Reactivity towards oxygen and cation distribution of manganese iron spinel Mn3-xFexO4 (0<=x<=3) fine powders studied by thermogravimetry and IR spectroscopy

The thermal behaviour in oxygen and the cation distribution of manganese iron spinels Mn3-xFexO4 (0 less than or equal to x less than or equal to 3) obtained by a soft chemistry method have been determined by thermogravimetry and IR spectroscopy. The existence of two different cation distributions as function of composition x is established from a quantitative analysis by derivative thermogravimetry. For x > 1.50, the IR spectroscopy clearly distinguished the presence of Fe2+ and Mn2+ ions located in octahedral and tetrahedral sites, respectively. For x < 1.50, this method is also efficient for the differentiation of the Mn2+ ions located in both types of site of the spinel structure. Kinetic studies on the oxidation process of Mn2+ ions show that the oxidation proceeds by way of a diffusion-controlled reaction or by a nucleation-growth mechanism. The difference in the kinetic behaviour is interpreted with regard to a structural change from the cation-deficient spinel to multi-phase oxides initiated by the oxidation temperature.