Hydrogen diffusion on the tin-covered tungsten surface: A first-principles study

The diffusion behavior of hydrogen (H) on tin-covered tungsten (Sn-W) surfaces is of great significance for understanding the retention of H in Sn-W materials exposed to H plasma. In this work, the diffusion mechanism of H in Sn-W surfaces and the related interactions were studied through employing density functional theory. The results show that Sn atoms are strongly adsorbed on the W surface with energy of -5.10 ∼ -5.60 eV with surface reconstruction for W (100). The diffusion of H atom from top of Sn to interior on W (100) and W (110) surface requires energy barrier of at least 1.02 eV and 0.76 eV, respectively. More interestingly, the Sn converging on W surface can reduce the diffusion energy barriers of H atom on or into the W surface, which indicates that the diffusion of H atom became easier in comparison to naked W surface. The facilitation effect of Sn on H atom diffusion can be attributed to the strong interaction between the Sn atom and W surface as proved by the calculated charge density and interaction energy. However, the coverage of Sn has little impact on the diffusion of H atom on or into the W sub-surface. As the depth increases, the influence of surface Sn atoms on the diffusion barrier of H decreases. The influence of liquid tin on the diffusion of hydrogen in solid tungsten will be helpful to understand the abnormal retention mechanism for liquid metal divertor.