The formation energy and bonding characteristics of small helium–vacancy clusters on the low-index surface of α-Fe by first principles calculations

The structure and energy of a small helium–vacancy (He–V) cluster on the low-index surface of α-Fe are investigated by an ab initio method. We report the stability characteristics of these He–V clusters. The He atom of the He–V cluster at the topmost surface atomic layers (SALs) will relax into the vacuum gap. Some of the surface He atoms at octahedral interstitial sites will relax into a more stable tetrahedral interstitial site. The deformation charge densities and electronic densities of the states explain the instability of the surface He atoms. The formation energies of the He–V clusters gradually increase from the topmost SAL to the bulk-like atomic layer. This indicates that all of the He–V defects considered tend to aggregate at the topmost SAL.