Topotactic Oxygen Release and Incorporation in AFeO3 with Fe4+, AFeO2.5 with Fe3+, and AFeO2 with Fe2+ (A = Ca and Sr): Dedicated to the Occasion of the 100th Birthday of Prof. John B. Goodenough

Oxygen contents in perovskite-structure Fe oxides can change in accordance with the valence states of Fe, i.e., AFeO 3 with Fe 4+ , AFeO 2.5 with Fe 3+ , and AFeO 2 with Fe 2+ (A = Ca and Sr). AFeO 3 has a fully oxygenated simple-perovskite structure, and the unusual high valence Fe 4+ in AFeO 3 is easily reduced to relatively stable Fe 3+ by releasing oxygen. On the other hand, AFeO 2 has an infinite-layer structure, and the unusual square-planar coordination of Fe 2+ in AFeO 2 changes to tetrahedral and octahedral Fe 3+ by incorporating oxygen. Sample weight measurements by thermogravimetry and corresponding phase analysis with synchrotron X-ray diffraction data revealed that the difference in the A-site cation strongly influenced the oxygen release and incorporation behaviors. In ambient air, topotactic changes of AFe 4+ O 3 → AFe 3+ O 2.5 ← AFe 2+ O 2 for both A = Ca and Sr can occur by releasing and incorporating oxygen in the perovskite structure frameworks. Nonstoichiometric phases with oxygen vacancies are present between SrFeO 3 and SrFeO 2.5 .