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

The coexistence of three valence states of Mn ions, namely, +2, +3, and +4, in substituted magnetoplumbite-type BaFe_{12–<i>x</i>}Mn_<i>x</i>O₁₉ 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 <i>x</i> = 1.5 and <i>x</i> = 1.7 led to Mn²⁺:Mn³⁺:Mn⁴⁺ atomic ratios of 0.16:0.51:0.33 and 0.19:0.57:0.24

Ti-Substituted BaFe₁₂O₁₉ Single Crystal Growth and Characterization

Ti-substituted barium hexaferrite BaFe₁₂O₁₉ single crystals BaFe_{12–<i>x</i>}Ti_<i>x</i>O₁₉ with <i>x</i> up to 1.3 and sizes 2–8 mm were grown by spontaneous crystallization from molten sodium carbonate flux. The distribution of Ti on different crystallographic sites was determined from single crystal X-ray diffraction data. For low Ti contents up to <i>x</i> = 0.8 the unit cell expands; on further increase of the Ti amount the unit cell starts to shrink. This behavior for low Ti contents is most likely due to a reduction of Fe³⁺ to Fe²⁺ for charge balance. At higher Ti concentrations, supposedly vacancies in the transition metal substructure are formed. An increasing Ti concentration results in a monotonous reduction of the Curie temperature from 452 to 251 °C and the saturation magnetization at room temperature from 64.8 to 24.8 emu/g for powder samples and from 70.0 to 60.1 emu/g for single crystals (for <i>x</i> up to 0.78)