Topotactic Oxygen Release and Incorporation in AFeO₃ with Fe⁴⁺, AFeO_2.5 with Fe³⁺, and AFeO₂ with Fe²⁺ (A = Ca and Sr): Dedicated to the Occasion of the 100th Birthday of Prof. John B. Goodenough

Abstract

Oxygen contents in perovskite-structure Fe oxides can change in accordance with the valence states of Fe, i.e., AFeO3 with Fe4+, AFeO2.5 with Fe3+, and AFeO2 with Fe2+ (A = Ca and Sr). AFeO3 has a fully oxygenated simple-perovskite structure, and the unusual high valence Fe4+ in AFeO3 is easily reduced to relatively stable Fe3+ by releasing oxygen. On the other hand, AFeO2 has an infinite-layer structure, and the unusual square-planar coordination of Fe2+ in AFeO2 changes to tetrahedral and octahedral Fe3+ 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 AFe4+O3 → AFe3+O2.5 ← AFe2+O2 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 SrFeO3 and SrFeO2.5.

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

“…It is noted that the 3C-type BaFeO 3 starts to release oxygen at a temperature as low as 130 °C. It is also interesting to note that this temperature is lower than the corresponding temperatures for the isostructural CaFeO 3 and SrFeO 3 , which start to release oxygen at about 300 °C …”

“…It is also interesting to note that this temperature is lower than the corresponding temperatures for the isostructural CaFeO 3 and SrFeO 3 , which start to release oxygen at about 300 °C. 19 When the 3C-type BaFeO 3 is heated in air, the sample weight starts to decrease at about 130 °C and reaches about 96.7% of the initial weight above 550 °C after a plateau at 97.6% around 450 °C. The result demonstrates that the 3Ctype BaFeO 3 changes to stable BaFeO 2.5 above 550 °C via BaFeO 2.67 around 450 °C.…”

“…Octahedrally oxygen-coordinated Fe 4+ in A FeO 3 changes to octahedrally and tetrahedrally oxygen-coordinated Fe 3+ in A FeO 2.5 and to square-planar oxygen-coordinated Fe 2+ in A FeO 2 . It is also important to note that oxygen release from A FeO 3 begins at very low temperatures, like 300 °C, rather than at high temperatures like the 800 °C at which it typically begins in many oxide ceramics . Because the Fe 4+ valence state is unusually high compared to typical valence states like Fe 3+ and Fe 2+ in oxides, its electronic instability tends to be relieved to reduce the valence state by releasing oxygen from the structure.…”

From Fundamentals to Next-Generation Technology - JES/JSS Focus Issue In Honor of John Goodenough: A Centenarian Milestone

“…Schemes for tuning topological effects in the MnBi 2 Te 4 compound are presented by Yan, 11 while Chakhalian and Middey 12 and Lokshin et al 13 discuss the complexity of effects arising from details of orbital occupancy in rare-earth nickelates in comparison with cuprates. Shimakawa and coauthors 14 examine the effects of cation size on the oxygen stoichiometry and iron valance states in the alkaline earth-based iron compound AFe 3-δ . Creating a bridge between fundamental insight and technological application, Harris and Andalib 15 connect the implications of the Goodenough-Kanamori-Anderson (GKA) rules, as operative in magnetic ceramics, to current and future wireless communication technologies.…”