Helium Impurities and Interactions in Lithium

We investigate helium interactions with lithium using density functional theory. Like other body-centered cubic (bcc) metals, the lowest-energy site for interstitial helium is a tetrahedral site. However, helium in lithium shows a higher substitutional formation energy than either the tetrahedral or octahedral interstitial formation energies, which is unique. The calculated migration energies of helium in lithium are also very low (~ 1 meV), an order of magnitude lower than those in other bcc metals. We find an increase in the binding energy to a vacancy as the number of helium atoms bound to it increases. The extremely low migration energies suggest that helium transport in lithium will be very fast.