Oxygen Release and Incorporation Behaviors Influenced by A-Site Cation Order/Disorder in LaCa2Fe3O9 with Unusually High Valence Fe3.67+

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.