Three Oxidation States of Manganese in the Barium Hexaferrite BaFe_{12–<i>x</i>}Mn_<i>x</i>O₁₉

Abstract

The coexistence of three valence states of Mn ions, namely, +2, +3, and +4, in substituted magnetoplumbite-type BaFe 12– x Mn x O 19 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 BaFe 12 O 19 is made possible by the fact that the charge disproportionation of Mn 3+ into Mn 2+ and Mn 4+ requires less energy than that of Fe 3+ into Fe 2+ and Fe 4+, related to the smaller effective Coulomb interaction of Mn 3+ (d 4 ) compared to Fe 3+ (d 5 ). The different chemical environments determine the location of the differently charged ions: with Mn 3+ occupying positions with (distorted) octahedral local symmetry, Mn 4+ ions prefer octahedrally coordinated sites in order to optimize their covalent bonding. Larger and more ionic bonded Mn 2+ 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 Mn 2+:Mn 3+:Mn 4+ atomic ratios of 0.16:0.51:0.33 and 0.19:0.57:0.24.