REACTIVITY OF THE SUBMICRON MOLYBDENUM FERRITES TOWARDS OXYGEN AND FORMATION OF NEW CATION DEFICIENT SPINELS

Valence states of molybdenum and iron ions and their cationic distribu tion on both octahedral (B) and tetrahedral (A) sites of the spinel st ructure of molybdenum-substituted magnetites, Fe3-xMoxO4, obtained by two ways (samples synthesized by wet process or ground samples synthes ized by the ceramic route) and which are oxidized in cation deficient spinels, have been performed by derivative thermogravimetry (DTG), X-r ay diffraction, IR and XPS spectrometries. It has been demonstrated th at in the case of the samples synthesized by wet processes, the Fe(B)2 +, Mo(B)3+, MO(B)4, Fe(A)2+, Mo(A)4, ions are successively oxidized in to Fe(B)3+ and Mo(A)6+ ions below 530-degrees-C. For the samples synth esized by the ceramic method, only the Fe(B)2, Mo(B)3+, Mo(B)4+ and Fe (A)2+ oxidations have been observed. In these both cases, a quantitati ve analysis based on the discrepancy in reactivity of these different oxidizable cations has been used to determine the distribution of cati ons between the sublattices. As a result of the oxidation of all the o xidizable cations, the molybdenum defect ferrites may have a very high number of vacancies. These may reach 0.71 for x=0.5. The possibility to oxidize the Fe(B)2+ and Mo(B)3+ ions selectively, permits on the on e hand to confirm the desummations of the DTG curves, and on the other hand to study the oxidation kinetics of these two ions. It has been a lso possible to show that the profile of the experimental DTG peaks ob tained after desummation is comparable to the DTG peaks calculated fro m the isothermal kinetic parameters. Finally, the study of the cationi c distribution of the oxidized phases has shown that the Mo6+ ions, in itially in tetrahedral coordination can remain in octahedral site as t he octahedral sites are also occupied by Fe2+ or Co2+ ions.