Redox Asymmetry Enables Fe-H Bonds in Perovskite Oxyhydrides

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 BaFe_0.5Ta_0.5O_2.7H_0.3, in which Fe is selectively reduced while Ta remains pentavalent. Density functional theory calculations reveal that O^2-/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.