Predicted vacancy cluster structures in MgO and their interaction with helium

Atomistic simulation calculations were performed to predict defect energies of vacancy clusters and dissociation energies of clusters with helium. The computation technique was based on pair potentials and the Mott-Littleton methodology was used as implemented in the CASCADE code. Both the full char ge and the partial charge model were employed. The partial charge model sho ws better agreement with experimental data reported in the literature. The energetically most favourable spatial configurations for clusters composed of up to eight vacancies were determined and the associated formation energ ies calculated. The results indicate that the three-dimensional growth of v acancy clusters in MgO is energetically favourable. Next, the activation en ergies for dissociation of helium atoms from these vacancy clusters were ca lculated. The activation energy for dissociation of a helium atom from vaca ncy clusters larger than a monovacancy is approximately 3.6 eV for the part ial charge model. Remarkably, the activation energy for dissociation of a h elium atom from a monovacancy was found to be higher, 3.9 eV for the partia l charge model. This last energy is enhanced by lattice relaxation around t he monovacancy. The dissociation energies from large vacancy clusters is in good agreement with the permeation energy of 3.3 +/- 0.3 eV obtained exper imentally with a permeation experiment in which helium release was monitore d with neutron depth profiling (NDP). (C) 2000 Elsevier Science B.V. All ri ghts reserved.