Hexagonal Perovskite Ba₄Fe₃NiO₁₂ Containing Tetravalent Fe and Ni Ions

BaFe_xNi_1–xO₃ with end members of BaNiO₃ (x = 0) and BaFeO₃ (x = 1), which, respectively, adopt the 2H and 6H hexagonal perovskite structures, were synthesized, and their crystal structures were investigated. A new single phase, Ba₄Fe₃NiO₁₂ (x = 0.75), that adopts the 12R perovskite structure with the space group R3̅m (a = 5.66564(7) Å and c = 27.8416(3) Å), was found to be stabilized. Mössbauer spectroscopy results and structure analysis using synchrotron and neutron powder diffraction data revealed that nominal Fe³⁺ occupies the corner-sharing octahedral site while the unusually high valence Fe⁴⁺ and Ni⁴⁺ occupy the face-sharing octahedral sites in the trimers, giving a charge formula of Ba₄Fe³⁺­Fe⁴⁺₂Ni⁴⁺O_11.5. The magnetic properties of the compound are also discussed.