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

Abstract

Fully oxygenated perovskite BaFeO 3 containing unusually high-valence Fe 4+ shows three crystal polymorphs with the same chemical composition. The 3C-type BaFeO 3 has a simple cubic perovskite structure consisting of corner-sharing FeO 6 octahedra, while the 6H-and 12R-type BaFeO 3 have hexagonal perovskite structures consisting of both corner-sharing and facesharing FeO 6 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 BaFeO 3 releases oxygen topotactically from the corner-shared sites of the FeO 6 octahedra at a temperature as low as 130 °C. In contrast, the 6H-and 12R-type BaFeO 3 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 Fe 4+ can be stabilized without extreme conditions.

Exploring Reversible Redox Behavior in the 6H-BaFeO3−δ (0 < δ < 0.4) System: Impact of Fe3+/Fe4+ Ratio on CO Oxidation

“…After the preparation of this manuscript and before sending it for publication, our attention has been drawn to an article about the oxygen release and incorporation in BaFeO 3 polymorphs (Watanabe et al). 34 A fully oxygenated 6H-BaFeO 3 sample has been prepared by using high-pressure (8 GPa) and high-temperature (1000 °C) conditions with KClO 4 as the oxidizing agent. These authors have described that sample 6H loses weight when heated in air to 600 °C, obtaining a composition close to BaFeO 2.8 .…”

Exploring Reversible Redox behavior in the 6H-BaFeO3-d (0 &lt; d &lt; 0.4) System. Impact of Fe3+/Fe4+ Ratio on Catalytic Properties.

“…After the preparation of this manuscript and before sending it for publication, our attention has been drawn to an article about the oxygen release and incorporation in BaFeO3 polymorphs (Watanabe et al, Accepted February 6, 2024) 33 . A fully oxygenated 6H-BaFeO3 sample has been prepared by using high-pressure (8Gpa) and high-temperature (1000 ºC) conditions with KClO4 as oxidizing agent.…”