The behaviors of helium atoms in tantalum, rhenium and osmium

We present systematic first-principles studies of occupancy, diffusion, and segregation of He atoms, as well as underlying evolution mechanism of He clusters in Ta, Re and Os to compare to that in W. It is found that single interstitial He atom prefers tetrahedral (T) site in Ta and W and basal octahedral (BO) site in Re and Os. Single He atom prefers the diffusion path from one T site to the closest T site in Ta and W, and the diffusion path from one BO site to the next BO site passing an octahedral site along 〈0001〉 direction in Re and Os. The diffusion of single He atom in Ta, Re and Os is relatively difficult than that in W. He atoms are energetically favorable to segregate at interstitial sites forming Hen clusters in Ta, W, Re and Os, and form close-packed He monolayer structure between (110) planes in Ta and W. The binding strength of Hen cluster in Ta, Re and Os is relatively small than that in W (W>Os>Re>Ta). Compared to interstitial sites, He atoms are more energetically favorable to aggregate in vacancy cluster forming VacmHen. The binding strength order of VacmHen cluster in the systems obeys W≃Os>Re>Ta.