Oxygen Release and Incorporation Behaviors Influenced by A-Site Cation Order/Disorder in LaCa₂Fe₃O₉ with Unusually High Valence Fe^3.67+

Abstract

Fully oxygenated perovskites with the same chemical composition crystallize in different polymorphs, o-LaCa2Fe3O9 with A-site triple-layer order and d-(LaCa2)­Fe3O9 with A-site disorder. Both compounds contain unusually high valence Fe3.67+ and, when they are heated, release oxygen to reduce such an unusual valence state. Thermogravimetry analysis revealed that the ordered/disordered arrangements of the A-site cations in the perovskite structures strongly influence the stability of unusually high valence Fe ions at the B-site. The A-site-ordered o-LaCa2Fe3O9 releases its oxygen topotactically and selectively from the FeO6 octahedra between the Ca layers above 400 °C, and the released oxygen is not incorporated back on cooling in air, Ar, or O2 atmospheres. On the other hand, oxygen in d-(LaCa2)­Fe3O9 is released from and incorporated into the rigid octahedra reversibly when the compound is heated in air or O2. More importantly, the oxygen-deficient d-(LaCa2)­Fe3O8 obtained by heating d-(LaCa2)­Fe3O9 in Ar incorporates extra oxygen to increase the valence state of Fe in an unusually high value even under ambient conditions. Once the A-site cation disorder structure framework is established, unusually high valence states, which usually require extreme conditions, can be stabilized by incorporating extra oxygen into the structure even under ambient conditions.

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

“…The A-site-disordered simple perovskite (La 1/3 Ca 2/3 )­FeO 3 , containing unusually high-valence Fe 3.67+ , reversibly releases oxygen and incorporates oxygen in air. In contrast, A-site-ordered triple perovskite LaCa 2 Fe 3 O 9 with the same chemical composition releases oxygen but does not incorporate oxygen in air. − …”

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

“…33 Besides, both Asite and B-site cation ordering in the perovskite structures are also reported to significantly influence the oxygen release and incorporation behaviors. 34,35 In this study, we have investigated the oxygen release from AFeO 3 and the oxygen incorporation into AFeO 2 (A = Ca and Sr). The unusual high valence Fe 4+ in AFeO 3 is easily reduced to relatively stable Fe 3+ by releasing oxygen, while the unusual squareplanar coordination of Fe 2+ in AFeO 2 changes to tetrahedral and octahedral Fe 3+ by incorporating oxygen in topotactic manners.…”