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 particles with a spherical shape were prepared at low temperature from oxalic precursors. Because of their small crystallite size (about 50 nm), these ferrite particles can be oxidized below 400 degrees C giving cubic deficient spinels having both Mn3+ and Mn4+ ions because of oxide-reduction phenomena, Information about the valence state of the manganese and cobalt ions and cation distribution between tetrahedral (A) and octahedral (B) sites are derived from lattice parameter variation, IR spectroscopy, thermogravimetric analysis, and electrical conductivity, A kinetics study of the oxidation of Mn3+ ions shows that oxidation proceeds by cation diffusion through a topotactic reaction with a constant 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.