1. Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
2. University of Rennes 1, Sciences Chimiques de Rennes UMR CNRS 6226, Campus de Beaulieu Bâtiment 10B, Rennes cedex 35042, France
3. Graduate School of Human and Environmental Studies, Kyoto University, Sakyo, Kyoto 606-8501, Japan
4. Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
5. Research Institute for Production Development, 15 Morimoto, Shimogamo, Sakyo, Kyoto 606-0805, Japan
6. Institute Laue Langevin, BP 156, 38042, Grenoble, France

Correspondence to: H. Kageyama¹ Correspondence and requests for materials should be addressed to H.K. (Email: kage@kuchem.kyoto-u.ac.jp).

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 metal¹. 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, 3, 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, 6, 7, 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 SrFeO_2.5, and start at the relatively low temperature of 400 K, making the material appealing for a variety of applications, including oxygen ion conduction, oxygen gas absorption and catalysis.

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