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 BaFeO_2.5 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.