattice con- stants of the samples were calculated by extrapolating the values of a, vs.the Nelson-Riley function, cos2 8/ sin 8 + cos28/8, to zero using the least-squares All of the Mossbauer spectra were recorded at room tem- perature with a 57C0source diffused in metallic Rh, which was oscillated in constant acceleration mode. The spectra were calibrated with a thin absorber comprising an a-Fe foil. The chemical compositions of the products were determined by induced coupled plasma atomic emission spectroscopy (ICP-AES) ( Seiko Instruments model SPS 7000) for analysis of the Zn2 + and Fetotal contents, and colorimetry' (Hitachi Model Photospectrometer 124) with 2,2'-bipyridine for the Fe2 and Fetotal molar ratio.The surface areas of the samples + were determined by the BET method using N, adsorption (Yuasa Ionics model Quantasorb). Reactivity of Oxygen-deficient Magnetite and ZnII-bearing Ferrite towards CO, Decomposition Magnetite or Zn"-bearing ferrite (1.00 g) was placed in a quartz cell (20 mm in diameter and 200 mm long), as in pre- vious The cell was heated to 300°C in an electric furnace while the reaction cell was evacuated with an oil rotary pump. After evacuating the reaction cell for 5 min at 300 "C, H, gas was passed through the ferrite at a flow rate of 0.018 dm3 min-' for 2 h at 300°C. After the H,-reduction process, the reaction cell was evacuated and CO, gas (2 x dm3 or 8.32 x lo-' mol) was introduced with a microsyringe (recorded as a zero reaction time).The internal gas species were determined by gas chromatography with a thermal conductivity detector (TCD) (Shimadzu model GC-gA, using Porapak Q and molecular sieve 13X as adsorbents). The solid sample was quenched after the reac- tion by quickly placing the reaction cell into a refrigerant of ice. The solid phases of the reduced samples were analysed using Mossbauer spectroscopy. The solid phases of the sample before and after the CO, decomposition reactions were identified by X-ray diffractometry with Cu-Ka radi- ation. The amounts of carbon deposited on the samples were measured using an elemental analyser (Perkin-Elmer model 2400 CHN).The surface of the quenched samples after decomposition of CO, was analysed by means of FTIR spec- troscopy (Shimadzu model FT-IR 8500) with the KBr disc technique. Results and Discussion Characterization of Magnetite and Zn"-bearing Ferrite Only peaks of the cubic spinel structure appeared in the X-ray diffraction (XRD) patterns of the prepared magnetite and Zn"-bearing ferrites. The lattice constants, chemical com- positions and BET surface areas of the samples are shown in Table 1. The surface area was smallest for a Zn :Fe mole ratio of 0.0 : 1 (magnetite) and largest for a Zn : Fe mole ratio of 0.495 : 1. The lattice constant increased as Zn" content in the reaction solution increased. Chemical analysis showed that Zn" content in the sample increased with an increase in the Zn" content in the reaction solution.These findings confirm the Fe" ions in the spinel ferrite were replaced by Zn" ions and that the amount of Zn" ions on a lattice point increased with increasing Zn :Fe mole ratio of the reaction solution. The accuracy of the lattice constant of the sample with a Zn :Fe mole ratio of 0.495 : 1 was low, owing to the smaller crystal grain size compared with that of the other samples. The chemical compositions of the samples with Zn :Fe mole ratios of 0 : 1 (magnetite) and 0.0964 : 1 showed deviations from the stoichiometric composition. These com- positions could be expressed as 0.70Fe,0,-O.30y-Fe,O3 and 0.264ZnFe,04-0. 