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Infinite-layer iron oxide with a square-planar coordination

Abstract

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 structures2,3,4. 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 oxides5,6,7,8, 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 D4h 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 400 K, making the material appealing for a variety of applications, including oxygen ion conduction, oxygen gas absorption and catalysis.

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Figure 1: Structural transformation via a topotactic route.
Figure 2: Structural characterization of SrFeO 2 by Rietveld refinement of high-resolution neutron diffraction at room temperature.
Figure 3: Temperature evolution of the magnetic order in SrFeO2.

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Data deposits

Atomic coordinates and structure factors for the crystal structure of SrFeO2 have been deposited with the ICSD database under accession codes 418603, 418605 and 418606.

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Acknowledgements

We thank Y. Kiuchi, H. Ueda, M. Isobe and Y. Ueda for their help in EDS and thermogravimetric measurements and K. Kato for his help in the synchrotron X-ray experiments at SPring-8. This work was supported by Young Scientists A (H.K.), the Grant-in-Aid for Scientific Research on Priority Areas (H.K. and K.Y.) and Scientific Research S (M.T.) from MEXT. See the Supplementary Notes for more details.

Author Contributions H.K. designed the study in collaboration with W.P, with M.T.’s help; C.T. performed the initial synthesis and proposed the structural model; Y.T. and T.W. optimized the synthetic conditions, performed chemical characterizations, X-ray diffraction measurements and corresponding structural refinement; N.H. conducted the Mössbauer experiment, with M.T.’s help; M.C., C.R. and W.P. performed the neutron diffraction measurements and M.C. and W.P. performed the corresponding structural refinement; All the authors discussed the results; H.K. wrote the manuscript, with comments mainly from M.T. and W.P.

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Correspondence to H. Kageyama.

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Supplementary Information

This file contains Supplementary Figures S1-S4 with Legends, Supplementary Table S1 with Legend, and Supplementary Notes. (PDF 1324 kb)

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Tsujimoto, Y., Tassel, C., Hayashi, N. et al. Infinite-layer iron oxide with a square-planar coordination. Nature 450, 1062–1065 (2007). https://doi.org/10.1038/nature06382

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