New solid solutions with the R-type hexaferrite structure, BaFe4-xMxM’2O11 (M = In, Sc; M’ = Ti, Sn)

Hexaferrites are a family of complex iron oxides with hexagonal structures. Novel compositions with the R-type hexaferrite structure, BaFe_4-xM_xM’₂O₁₁ (M = In, Sc; M’ = Ti, Sn; x = 0.0–1.8), are synthesized and characterized. Structural analyses using powder neutron diffraction reveal that trivalent iron cations are distributed among all the available M sites (octahedral and trigonal bipyramidal sites), with site preference varying with the composition. Ferrimagnetic behavior is observed for all the compounds, and the observed magnetic hysteresis loop indicates that the compounds are soft magnets. The color of the new solid solution can be tuned from dark reddish brown to reddish-orange and yellowish-orange as In or Sc substitutions increase. The origin of colors in these ferrite-based solid solutions results from the combination of ligand-to-metal charge transfer and electron-pair transitions across the face-shared octahedra, as seen in the hematite (Fe₂O₃). Reducing Fe content in the parent compound improves the diffuse reflectance in the near-infrared range, suggesting potential applications as cool pigments.