First-principles study of helium solution and diffusion in tungsten borides

The behavior of helium (He) in plasma facing materials (PFMs) is crucial for the performance of PFMs under neutron irradiation in a fusion environment. First-principles density functional theory calculations have been performed to model the solution and diffusion of He in six stoichiometric ground-state tungsten borides (W _xB _y), including W ₂B, WB, WB ₂, W ₂B ₅, WB ₃ and WB ₄ . The solution energy of He interstitial in W borides linearly decreases with increasing B content, except for the deviation in WB and WB ₃. The mechanical and electronic contributions to He solubility in W _xB _y have been discussed. The calculated diffusion energetics indicates that He transport in W ₂B ₅, WB ₃ and WB ₄ is much more difficult than in W ₂B, WB and WB ₂. Subsequently, we investigate the influence of vacancies on the He behavior. Except for WB ₃, He is more strongly bound in W vacancies rather than B vacancies and the He binding energy to a W vacancy generally decreases with increasing B content in W _xB _y. These findings can be used to predict the potential influence of He on the properties of different W borides in the fusion environment.