588Fe ,O4-0.224y-Fe20 , respectively, assuming that the compounds are solid solutions of stoichio- metric Zn"-bearing ferrite, ZnxFe3 -x04, and maghemite, y-Fe,03 .The samples with Zn, Fe mole ratios of 0.334: 1 and 0.495 :1 showed nearly stoichiometric compositions. Characterization of Oxygen-deficient Magnetite and Zn"- bearing Ferrite The XRD patterns of the H,-reduced magnetite and Zn"- bearing ferrites showed only peaks assigned to the cubic spinel structure. The lattice constants of these compounds increased upon reduction with H, for 2 h (Table 1). The lattice constant increased as Zn" content in the oxygen- deficient Zn"-bearing ferrite increased. These lattice constants J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 were larger than those of the corresponding spinels of stoi- chiometric composition. All of the reduced samples were oxygen-deficient compared with the stoichiometric composi- tion.These changes in chemical composition indicate that some Fe3+ ions in the Zn"-bearing ferrites were reduced to Fe2+ ions. These ferrites are unstable in air and easily oxi- dized at room temperature. Qian et aL9 have studied the sta- bility of Zn"-bearing ferrite with a Zn :Fe mole ratio of 0.5 : 1 in an H, atmosphere, and found that the Zn" ferrite decreased in weight in an H, flow while heating at a rate of 30°C h-'. They reported that the reduction of Zn" ferrite took place at 495°C where the solid phase gave a mixture of a-Fe and ZnO. Our recent study on the reduction of Zn" ferrite, ZnFe,O,, at 300°C '* showed that Zn" ferrite was reduced to a mixture of ZnO and FeO (Wustite) after 12 h H, reduction at a flow rate of 0.20 dm3 min-'.Therefore, the H,-reduced Zn"-bearing ferrite prepared in the present study was the oxygen-deficient Zn"-bearing ferrite. This is a new metastable phase of Fe-Zn oxide. We could thus prepare oxygen-deficient Zn"-bearing ferrites with spinel structure. Miissbauer Spectra Magnetite The Mossbauer spectra of magnetite are similar to those reported previo~sly.~ The spectrum gave the usual two sextets. A sextet with a smaller isomer shift was assigned to the Fe3+ ions in the tetrahedral site (site A) and the other was assigned to the Fe2+ and Fe3+ ions in the octahedral site (site B).The Mossbauer parameters are presented in Table 2 The site A to site B area ratio (AJAB) was 0.85, which is larger than that of stoichiometric magnetite. The larger value is due to the presence of Fe3+ ions which do not contribute to the electron hopping between B sites. The absorption due to these Fe3+ ions does not contribute to the site B spectra but on to the site A spectra." The Mossbauer patterns and the parameters of oxygen- deficient magnetite were similar to that of the unreduced magnetite (Table 2).3 However AJAB was 0.537, which is larger than that when the composition is stoichiometric (0.5). Since no other phase was observed in the XRD and Moss- bauer spectrum of the oxygen-deficient magnetite, the excess Fe2+ ions must be one of the constituents of the oxygen- deficient magnetite.These excess ions may be distributed among the interstices of the spinel structure. Thus, the increase in AJAB can be ascribed to the contribution of Fe2+ ions in the tetrahedral interstices which have migrated from the B sites. Unreduced Zn"-bearing Ferrite Mossbauer spectra of the Znn-bearing ferrite with a Zn :Fe mole ratio of 0.0964 : 1 (a), 0.334 : 1 (b) and 0.495 : 1 (c) are shown in Fig. 1. The parameters in Table 2 were evaluated by fitting the full spectra to two sextet subspectra with Lorentz- ian lineshapes. The spectrum of sample (a) consisted of a sharper sextet with a smaller isomer shift and a broader one with a larger isomer shift. The sextet with lower isomer shift Table 1 Chemical compositions, lattice constants and surface areas of Zn"-bearing ferrites before and after H2 reduction chemical composition lattice constant, ao/nm before reduction after reduction 0.8391 0.8399 0.8406 0.8409 0.8420 0.8435 0.8449 0.8450 Zn :Fe 0: 1 0.0964: 1 0.334 : 1 0.495 : 1 before reduction Fe3.0004.09 zn0.264Fe2.7404.08 zn0.752Fe2.2504.00 zn0.995Fe2.0104.00 after reduction Fe3.0003.93 zn0.264Fe2.7403.97 zn0.752Fe2.2503.94 Zn0.995Fe2.0103.96 surface area/m2 g-' 8.07 16.63 13.00 29.45 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Mossbauer parameters of magnetite and Zn"-bearing ferrites before and after H, reduction isomer shift/mm s -quadrupole splitting/mm s -magnetic hyperfine field/MA m -Zn : Fe AJAB A B before