Characteristic fast H^- ion conduction in oxygen-substituted lanthanum hydride

Fast ionic conductors have considerable potential to enable technological development for energy storage and conversion. Hydride (H^-) ions are a unique species because of their natural abundance, light mass, and large polarizability. Herein, we investigate characteristic H^- conduction, i.e., fast ionic conduction controlled by a pre-exponential factor. Oxygen-doped LaH₃ (LaH_3-2xO_x) has an optimum ionic conductivity of 2.6 × 10^-2 S cm^-1, which to the best of our knowledge is the highest H^- conductivity reported to date at intermediate temperatures. With increasing oxygen content, the relatively high activation energy remains unchanged, whereas the pre-exponential factor decreases dramatically. This extraordinarily large pre-exponential factor is explained by introducing temperature-dependent enthalpy, derived from H^- trapped by lanthanum ions bonded to oxygen ions. Consequently, light mass and large polarizability of H^-, and the framework comprising densely packed H^- in LaH_3-2xO_x are crucial factors that impose significant temperature dependence on the potential energy and implement characteristic fast H^- conduction.