CATIONIC DISTRIBUTION AND OXIDATION MECHANISM OF TRIVALENT MANGANESE IONS IN SUBMICROMETER MNXCOFE2-XO4 SPINEL FERRITES

Submicrometer Mn2CoFe2-xO4 (0 less than or equal to x < 1) spinel part icles with a spherical shape were prepared at low temperature from oxa lic precursors. Because of their small crystallite size (about 50 nm), these ferrite particles can be oxidized below 400 degrees C giving cu bic deficient spinels having both Mn3+ and Mn4+ ions because of oxide- reduction phenomena, Information about the valence state of the mangan ese and cobalt ions and cation distribution between tetrahedral (A) an d octahedral (B) sites are derived from lattice parameter variation, I R spectroscopy, thermogravimetric analysis, and electrical conductivit y, A kinetics study of the oxidation of Mn3+ ions shows that oxidation proceeds by cation diffusion through a topotactic reaction with a con stant chemical diffusion coefficient for x < 0.60 and variable above x = 0.80 and an activation energy close to 115 kJ mol(-1) but, however, depending on manganese substitution content. (C) 1996 Academic Press, Inc.