Hydrogen-Induced\nTopotactic Phase Transformations\nof Cobaltite Thin Films

Manipulating physical\nproperties through ion migration\nin complex\noxide thin films is an emerging research direction to achieve tunable\nmaterials for advanced applications. While the reduction of complex\noxides has been widely reported, few reports exist on the modulation\nof physical properties through a direct hydrogenation process. Here,\nwe report an unusual mechanism for hydrogen-induced topotactic phase\ntransitions in perovskite La<sub>0.7</sub>Sr<sub>0.3</sub>CoO<sub>3</sub> thin films. Hydrogenation is performed upon annealing in\na pure hydrogen gas environment, offering a direct understanding of\nthe role that hydrogen plays at the atomic scale in these transitions.\nTopotactic phase transformations from the perovskite (P) to hydrogenated-brownmillerite\n(H-BM) phase can be induced at temperatures as low as 220 °C,\nwhile at higher hydrogenation temperatures (320–400 °C),\nthe progression toward more reduced phases is hindered. Density functional\ntheory calculations suggest that hydroxyl bonds are formed with the\nintroduction of hydrogen ions, which lower the formation energy of\noxygen vacancies of the neighboring oxygen, enabling the transition\nfrom the P to H-BM phase at low temperatures. Furthermore, the impact\non the magnetic and electronic properties of the hydrogenation temperature\nis investigated. Our research provides a potential pathway for utilizing\nhydrogen as a basis for low-temperature modulation of complex oxide\nthin films, with potential applications in neuromorphic computing.