Search for metastable A-site ordered perovskite-type ferrites by high-pressure synthesis assisted by first-principles calculations

A perovskite-type iron oxide SrFeO₃ with Fe⁴⁺ in an unusually high valence state has attracted much attention for its versatile helimagnetic phases [1], despite its simple cubic structure with inversion symmetry. Some of the helimagnetic phases are topologically nontrivial multi-q spin textures, which are represented by the superposition of several propagation vectors along the <111> equivalents of the cubic lattice. The number and direction of q can be modulated in various ways by controlling the electron filling [2], Fe-O bond length [3], and dimensionality of the crystal structure [4,5]. In this study, we explored new A-site ordered perovskite-type iron oxides with Fe⁴⁺ by high-pressure synthesis, aiming to modulate the versatile helimagnetic phases of SrFeO₃ by introducing weak anisotropy through A-site ordering.

Since the target materials to synthesize were new compounds without any previous reports, we evaluated the thermodynamic stability under ambient and high pressures. Crystal structures of each candidate were optimized based on DFT calculations, followed by the construction of convex hulls. Two types of new oxygen-deficient A-site ordered perovskites were predicted to be stabilized by applying pressure, and they were successfully obtained by high-pressure synthesis. We also attempted topotactic ozone oxidation to those oxygen-deficient compounds to realize Fe⁴⁺ states. Whereas the oxygen-deficient samples are likely to be antiferromagnetic insulators whose transition temperatures are above 300 K, a potentially novel helimagnetic phase is found far below 300 K in the compounds after ozone oxidation.

[1] S. Ishiwata et al., Phys. Rev. B 101, 134406 (2020).

[2] M. Onose et al., Phys. Rev. Mater. 4, 114420 (2020).

[3] N. Hayashi et al., J. Phys. Soc. Japan 82, 113702 (2013).

[4] J. -H. Kim et al., Phys. Rev. Lett. 113, 147206 (2014).

[5] P. Adler et al., Phys. Rev. B 105, 054417 (2022).