Structure of Highly Divided Nonstoichiometric Iron Manganese Oxide Powders Fe3-xMnx3delta/4O4+delta

Highly divided iron manganese oxide powders, Fe3-xMnx3delta/4O4+delta, were prepared at low temperature (T<560°C) by the thermal decomposition of mixed oxalate precursors Fe1-deltaMndeltaC2O4, 2H2O (delta=x/3). The manganese-rich compounds (x1.5) have a complex structure that can be cubic, tetragonal, or a mixture of both tetragonal and cubic spinel phases that indicates a lack of miscibility existing in the Fe3O4–Mn3O4 phase diagram at low temperature. The structure strongly depends on the chemical composition but also on the nonstoichiometric coefficient (delta). It has been identified as one of the mineral "vredenburgite" and has never been reported before for powders prepared by chimie douce. The oxygen excess, determined by thermogravimetric analyses, is the highest (delta=0.4) when the oxide is prepared at 410°C. Theses oxides are defect spinel phases containing cationic vacancies.