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 x 10(-5) S cm(-1) for NdHO at 300 degrees 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 Ln(3) bottleneck expanded by the anion order, with a critical bottleneck radius of 1.18 angstrom. 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.