Anion ordering enables fast H^- conduction at low temperatures

The introduction of chemical disorder by substitutional chemistry into ionic conductors is the most commonly used strategy to stabilize high-symmetric phases while maintaining ionic conductivity at lower temperatures. In recent years, hydride materials have received much attention owing to their potential for new energy applications, but there remains room for development in ionic conductivity below 300°C. Here, we show that layered anion-ordered Ba_2-δH_3-2δ X (X = Cl, Br, and I) exhibit a remarkable conductivity, reaching 1 mS cm^-1 at 200°C, with low activation barriers allowing H^- conduction even at room temperature. In contrast to structurally related BaH₂ (i.e., Ba₂H₄), the layered anion order in Ba_2-δH_3-2δ X, along with Schottky defects, likely suppresses a structural transition, rather than the traditional chemical disorder, while retaining a highly symmetric hexagonal lattice. This discovery could open a new direction in electrochemical use of hydrogen in synthetic processes and energy devices.