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+(Fe20.54+Ge20.54+)2Ge324+O17 does not show a charge transition.