Helium atom diffusion in titanium tritides (TiT2-x, −0.02 ≤ x ≤ 0.25): A first-principles and kinetic Monte Carlo study

Abstract

The tritium (T, or 3H) and helium (3He) diffusion properties are critical to the helium bubble nucleation and growth behaviors in tritium storage materials such as titanium tritide (TiT2-x). Hereby, the possible atom migration paths of T and He in TiT2-x as well as their barriers and prefactors are calculated by density functional theory calculations, and are further applied in kinetic Monte Carlo simulations to determine the overall diffusion rates. The correlations of diffusion coefficients of the He atom with temperature in different TiT2-x (0 ≤ x ≤ 0.25) are shown as Arrhenius plots. The results show that two diffusional regimes, migration more via tetrahedral vacancies or octahedral interstitials, are presented depending on temperature and composition for both stoichiometric and non-stoichiometric tritides. Comparing with T, He exhibits similar diffusion mechanisms but faster diffusion rates. Our results suggest that the tritium (or vacancy) concentration, temperature, and axial strain are effective for tuning the diffusion mechanism and rate of a helium atom in titanium tritides.