A first-principles study of surface stability and H behavior near surfaces of W borides*

Understanding the behavior of tungsten boride (W_xB_y) surfaces in a fusion reactor environment is necessary since W_xB_y may form on W divertor surfaces due to boronization treatments. The surface stability of W_xB_y (I4₁/amd-WB, P6₃/mmc-WB₂ and I4/m-W₂B) with low-index orientations, as well as hydrogen (H) energetics near W_xB_y surfaces has been investigated by density functional theory calculations. For single element terminated W_xB_y surfaces, B terminated surfaces are more stable than W terminated due to the significant reconstruction of B. The H surface adsorption energy and activation energy of H diffusion penetration below W_xB_y surfaces are mainly related to the outermost termination of surfaces. Specifically, the WB(001) surface terminated with two B layers, referred to as WB(001)-T_BB, has higher H adsorption ability and lower H diffusion probability on the surface than other terminations, which results from the significant charge transfer from B to H. However, B atoms on the WB₂(0001)-T_BB surface decrease both H adsorption and diffusion abilities on the surface, but enhance H diffusion below the surface in comparison to the W terminated WB₂(0001) surface. H would be trapped and diffuse within atomic surface gaps on the WB₂(2-1-10) surface, while H below the surface layer would jump along the [0001] direction rather than diffuse into bulk. The surface diffusion activation energy of H on the W₂B(001) surface varies only slightly with the termination. Once H crosses the surface layer of W₂B(001) with either termination, H prefers to diffuse into the bulk. Correspondingly, we predict that both WB₂(0001) and WB₂(2-1-10) surfaces likely have higher H retention than other evaluated W_xB_y surfaces.