Synergetic effect of H and He with vacancy in vanadium solid from first-principles simulations

Using first-principles calculations, we investigate the stability and energetics of H-He-vacancy complexes and He-He/He-H/H-H interactions in bulk vanadium to explore the synergetic effect of H and He impurities with vacancy. Inside vacancy space, He prefers to occupy octahedral site rather than vacancy center, different from of the cases of bcc Fe and W. Equilibrium He-H (2.33 Å) and H-H (2.45 Å) distances in the vacancy are longer than He-He distance (1.75 Å) and they exhibit a weak attraction. One He-vacancy complex can trap up to five H atoms and the stable configurations with different amount of trapped H atoms are discussed in details. If a He atom occupies vacancy center, formation of H2 molecule in the He-vacancy complex is almost impossible. Moreover, formation energy of a new vacancy around the H-He-vacancy complex does not remarkably reduce with increasing number of H atoms. We thus suggest that creation of a new vacancy around the He-complex via H aggregation is thermodynamically difficult.