Mechanistic Insights into Hydride Incorporation in BaZr_1–<i>x</i>In_<i>x</i>O_3−δ-Based Perovskite Oxyhydrides

Metal oxyhydrides have emerged as promising candidates for next-generation electrochemical devices due to their unique combination of hydride (H–) ion conductivity and catalytic activity for carbon dioxide and nitrogen reduction. However, their limited stability under ambient hydrogen pressure (pH2) poses a significant challenge. In this study, we demonstrated that BaZr1–xInxO3–0.5x (BZI) undergoes topochemical transformation into a thermodynamically stable oxyhydride phase under ambient pH2 across a broad composition range. Indium-rich compositions (x = 0.5, 0.6, and 0.7) successfully formed oxyhydrides─BaZr0.5In0.5O2.25H0.5, BaZr0.4In0.6O2.18H0.23, and BaZr0.3In0.7O2.17H0.11─via the sequential formation of oxygen vacancy (VO) and subsequent H– ion incorporation. Conversely, the Zirconium-rich composition (x = 0.3) formed only VO and stabilized as an oxygen-deficient phase, BaZr0.7In0.3O2.70, without hydride uptake. Combined experimental and density functional theory (DFT) analyses reveal that the extent of In reduction, reflected by an increase of the Fermi level following VO donor formation, governs the thermodynamic driving force for H– incorporation. DFT results show that a greater initial Fermi energy rise facilitates greater H– uptake, which subsequently lowers Fermi energy upon H– incorporation, establishing an equilibrium determined by defect formation energetics. Furthermore, DFT and in situ X-ray absorption spectroscopy confirm that the oxyhydride phase preferentially forms neutral oxygen vacancies (VO×) during dehydrogenation, in contrast to the commonly assumed doubly charged species (VO••). This reversible transformation between a pure oxyhydride and an oxyhydride–electride phase, dependent on pH2, highlights the tunable defect chemistry and electronic structure of perovskite oxyhydrides and offers new design principles for stable and fast hydride ion conductors.