Quantum tunneling effects on hydrogen transport in lanthanum trihydrides

Ionic conductivity in solids is a topic of great interest in the fields of physics, materials science, and energy applications. Previous studies have primarily focused on the activation energy of ion transport based on classical transition state theory, lacking considerations from the perspective of nuclear quantum effects. Herein, by considering the effects of zero-point energy and quantum tunneling, we examine the quantum behaviors of hydrogen migration in lanthanum trihydrides (LaH₃), through the two dominant pathways-concerted migration and single-ion migration. Our first-principles calculations based on instanton rate theory indicate that the quantum rate constants diverge significantly from their classical counterparts at low temperatures. We predict that quantum tunneling becomes dominant over thermal diffusion for concerted hydrogen migration at liquid nitrogen temperature, and emerges even at room temperature when concerted transport is suppressed. We also demonstrate the tuning of migration rates by strain, and the sensitivity of the quantum tunneling rate to the energy barrier geometry. Our findings depict a complete quantum picture of hydrogen transport in lanthanide hydrides and provide a new perspective on ionic conductivity of solid materials.