(Sr1-xBax)FeO2 (0.4 ≤ x ≤ 1): A New Oxygen-Deficient Perovskite Structure

Topochemical reduction of (layered) perovskite iron oxides with metal hydrides has so far yielded stoichiometric compositions with ordered oxygen defects with iron solely in FeO4 square planar coordination. Using this method, we have successfully obtained a new oxygen-deficient perovskite, (Sr1-xBax)FeO2 (0.4 <= x <= 1.0), revealing that square planar coordination can coexist with other 3-6-fold coordination geometries. This BaFeO2 structure is analogous to the LaNiO2.5 structure in that one-dimensional octahedral chains are linked by planar units, but differs in that one of the octahedral chains contains a significant amount of oxygen vacancies and that all the iron ions are exclusively divalent in the high-spin state. Mossbauer spectroscopy demonstrates, despite the presence of partial oxygen occupations and structural disorders, that the planar-coordinate Fe2+ ions are bonded highly covalently, which accounts for the formation of the unique structure. At the same time, a rigid 3D Fe-O-Fe framework contributes to structural stabilization. Powder neutron diffraction measurements revealed a G-type magnetic order with a drastic decrease of the Neel temperature compared to that of SrFeO2, presumably due to the effect of oxygen disorder/defects. We also performed La substitution at the Ba site and found that the oxygen vacancies act as a flexible sink to accommodate heterovalent doping without changing the Fe oxidation and spin state, demonstrating the robustness of this new structure against cation substitution.