Exploration of Metastable A-Site-Ordered Perovskites (Ca,Ba)FeO_3-δ by Computationally Guided Multistep Synthesis

Perovskite-type iron oxides with Fe⁴⁺ ions have attracted a great deal of attention for their versatile helimagnetic phases. While the introduction of a layered A-site ordered structure to AFeO₃ with Fe⁴⁺ ions potentially leads to novel helimagnetic phases, the synthetic pathway spanning a high-pressure range is apparently difficult to elucidate. Here, we explored new A-site ordered perovskite-type iron oxides (Ca,Ba)FeO_3-δ with Fe⁴⁺ ions with the support of first-principles calculations evaluating thermodynamic stability at selected pressures and chemical compositions. Among the six types of putative A-site ordered perovskites with and without oxygen vacancy, only two types of oxygen-deficient perovskites CaBaFe₂O_6-δ and Ca(Ba_0.9Ca_0.1)₂Fe₃O_9-δ (δ ∼ 1) were successfully obtained by high-pressure synthesis, consistent with the DFT-based convex-hull calculations. Considering the evaluated stability of the putative perovskites at selected pressures, we adopted low-temperature topotactic oxidation using ozone at ambient pressure and obtained the oxidized perovskites CaBaFe₂O_6-δ (δ ∼ 0.4) and Ca(Ba_0.9Ca_0.1)₂Fe₃O_9-δ (δ ∼ 0.6), potentially showing novel helimagnetic phases. This study demonstrates that computational visualization of multistep synthetic pathways involving high pressure can accelerate the search for new metastable perovskites with rich magnetic phases.