Mechanistic Insights into Hydride Incorporation in BaZr1–xInxO3−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 (p_H2) poses a significant challenge. In this study, we demonstrated that BaZr_1–xIn_xO_3–0.5x (BZI) undergoes topochemical transformation into a thermodynamically stable oxyhydride phase under ambient p_H2 across a broad composition range. Indium-rich compositions (x = 0.5, 0.6, and 0.7) successfully formed oxyhydrides─BaZr_0.5In_0.5O_2.25H_0.5, BaZr_0.4In_0.6O_2.18H_0.23, and BaZr_0.3In_0.7O_2.17H_0.11─via the sequential formation of oxygen vacancy (V_O) and subsequent H^– ion incorporation. Conversely, the Zirconium-rich composition (x = 0.3) formed only V_O and stabilized as an oxygen-deficient phase, BaZr_0.7In_0.3O_2.70, 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 V_O 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 (V_O^×) during dehydrogenation, in contrast to the commonly assumed doubly charged species (V_O^••). This reversible transformation between a pure oxyhydride and an oxyhydride–electride phase, dependent on p_H2, highlights the tunable defect chemistry and electronic structure of perovskite oxyhydrides and offers new design principles for stable and fast hydride ion conductors.