Oxygen Release and Incorporation Behaviors in BaFeO₃ Polymorphs with Unusually High-Valence Fe⁴⁺

Fully oxygenated perovskite BaFeO3 containing unusually high-valence Fe4+ shows three crystal polymorphs with the same chemical composition. The 3C-type BaFeO3 has a simple cubic perovskite structure consisting of corner-sharing FeO6 octahedra, while the 6H- and 12R-type BaFeO3 have hexagonal perovskite structures consisting of both corner-sharing and face-sharing FeO6 octahedra. The compounds readily release oxygen into the air to reduce the high-valence state of the Fe ions, but the oxygen release behaviors strongly depend on the crystal structure. The 3C-type BaFeO3 releases oxygen topotactically from the corner-shared sites of the FeO6 octahedra at a temperature as low as 130 °C. In contrast, the 6H- and 12R-type BaFeO3 preferentially release oxygen from the face-shared sites above 320 and 460 °C, respectively, although they include the corner-shared sites in the crystal structures. The resultant oxygen-deficient 3C-type BaFeO2.5 does not incorporate back oxygen in air, whereas the 12R-type hexagonal structure shows completely reversible oxygen release and incorporation in air. Once the 12R-type structure is established, unusually high-valence states such as Fe4+ can be stabilized without extreme conditions.