Ionic Valences in Manganese-Iron Spinels

Abstract

Because both manganese and iron assume multiple valences in oxide structures, it is possible to fabricate spinels over a continuous range of composition from Mn₃O₄ to Fe₃O₄. Magnetic‐moment data for samples fabricated within this range are reported and an interpretation of the data in terms of the distribution of ions of manganese and iron is presented. The data agree with previous indications that manganese ferrite, MnFe₂O₄, is an almost completely normal spinel with Mn²⁺ on A sites and Fe³⁺ on B sites. In addition, the usual assumption that excess iron replaces manganese as Fe²⁺ on B sites is substantiated. However, the data are not compatible with the previously held assumption that Fe³⁺ is replaced by excess manganese as Mn³⁺. Instead, the variation of magnetic moments and Curie temperatures with composition is interpreted in terms of a model in which excess manganese replaces Fe³⁺ as Mn⁴⁺ and an equal amount of iron is converted to Fe²⁺. When the manganese content gets so high that all the iron is Fe²⁺ (Mn₂FeO₄), manganese then replaces the iron as Mn³⁺ and reconverts an equal amount of Mn⁴⁺ to Mn³⁺, and the Mn³⁺, because of its tetragonal nature, introduces spinel twinning. It is proposed that the Mn⁴⁺–Fe²⁺ pair in octahedral coordination is stabilized with respect to a Mn³⁺–Fe³⁺ pair because of the interaction of the manganese orbitals with the 2p orbitals of the surrounding oxygen anions.