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 LaH(3) (LaH(3)(−2x)O(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)(−)(2x)O(x) are crucial factors that impose significant temperature dependence on the potential energy and implement characteristic fast H(−) conduction.