reduction after reduction 0: 1{ 0.850 0.537 0.290 0.273 0.662 0.662 0.0964: 1 before reduction after reduction 0.458 0.311 0.307 0.295 0.593 0.603 0.334: 1 before reduction after reduction a a 0.495 : 1 before reduction after reduction 0.352 0.357 Spectrum could not be decomposed to one sextet and one doublet.may correspond to the site A spectrum and the other one to the site B spectrum. AJAB of the subspectra was 0.458. If all of the Fe ions in site B contribute only to the site B sub-spectrum, AJAB should be 0.368. This may be caused by some Fe3+ ions in site B. If the Zn"-bearing ferrite is not a solid solution of stoichiometric Zn"-bearing ferrite and maghemite (y-Fe203) but a mixture of them, the chemical composition of the sample with a Zn: Fe mole ratio of 0.0964 : 1 can be expressed as 0.851Zn0.,loFe2~6904~oo-0.2~~y-Fe2~3. A,/AB of the sub- spectra can be calculated according to the pair-wise localized electron-hopping model of Daniel and Rosencwaig l1 using -10 -5 0 5 10 velocity/mm s -Fig.1 Mossbauer spectra of Zn"-bearing ferrite with a Zn : Fe mole ratio of (a)0.0964 : 1, (b)0.334 : 1 and (c)0.495 : 1. (-) Least-squares fit. A B A B 0.0252 -0.0256 -38.86 -36.80 0.000392 -0.00352 -38.90 -36.85 0.0371 -0.0101 -38.54 -35.46 0.0272 0.00926 -38.30 -35.52 0.414 0.394 the follow equation (all Fe ions in site A of Zno~31Fe2~6904~oo) + (all Fe ions in y-Fe203) AJAB = (1)all Fe ions in site B of Zno~31Fe2~6904~oo The calculated AJAB was 0.664, which is larger than the empirical value of 0.458.Hence, the Zn"-bearing ferrite is not a physical mixture of stoichiometric Zn"-bearing ferrite and y-Fe,O, , but the solid solution. The spectrum of the sample with a Zn : Fe mole ratio of 0.334 : 1 comprised a broad sextet and a sharp doublet [Fig. l(b)]. The latter was ascribed to a quadrupole splitting. The broad sextet could not be resolved and the Mossbauer parameters are not given in Table 2. The spectrum of sample (c) showed only a quadrupole splitting. The appearance of quadrupole splitting patterns for samples (b) and (c) may be due to domain-wall oscillations, as suggested by Srivastava et a1.12.13 Oxygen-deficient Zn"-bearing Ferrite The Mossbauer spectra of the H,-reduced sample with Zn : Fe mole ratios of (a') 0.0964 : 1, (b')0.334 : 1 and (c') 0.495 : 1 are shown in Fig.2. The spectra were similar to that of the unreduced Zn"-bearing ferrite, indicating that the syn- thesized H,-reduced Zn"-bearing ferrite contains no metallic Fe component. The spectra of sample (a')could be separated into two sextets with Lorentzian lineshapes. The site B spec-trum became sharp after H, reduction. This indicates an increase in the number of Fe ions which take part in the elec- tron hopping between the B sites. The isomer shift, quadru- pole splitting and magnetic hyperfine field were nearly equal to those of the unreduced samples (Table 2).This suggests that the Fe2+ : Fe3+ mole ratio in site B is not changed. AJAB was 0.311, which was smaller than that of the unre- duced sample (0.458) and that of the sample with stoichio- metric composition (0.368).Therefore, the amount of Fe ions in site A decreased and the amount of Fe ions in site B increased, while the Fe2+ : Fe3+ mole ratios in site B remained constant. Therefore, some of the Fe3+ ions in site A were allowed to migrate to site B, and some of the migrant Fe3+ ions were reduced to Fe2+ ions. The pattern of migra- tion during the formation of oxygen-deficient Zn"-bearing ferrite may be different from that of oxygen-deficient magne- tite. As mentioned above, Fe ions in site B of magnetite will migrate to site A.This difference may be due to the presence of Zn ions in site A. The Mossbauer spectrum of sample (b') showed a broad sextet and a sharp doublet (Fig. 2). The shape of the pattern was very similar to that of the unreduced sample. The spectra 1174 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 10 I -10 -5 0 5 10 velocity/mm s-l Fig. 2 Mossbauer spectra of oxygen-deficient ZnII-bearing ferrite with a Zn : Fe mole ratio of (a') 