Thermal behavior and cation distribution in nanosized Mo-Co ferrite spinels Mo 0.5Co yFe 2.5− yO 4 (0 ≤ y ≤ 1) studied by DTG, FT–IR and DC conductivity

Abstract

a cobalt content varying in a large domain of composition (0 ≤ y ≤ 1) has been studied by X-ray diffraction, infrared spectroscopy, thermal analyses (DTG and DSC) and electrical conductivity. Because of their small crystallite size (about 40 nm), these ferrites can be oxidized between 100 and 500°C while retaining the spinel structure. Based essentially on a quantitative determination of the cation distribution on the octahedral (B) and tetrahedral (A) sites by derivative thermogravimetry (DTG), it has been found that Fe 2+ B, Mo 3+ B, Mo 4+ B and Fe 2+ A are successively oxidized into Fe 3+ and Mo 6+ ions. Moreover, for high cobalt content, stoichiometric spinels have a low proportion of these ions in valency state 3+. FT–IR spectroscopy indicated, as a result of the oxidation in cation-deficient spinels, the change in the coordination around Mo 6+ ions from A to B-sites with cobalt substitution. The conductivity evolution during oxidation has also been related to the change in the cation distribution occurring for high cobalt content causing the hopping mechanism to be destroyed.