[SY-B6] Isotope effect on quantum diffusion of interstitial hydrogen in face-centered cubic metals
Safe control of tritium is one of the technological challenges in the development of next-generation deuterium-tritium fusion reactors where tritium is used as nuclear fuel. From a scientific viewpoint, measuring the rate of diffusion of hydrogen isotopes in metals is important for understanding the isotope effect on the kinetics of hydrogen transport and hydrogen-induced deterioration of materials. However, a consensus on the physical mechanisms and numerical values of diffusivities of hydrogen isotopes in metals is still lacking. In this study, we clarified the site preference and diffusion rate of interstitial hydrogen and tritium in several face-centered cubic (fcc) metals, such as palladium, aluminum, and copper, by performing ab initio path-integral molecular dynamics (PIMD) modeling in the framework of density functional theory. This was necessary as the hydrogen atom has sufficiently low mass that it exhibits significant nuclear quantum delocalization and zero-point motion even at ambient temperature. The nuclear quantum effect on the activation free energies for hydrogen migration was characterized according to the PIMD-based free energy profiles obtained from the thermodynamic integration of the centroid force along the migration path at 75-1200 K. We found that the nuclear quantum effects significantly affected the activation barrier for hydrogen migration and the difference between the energies of the hydrogen atom at the octahedral and tetrahedral interstitial sites even at ambient temperature. Consequently, we revealed the role of quantum fluctuations on the reversed isotope dependence for hydrogen diffusion in certain fcc metals.