Layered Hexagonal Perovskite Oxides 21R Ba₇Fe₅Ge₂O₂₀ and 12H Ba₆Fe₃Ge₃O₁₇

Two hexagonal-perovskite-structure oxides, 21R Ba₇Fe₅Ge₂O₂₀ and 12H Ba₆Fe₃Ge₃O₁₇, were obtained by synthesis with a high-pressure and high-temperature technique. The Fe-containing hexagonal-perovskite-structure units are sandwiched by nonmagnetic GeO₄ tetrahedral layers in the structures, and thus both compounds show two-dimensional ferrimagnetic behaviors due to intra- and interunit magnetic interactions. 21R Ba₇Fe₅Ge₂O₂₀ has the ionic formula Ba₇Fe1₂³⁺Fe2⁴⁺Fe3₂⁴⁺Ge4₂⁴⁺O₂₀ at room temperature, and unusually high valence Fe⁴⁺ in the trimers undergoes charge disproportionation, Fe2⁴⁺ + 2Fe3⁴⁺ → Fe2^(4+2δ)+ + 2Fe3^(4-δ)+, at low temperatures. In contrast, 12H Ba₆Fe₃Ge₃O₁₇ with ionic formula Ba₆Fe1₂³⁺(Fe2_0.5⁴⁺Ge2_0.5⁴⁺)₂Ge3₂⁴⁺O₁₇ does not show a charge transition.