The structure and energy of He impurities and vacancies on the (001) surface of bcc iron were investigated using an ab initio method. Three cases for a He atom at the surface were found: some of the He atoms at surface atomic layers relaxed into the vacuum gap; some of the surface He atoms at octahedral interstitial sites relaxed into more stable tetrahedral interstitial sites; some of the surface He atoms remained at tetrahedral interstitial sites. The instability of the He atom at the surface system could be explained by the deformation mechanism of charge densities and electronic densities of states. It was found that the formation energy of the point defects, from the uppermost surface atomic layers to bulk-like atomic layers increased gradually. The formation energies of a monovacancy in the first five uppermost surface atomic layers were equal to 0.33, 1.56, 2.04, 2.02 and 2.11eV.

Bonding Character and Formation Energy of Point Defects of He and Vacancy on (001) Surface of BCC Iron by First Principle Calculations. J.Cai, D.Lu: Acta Metallurgica Sinica, 2013, 26[1], 25-32