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 charge and the partial charge model were employed. The partial charge model shows 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 energies calculated. The results indicate that the three-dimensional growth of vacancy clusters in MgO is energetically favourable. Next, the activation energies for dissociation of helium atoms from these vacancy clusters were calculated. The activation energy for dissociation of a helium atom from vacancy clusters larger than a monovacancy is approximately 3.6 eV for the partial charge model. Remarkably, the activation energy for dissociation of a helium atom from a monovacancy was found to be higher, 3.9 eV for the partial charge model. This last energy is enhanced by lattice relaxation around the monovacancy. The dissociation energies from large vacancy clusters is in good agreement with the permeation energy of 3.3 +/- 0.3 eV obtained experimentally with a permeation experiment in which helium release was monitored with neutron depth profiling (NDP). (C) 2000 Elsevier Science B.V. All rights reserved.