Diffusion Mechanisms of Hydrogen in Metals Explored via Path-Integral Methods

Abstract

Lighter isotopes normally diffuse faster than heavier ones; however, this is not necessarily the case for H. That is one of the reasons why the prediction of the kinetics of H-isotope transport and reaction in and on crystals remains a fundamental challenge in materials physics. Actually, the peculiar isotope effect experimentally observed on H diffusivities in face-centered cubic (fcc) metals has been a long-standing unsolved problem. Using a state-of-the-art theoretical approach to exploring the quantum mechanical nature of both electrons and nuclei, we succeeded in predicting the H-isotope diffusivities in fcc Pd over a wide temperature range. We found that the temperature dependence of the diffusivities has an unusual “reversed S” shape on Arrhenius plots. This irregular behavior, which stems from the competition between different nuclear quantum effects with different temperature dependencies, unravels the mechanism of anomalous crossovers between the normal and reversed isotope effects. Our approach is broadly applicable to assessing the quantum behavior of H isotopes in a range of materials.