Valence and Distribution of Manganese Ions in Ferrospinels

Abstract

Anomalous values of the magnetic moments observed for ferrospinels containing manganese have suggested that on octahedrally ligated spinel sites the ion pair $[{\mathrm{Fe}}^{\text{+2}}{\mathrm{Mn}}^{\text{+3}}]$ is more stable than $[{\mathrm{Fe}}^{\text{+3}}{\mathrm{Mn}}^{\text{+2}}]$ . Such behavior is opposed to that observed in aqueous solutions, and to what is usually expected in oxidic compounds. The magnetic measurements, however, cannot be interpreted unequivocally, since they can also be explained by invoking spin quenching, or deviations from the simple antiparallel spin-only coupling scheme such as Yafet-Kittel angular coupling. A crystallographic rather than a magnetic criterion was devised to elucidate the valence behavior in these compounds. When a sufficient fraction of the octahedrally ligated spinel sites are occupied by ${\mathrm{Mn}}^{\text{+3}}$ , a macroscopic tetragonal distortion, due to the Jahn-Teller effect, occurs. For appropriately selected spinel systems, the compositions for which this distortion is observable at room temperature depend on whether $[{\mathrm{Fe}}^{\text{+3}}{\mathrm{Mn}}^{\text{+2}}]$ or $[{\mathrm{Fe}}^{\text{+2}}{\mathrm{Mn}}^{\text{+3}}]$ are the stably coexisting species. Various compositions in the system ${\mathrm{Zn}}_{\text{1−0.5x}}{\mathrm{Ge}}_{\text{0.5x}}[{\mathrm{Fe}}_x{\mathrm{Mn}}_{\text{2−x}}]{\mathrm{O}}_4$ were synthesized, and x-ray powder diffraction photographs were made to determine whether they were cubic or tetragonal. The findings confirmed that the stable ion pair on octahedrally ligated sites is indeed $[{\mathrm{Fe}}^{\text{+2}}{\mathrm{Mn}}^{\text{+3}}]$ . Furthermore, the axial ratios observed for the tetragonal compositions are in good agreement with those calculated by Wojtowicz, provided that the valence assignment $[{\mathrm{Fe}}^{\text{+2}}{\mathrm{Mn}}^{\text{+3}}]$ is assumed. The reversal of valence stability can be explained reasonably as arising from crystal field effects. There is evidence that on octahedrally ligated spinel sites at elevated temperatures, and on tetrahedrally ligated sites, the ion pair $[{\mathrm{Fe}}^{\text{+3}}{\mathrm{Mn}}^{\text{+2}}]$ is more stable than $[{\mathrm{Fe}}^{\text{+2}}{\mathrm{Mn}}^{\text{+3}}]$ . The method of predicting the actual valence and ionic distributions in manganese-containing ferrospinels is discussed.