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.