Theoretical Study on the Stability of Helium in bcc and fcc Transition Metals

The relative stability of single He defect in bcc and fcc metals is investigated using ab initio calculations based on density functional theory (DFT). The calculations reveal that for all metals involved, except for V and Nb, the substitutional position is the most stable. Furthermore, the tetrahedral site is energetically more favorable for the He interstitial than the octahedral site in the bcc metals, but the relative stability of He defects in the fcc metals is not definitive. The He formation energies in bcc Fe and fcc Ni at the tetrahedral and octahedral sites with or without spin polarization are investigated. It is interesting to find that the magnetism of host atoms does not directly affect the relative stabilities of He interstitial sites.