Exploring\nTopochemical Oxidation Reactions for Reversible\nTuning of Thermal Conductivity in Perovskite Fe Oxides

We present a study on the reversibility of thermal conductivity\nin iron oxides through topochemical oxygen exchange between brownmillerite\n(BM) (Ca,Sr)FeO<sub>2.5</sub> and perovskite (PV) (Ca,Sr)FeO<sub>3.0</sub>. By using different oxidation methods, including gas phase (O<sub>2</sub>/O<sub>3</sub>), liquid phase (NaOCl in H<sub>2</sub>O), and\nsolid electrolyte (Y<sub>2</sub>O<sub>3</sub>:ZrO<sub>2</sub>), we\ndemonstrate that the oxidation pathway has a critical influence on\nthe reversibility of the ionic-exchange process. Cyclic oxidation\nand reduction using O<sub>2</sub>/O<sub>3</sub> or NaOCl lead to an\nimportant accumulation of structural defects, undermining the reversibility\nof thermal conductivity. In the case of wet oxidation, we demonstrate\nan inherent tendency of negative charge-transfer oxides toward amorphization\nand elucidate the origin of this effect. Conversely, the electrochemical\ninjection of the O<sup>2–</sup> ions via a Y<sub>2</sub>O<sub>3</sub>:ZrO<sub>2</sub> solid electrolyte reduces structural damage\nsignificantly, enhancing both reversibility and durability. This study\nunderscores the importance of selecting appropriate topochemical oxygen\nexchange methods to maintain structural integrity and optimize functional\nperformance in oxide-based tunable devices.