Three Oxidation States of Manganese in the Barium Hexaferrite BaFe_12-xMn_xO₁₉

The coexistence of three valence states of Mn ions, namely, +2, +3, and +4, in substituted magnetoplumbite-type BaFe_12-xMn_xO₁₉ was observed by soft X-ray absorption spectroscopy at the Mn-L_2,3 edge. We infer that the occurrence of multiple valence states of Mn situated in the pristine purely iron(III) compound BaFe₁₂O₁₉ is made possible by the fact that the charge disproportionation of Mn³⁺ into Mn²⁺ and Mn⁴⁺ requires less energy than that of Fe³⁺ into Fe²⁺ and Fe⁴⁺, related to the smaller effective Coulomb interaction of Mn³⁺ (d⁴) compared to Fe³⁺ (d⁵). The different chemical environments determine the location of the differently charged ions: with Mn³⁺ occupying positions with (distorted) octahedral local symmetry, Mn⁴⁺ ions prefer octahedrally coordinated sites in order to optimize their covalent bonding. Larger and more ionic bonded Mn²⁺ ions with a spherical charge distribution accumulate at tetrahedrally coordinated sites. Simulations of the experimental Mn-L_2,3 XAS spectra of two different samples with x = 1.5 and x = 1.7 led to Mn²⁺:Mn³⁺:Mn⁴⁺ atomic ratios of 0.16:0.51:0.33 and 0.19:0.57:0.24.