Atomistic simulation calculations were used to predict the defect energies of vacancy clusters, and the dissociation energies of clusters with He. The technique was based upon pair potentials, and the Mott-Littleton method was implemented in the computer code. Both the full-charge and the partial-charge models were used. The partial-charge model gave better agreement with published experimental data. The energetically most favourable spatial configurations, for clusters composed of up to 8 vacancies, were determined and the associated formation energies were calculated. The results indicated that the 3-dimensional growth of vacancy clusters was energetically favoured. The activation energies for the dissociation of He atoms from these vacancy clusters were calculated. The activation energy for the dissociation of a He atom from a vacancy cluster larger than a monovacancy was about 3.6eV for the partial charge model. The activation energy for the dissociation of a He atom from a monovacancy was found to be higher (3.9eV) for the partial-charge model. The latter energy was enhanced by lattice relaxation around the monovacancy. The energy of dissociation from large vacancy clusters was in good agreement with the permeation energy (3.3eV) which had been obtained experimentally by using a permeation experiment in which He release was monitored via neutron depth-profiling.

Predicted Vacancy Cluster Structures in MgO and their Interaction with Helium. G.Busker, M.A.Van Huis, R.W.Grimes, A.Van Veen: Nuclear Instruments and Methods in Physics Research B, 2000, 171[4], 528-36