TRIVALENT CATION SUBSTITUTION OF PULVERULENT COBALT-IRON MOLYBDATES CO1-XFEXMOO4

Different mixed cobalt-iron molybdates Co1-xFexMoO4 (O less than or eq ual to x less than or equal to 1) have been prepared by means of a cer amic process. The oxidation of pulverulent samples leads to ferric mol ybdate Fe-2(oO(4))(3), spinelle Co3O4 and cobalt molybdate CoMoO4. Aft er a strong grinding which reduces the grain size (about 0.1 mu m) and induces crystallographic defects in the grains, the cobalt-iron molyb dates can be partially oxidized into cation-deficient phases. This oxi dation corresponds to an isostructural substitution of part of the Fe2 + and Co2+ ions by Fe3+ and Co3+ ions and the creation of cation vacan cies. The concomitant presence of trivalent cations and associated cat ion Vacancies in the lattice stabilizes the highest-temperature variet y of the cobalt or iron molybdates, i.e., the a variety. (C) 1998 Else vier Science S.A. All rights reserved.