The defect structure of MgO containing trivalent cation solutes: shell model calculations

Abstract

We have used the HADES program to calculate the energies of various defect aggregates found in MgO containing Al³⁺ and Fe³⁺ solutes and compensating cation vacancies. Calculated energies of substitution are compared with heats of solution derived from phasediagram data; from the accuracy of these results, we deduce the validity of the models used for the lattice simulations. We find that our models provide a satisfactory description for Al³⁺ but are a less precise representation of crystals containing Fe³⁺; the models used, however, bracket a reasonable range of solute behaviour and important trends are unaffected by reasonable changes in the interionic potentials. The simplest vacancy-solute dimer can have either a 〈1 0 0〉 or 〈1 1 0〉 orientation; the two constituent defects are closest when the dimer has a 〈1 1 0〉 axis, but the 〈1 0 0〉 dimer is more stable because of the large displacement and polarization of the oxygen ion between the trivalent ion and vacancy. Trimers with either orientation are about twice as stable as the corresponding dimers. Complex aggregates of solutes and vacancies, which adopt configurations that form nuclei of the mixed-oxide spinel structure, are even more stable and the stability increases with cluster size. Thus we conclude that such clustering is an important phenomenon at low homologous temperatures. Calculated interstitial formation energies in MgO are large (>10eV) and our results for the activation energies for solute motion are of the order of 2 eV.