(Sr_1–<i>x</i>Ba_<i>x</i>)FeO₂ (0.4 ≤ <i>x</i> ≤ 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, (Sr_1–<i>x</i>Ba_<i>x</i>)­FeO₂ (0.4 ≤ <i>x</i> ≤ 1.0), revealing that square planar coordination can coexist with other 3–6-fold coordination geometries. This BaFeO₂ 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. Mössbauer 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 <i>G</i>-type magnetic order with a drastic decrease of the Néel 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