0.0964: 1 (b') 0.334: 1 and (c') 0.495 : 1. (-) Least-squares fit. could not be fitted to one Lorentzian sextet and one quadru- pole splitting pattern. The details are not clear and need to be studied. The Mossbauer spectrum of sample (c')shows only a quad- rupole splitting (Fig. 2). The spectra could be analysed as a single quadrupole splitting.The Mossbauer parameters were nearly equal to that of the unreduced Zn"-bearing ferrite. This indicates that changes in the Fe2+ : Fetota, do not affect the parameters in the region of Zn : Fe, where their spectra consist only of quadrupole splitting. CO, Decomposition with Oxygen-deficient Zn"-bearing Ferrites Decomposition of CO, was studied on sample (Q') at 300°C (Fig. 3). The initial CO, content in the reaction cell was 8.2 kPa. The CO, content decreased and became <lo% of the initial amount of CO, injected after 30 min and decreased to 30% after 4 h. On the other hand, CO was formed imme- diately after injection of CO, and its content increased up to 10% of the injected CO,. The amount of CO gradually decreased and became <3% of the initial CO, amount after 4 h.No other gases were observed during the reaction. The lattice constant of the ferrite was restored to that prior to H,-reduction after CO, decomposition. Fig. 4 and 5 show the time variations of the gas content during the reaction of CO, with samples (b')and (c'),respectively. A similar pattern for decrease in CO, and evolution of CO could be seen for 0 0 b 0" 0,." 0 100 200 ti me/mi n Fig. 3 Changes in the composition of CO, (0) duringand CO (0) the CO, decomposition reaction with oxygen-deficient Zn"-bearing ferrite (Zn : Fe-0.0964 : 1) at 300°C both samples. The amount of CO evolved was small with sample (c'). Carbon particles were observed after dissolving the reacted ferrites.The amount of carbon deposited was determined by dissolving reacted ferrites in HC1 solution, 0.874, 0.478 and 0.312 mg for samples (a'), (b') and (c'), respectively. The amounts of carbon were small compared with those esti- mated from the gas content. No peaks for CO and carbon- ates were observed in the IR spectrum of the ferrite after C02 0""b----0 0 0 100 200 time/min Fig. 4 Changes in the composition of CO,(O) and CO (0)during the CO, decomposition reaction with the oxygen-deficient Zn"- bearing ferrite (Zn : Fe = 0.334 : 1)at 300 "C I" I b - IY n 4-l "n 0 100 200 time/m i n Fig. 5 Changes in the composition of CO, (0)and CO (0)during the CO, decomposition reaction with oxygen-deficient Zn"-bearing ferrite (Zn : Fe = 0.495 : 1) at 300°C J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 decomposition. The carbonate compound (CaCO,) cannot be detected by our equipment when the C : ferrite weight ratio is below 0.001 : 1 on a weight basis. Therefore, the amount of carbon deposited after CO, decomposition could not be detected by means of IR spectroscopy, because the maximum amount of carbon deposited is 1.00 mg in the present experi- ment. In situ surface and bulk investigations of the sample during the CO, decomposition reaction will provide informa- tion on the intrinsic physicochemical properties of the ferrite. Further work is in progress. The smaller recovery of carbon can be explained as follows.Two types of carbon deposited on the metallic Ni catalyst have been rep~rted.'~ The c1 form of carbon is considered to consist of isolated carbon atoms on the surface and the p form is assigned to polymerized carbon. In the present work, at least two types of carbon were probably deposited on the surface. The elementary carbon could not be collected in the present procedure of dissolving the sample in HCl solution, because it would be washed out. Therefore, the reaction of oxygen-deficient Zn"- bearing ferrite with CO, may be expressed as the following stepwise processes : physical adsorption [eqn. (I)], chemical decomposition of adsorbed CO, to toad and Olattice[eqn. (11)] and further decomposition of adsorbed CO to carbon Csurface and Olattice (eqn. (WI.c02 c02 ad 'O2 ad Oad + Olattice (11) COad +Csurface + Olattice (111) The relationship between the amount of carbon deposited and the Zn :Fe shows that an increase in Zn content resulted in a lowering of the amount of deposited carbon [Fig. 6(a)]. This is in good agreement with the change in electron con- ductivity calculated from the electric resistivity data by Dobson et a1." [Fig. qb)].This can be interpreted in forms 0 1.2 EiTc,.