Oxygen release/incorporation behavior in hexagonal perovskite BaFeO3 explored by 18O/16O isotope exchange reactions

Abstract

The 12R-type hexagonal perovskite BaFeO₃ containing unusually high-valence Fe⁴⁺ has a characteristic feature of completely reversible oxygen release and incorporation up to 700 °C in air. Once the structure is established, the unusually high-valence Fe⁴⁺ state can be stabilized without extreme conditions like a strongly oxidizing atmosphere. During annealing the samples in an ¹⁸O₂ gas atmosphere, almost all oxygen (¹⁶O) atoms in the 12R-type BaFeO₃ are exchanged with the ¹⁸O isotope in an equilibrium process. Thermogravimetric analysis with mass spectrometry in an air-like atmosphere reveals that ¹⁸O-exchange-treated 12R-type BaFeO₃ releases ¹⁸O upon heating and then incorporates ¹⁶O on cooling, and the sample reversibly recovers to the initial 12R-type structure. Furthermore, such oxygen release starts at as low as 400 °C, suggesting oxide-ion mobility at such a low temperature. Oxide ions that connect Fe-centered octahedra through both shared corners and shared faces contribute to the oxide ion mobility.