First-principles study of hydrogen diffusion and self-clustering below tungsten surfaces
The diffusion and self-clustering nucleation behavior of hydrogen (H) without vacancies below tungsten (W) surfaces are important for understanding the retention of deuterium (D) in W crystals exposed to high-flux and low-energy D plasma. The H migration energy and binding energy of H to self-clusters near the W(100), W(110), and W(111) surfaces have been investigated by first-principles computer simulations using density functional theory. H diffusion from adsorption sites on the W(100), W(110), and W(111) surfaces into solute sites in the bulk requires energies of at least 1.21, 1.78, and 1.80 eV, respectively, while 0.27, 0.31, and 0.24 eV for the reverse process. In addition, the lateral diffusion of H between two subsurface layers below the W surfaces has been investigated. Two H atoms at a depth of 0.08 nm below the W(110) surface have the highest binding energy, followed by H atom pairs below the W(111) and W(110) surfaces. The nucleation and stability of H clusters depend on the surface orientation. A planar configuration between the first nearest neighbor {100} planes is energetically favorable for H self-clustering below the W(100), W(110), and W(111) surfaces. In conclusion, the thermal stability of a platelet containing 16 H atoms below the W surfaces at 300 and 600 K was also studied by ab initio molecular dynamics simulations, which indicate that the H platelet below a W(111) surface is more stable than that below either the W(100) or the W(110) surface.