Topochemical Hydrogenation of Perovskite-Type BaM _{1 <b>–</b> <i>x</i>} In <i> _x </i> O _3−δ (M = Ce, Ti)

Hydride-ion–conducting oxyhydrides have attracted increasing interest for electrochemical and energy-conversion applications owing to their strong reducing power and high hydride anion mobility; however, their practical deployment has been severely limited by poor thermal stability. In this work, we systematically investigate the hydrogenation behavior and oxyhydride phase stability of In-doped perovskite oxides BaCe 1– x In x O 3−δ and BaTi 1– x In x O 3−δ ( x = 0.5–0.7) under ambient hydrogen pressure to elucidate the materials parameters governing stable oxyhydride formation. Thermogravimetric analysis, temperature-programmed desorption/oxidation, neutron diffraction, and X-ray absorption spectroscopy demonstrate that BaCe 1– x In x O 3−δ undergoes a topochemical transformation into an oxyhydride phase stable up to 600 °C; in particular, BaTi 1– x In x O 3−δ decomposes above 500 °C without forming an oxyhydride. Neutron diffraction confirms that hydrogen in the BaCe 0.3 In 0.7 O 2.20 H 0.15 oxyhydride phase occupies oxygen anion sites as hydride anions. Density functional theory calculations reveal that oxyhydride formation in BaCe 1– x In x O 3−δ proceeds via preferential oxygen vacancy formation and subsequent hydride incorporation at In–O–In bridge sites, while unfavorable defect energetics destabilize the oxygen-deficient BaTi 1– x In x O 3−δ lattice. Comparative analysis across BaM 0.5 In 0.5 O 3−δ systems (M = Zr, Ce, Sn, Ti) establishes a strong correlation between oxyhydride thermal stability and the ionic character of the B-site metal–oxygen bond. These findings provide a clear design principle for developing thermally robust hydride-ion–conducting perovskite oxyhydrides suitable for high-temperature electrochemical and energy-conversion devices.