-g 0.8 n -0 0.4 0.0 0.0 0.2 0.4 Zn/Fe c I E T3r 4-.-.-> c, $2 U s 1 4-Q, Q, 00.0 0.2 0.4 Zn/Fe Fig. 6 Relationship between Zn : Fe mole ratio and (a)the amount of deposited carbon after CO, decomposition, (b) the electric conduc- tivity, based mainly on the data of Dobson et ~1.'~ .; of the relationship between Zn content in Zn"-bearing ferrite and the charge-transfer process.The electric resistivity increases slowly with a Zn : Fe mole ratio of up to about 0.25 and rapidly increases at a Zn : Fe mole ratio of >0.25 : 1. The variation of the electric resistivity is in agreement with the variation of the Mossbauer spectra with Zn : Fe mole ratio." As noted above, two sextet peaks were observed in a region where the mole ratio of Zn :Fe was <0.25, i.e. rapid electron hopping among B sites occurs. In the region of Zn : Fe > 0.25 : 1, a quadrupole splitting became the main feature of the absorption spectrum. Conclusion Oxygen-deficient Zn"-bearing ferrites were synthesized by reducing Zn"-bearing ferrites with H, gas for 2 h at 300°C.Their lattice constants (ao)were larger than those of the unre- duced forms. The Mossbauer spectrum of the sample with a Zn : Fe mole ratio of 0.0994 after H, reduction showed a sharpening of the B site spectrum. For the samples with Zn : Fe = 0.334 : 1 and 0.495 : 1, the Mossbauer spectra were slightly changed by H, reduction. These oxygen-deficient Zn"-bearing ferrites have been found to be reactive towards CO, decomposition at 300°C. The CO, oxygen was incorp- orated into the solid phase, accompanied by the deposition of carbon on the surface of the oxygen-deficient Zn"-bearing ferrite. CO, may be decomposed to carbon with the concomi- tant formation of CO.The amount of carbon deposited decreased as the mole ratio of Zn : Fe increased. The amount of deposited carbon decreased in the case of the reaction with samples of higher Zn :Fe mole ratio. This relationship between the amount of carbon deposited and the Zn content can be understood in terms of the electron conductivity of the Zn"-bearing ferrite. The present work was partially supported by a Grant-in-Aid for Scientific Research No. 03203216 from the Ministry of Education, Science and Culture, Japan. M. T. and K. A. are grateful for grants from Fellowships of the Japan Society for the Promotion of Science for Japanese Junior Scientists. References 1 Y. Tamaura and M. Tabata, Nature (London), 1990,346,255. 2 M. Tabata, Y. Nishida, T. Kodama, K. Mimori, T. Yoshida and Y. Tamaura, J. Muter. Sci., 1993,28,971. 3 M. Tabata, K. Akanuma, K. Nishizawa, T. Yoshida, M. Tsuji and Y. Tamaura, J. Muter. Sci., 1993,243,6753. 4 T. Kanzaki, J. Nakajima, Y. Tamaura and T. Katsura, Bull. Chem. SOC. Jpn., 1981,54,135. 5 J. B. Nelson and D. P. Riley, Proc. Phys. SOC. London, 1945, 57, 160. 6 A. Taylor and H. Sinclair, Proc. Phys. SOC. London, 1945, 57, 126. 7 R. H. Geiss, Adu. X-Ray Anal., 1961, 5, 71. 8 I. Iwasaki, T. Katsura, T. Ozawa, M. Yoshida, M. Mashima, H. Harashima and B. Iwasaki, Bull. Volcanol. SOC. Jpn. Ser. ZZ, 1960,5, 9. 9 Y-T, Qian, R. Kershaw, S. Soled, K. Dwight and A. Wold, J. Solid State Chem., 1984, 52, 211. 10 T. Kodama, M. Tabata, K. Tominaga, T. Yoshida and Y. Tamaura, J. Muter. Sci., 1993,243, 547. 11 J. M. Daniel and A. Rosencwaig, J. Phys. Chem. Solids, 1969, 30, 1561. 12 C. M. Srivastava, S. N. Shringi and R. G. Srivastava, Phys. Rev. B, 1976,14,2041. 13 C. M. Srivastava and M. J. Patni, J. Magn. Reson., 1974, 15,359. 14 J. G. McCarty and H. Wise, J. Catal., 1979,57,406. 15 D. C. Dobson, J. W. Linnett and M. M. Rahman, J. Phys. Chem. Solids, 1970, 31, 2727. Paper 3/0479OC; Received 9th August, 1993