Stoichiometric magnesium aluminate spinel, MgAl2O4, contained equimolar proportions of Al2O3 and MgO. Spinel could, however, exhibit significant deviations from this stoichiometric composition. There was considerable disagreement concerning which species compensate for either excess Al2O3 or MgO non-stoichiometry. Here, empirical and quantum mechanical (density functional theory) atomistic simulation techniques were used to investigate the defect chemistry accommodating non-stoichiometry. The incorporation of excess Al2O3 was found to be a lower energy process than the solution of excess MgO. Elevated magnesium and aluminium cation vacancy defect concentrations were predicted in Al2O3 rich spinels, whilst MgO excess was facilitated by a combination of oxygen vacancy and magnesium interstitial defects.

Non-Stoichiometry in MgAl2O4 Spinel. S.T.Murphy, C.A.Gilbert, R.Smith, T.E.Mitchell, R.W.Grimes: Philosophical Magazine, 2010, 90[10], 1297-305