Phase Transformation in Barium Orthoferrate, BaFeO 3‐ x

The compound BaFeO 3‐ x exists in many forms, the hexagonal phase having a wide range in oxygen content (BaFeO 2.63–2.92 ). The other phases have the limited compositions and the distinct structures of perovskite: triclinic I, BaFeO 2.50 ; triclinic II, BaFeO 2.64–2.67 ; rhombohedral I and II, BaFeO 2.62–2.64 ; and tetragonal, BaFeO 2.75–2.81 . The phases contain Fe 4+ ions correlating with the oxygen content. The hexagonal phase shows a continuous change in oxygen content with temperature down to BaFeO 2.63 . The perovskitelike phases, however, show characteristic transformations. The triclinic I phase oxidizes to the triclinic II form at 320° to 500°C and to the hexagonal form at 720° to 915°C in oxygen. These transformations are related to oxidation‐reduction of iron ions (Fe 3+ ⇌ Fe 4+ ).