For decades, ion transport in solids has been largely described as a classical barrier-climbing process: an ion requires sufficient activation energy to surmount an energy barrier before it can move. However, for hydrogen—the lightest element in the periodic table—this classical framework can break down.
In this new study, researchers from the Institute of Physics, Chinese Academy of Sciences, Fudan University and Peking University note that hydrogen in LaH 3 can tunnel through energy barriers rather than always surmounting them. Using first-principles calculations based on ring-polymer instanton theory, they uncovered a quantum mechanism for hydrogen transport: for the concerted migration, quantum tunneling becomes dominant at approximately 71 K—close to liquid-nitrogen temperature. Moreover, for the single-ion migration, the crossover temperature increases to 308 K, bringing quantum tunneling into the near-room-temperature range. Below these corresponding crossover temperatures, the instanton rate constants diverge significantly from their classical counterparts, demonstrating that nuclear quantum effects in hydrogen transport are not limited to extreme cryogenic conditions, but instead play a major role in more practical temperature regimes.
This study also clarifies why classical models fails to capture key aspects of the underlying physics. From the classical perspective, the migration rate is primarily governed by barrier height. When nuclear quantum effects are incorporated, however, barrier width becomes nearly as important. A narrow barrier is far more “tunneling-friendly” than classical descriptions would predict. It explains why the difference in rate constant between the concerted and single-ion pathways is vastly overestimated when quantum tunneling is overlooked. The calculations further indicate that the tunneling rate can be adjusted by modifying the barrier geometry through strain, suggesting that strain engineering could provide a practical means of controlling hydrogen motion in solid materials.
Taken together, these findings establish a quantum picture of hydrogen transport in LaH 3 and identify relatively high crossover temperatures for quantum tunning. The work suggests that nuclear quantum effects may need to be considered under far more practical conditions than commonly assumed, offering a novel perspective on ionic conductivity in hydrogen-containing solid materials and providing theoretical support for related functional material design and regulation.
Science Bulletin
Computational simulation/modeling