Redox Asymmetry Enables Fe–H Bonds in Perovskite Oxyhydrides

Abstract Transition-metal oxyhydrides have attracted considerable interest because incorporation of hydride anions endows oxides with distinctive structural, electronic, and catalytic functionalities. Although CaH2 topochemical reduction has enabled various transition-metal oxyhydrides (e.g., V, Ti, Co, and Ru), Fe-based oxides exclusively form oxygen-deficient phases such as SrFeO2, whereas oxides containing redox-inert cations such as Ta5+, Nb5+, or Hf4+ remain essentially unreactive. Here we show that combining Fe with such redox-inert cations in B-site solid-solution perovskites overcomes both limitations and enables the synthesis of BaFe0.5Ta0.5O2.7H0.3, in which Fe is selectively reduced while Ta remains pentavalent. Density functional theory calculations reveal that O2–/H– substitution stabilizes the structure by relieving the local strain associated with oxygen-vacancy formation. The resulting Fe–H bonds are remarkably robust, persisting in the mixed-valent Fe2+/Fe3+ configuration, whereas Fe–H bonds in molecular complexes are generally limited to lower oxidation states. Extension of this strategy to B-site Fe/Nb and Fe/Hf solid-solution perovskites likewise yields their corresponding oxyhydrides. These results establish a general design principle for stabilizing chemically robust Fe–H bonds in perovskites and other structures.