First-principles theory of the energetics of He defects in bcc transition metals

Helium defect properties in V, Nb, Ta, Mo, and W were studied using first-principles electronic structure calculations. The most stable position for the He in all bcc metals is a substitutional site; the tetrahedral interstitial position is more favorable than the octahedral position. The formation energy of He substitutional defect is nearly the same for all the metals, while the formation energy of He interstitial defect strongly depends on the electronic structure of the host and insignificantly on its atomic size. The obtained He formation energies were used to calculate He binding energy to the vacancy. For V, Nb, and Ta He-vacancy binding energy is about one-half of the vacancy formation energy; for Mo and W it is about 40% higher than the vacancy formation energy. Both pair potentials and effective-medium theory fall to reproduce the preference order or the relationship between the formation energies. Calculated He formation energies and He-vacancy binding energies improve understanding of He behavior and diffusion mechanisms in metals.