(Sr_(1-x)Ba_x)FeO_2 (0.4 ≤ x ≤ 1): A New Oxygen-Deficient Perovskite Structure

摘要

Topochemical reduction of (layered) perovskite iron oxides with metal hydrides has so far yielded stoichiometric compositions with ordered oxygen defects with iron solely in FeO_4 square planar coordination. Using this method, we have successfully obtained a new oxygen-deficient perovskite, (Sr_(1-x)Ba_x)FeO_2 (0.4 ≤ x ≤ 1.0) , revealing that square planar coordination can coexist with other 3- 6-fold coordination geometries. This BaFeO_2 structure is analogous to the LaNiO_(2.5) 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 Fe~(2+) 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 SrFeO_2, 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.