Infinite-layer iron oxide with a square-planar coordination

Conventional high- temperature reactions limit the control of coordination polyhedra in transition- metal oxides to those obtainable within the bounds of known coordination geometries for a given transition metal(1). For example, iron atoms are almost exclusively coordinated by three- dimensional polyhedra such as tetrahedra and octahedra. However, recent works have shown that binary metal hydrides act as reducing agents at low temperatures, allowing access to unprecedented structures(2-4). Here we show the reaction of a perovskite SrFeO3 with CaH2 to yield SrFeO2, a new compound bearing a square- planar oxygen coordination around Fe2(+). SrFeO2 is isostructural with 'infinite layer' cupric oxides(5-8), and exhibits a magnetic order far above room temperature in spite of the two-dimensional structure, indicating strong in-layer magnetic interactions due to strong Fe d to O p hybridization. Surprisingly, SrFeO2 remains free from the structural instability that might well be expected at low temperatures owing to twofold orbital degeneracy in the Fe2+ ground state with D-4h point symmetry. The reduction and the oxidation between SrFeO2 and SrFeO3 proceed via the brownmillerite- type intermediate SrFeO2.5, and start at the relatively low temperature of similar to 400 K, making the material appealing for a variety of applications, including oxygen ion conduction, oxygen gas absorption and catalysis.