Energetics of He and H atoms with vacancies in tungsten: First-principles approach

The formation energies of various defect configurations of He and H atoms in W were estimated based on the density functional theory. A special consideration was given to the coexistence of the He and H atoms at the presence of the vacancy and vacancy cluster in W. A single He atom favors a substitutional site, while a H atom spontaneously incorporates at an interstitial site with the negative formation energy. When He and H are present close to each other, they form an interstitial pair, occupying relaxed tetrahedral sites. When He, H and a vacancy coexist within a unit cell of W, however, He occupies the vacancy site then the He_sub-H_tet pair is predicted to be the lowest energy configuration. At the presence of a nearby vacancy cluster, He atoms occupy the vacant space while H atoms move slightly toward W.