Hydride Conductivity in an Anion-Ordered Fluorite Structure LnHO with an Enlarged Bottleneck

We report on the hydride (H^–) conductivity in fluorite-type LnHO oxyhydrides (Ln = lanthanide) using samples prepared under high pressure. It is found that, despite its “stoichiometric” composition, the anion-ordered phase (Ln = La, Nd) exhibits hydride conductivity (e.g., 2.3 × 10^–5 S cm^–1 for NdHO at 300 °C), while the anion-disordered one (Ln = Gd, Er) is an ionic insulator. The systematic structural analysis combined with computational calculations has revealed the indirect interstitial mechanism, where H^– anions migrate between the tetrahedral and octahedral sites through a triangular Ln₃ bottleneck expanded by the anion order, with a critical bottleneck radius of 1.18 Å. This study may offer a general guide for the design and control of suitable anion diffusion pathways for oxyhydrides and more generally mixed-anion compounds.