Defect processes in MgAl2O4 spinel

In perfect normal MgAl2O4 spinel the Mg2+ ions occupy tetrahedral 8a sites and Al3+ ions occupy octahedral 16d sites. In reality some cations are exchanged between the cation sublattices forming pairs of antisite defects and thus a degree of “inversion”. Here atomic simulation is used to investigate the influence that antisite defects have on the populations of other intrinsic defects, those associated with Schottky and Frenkel reactions. One consequence is that the total magnesium interstitial concentration is increased substantially over the aluminium interstitial concentration and the magnesium vacancy concentration is increased over the aluminium vacancy concentration but to a much smaller extent. The split structures of isolated interstitial defects and the stability of various defect clusters are also discussed.

Framework representation of a single unit cell of normal MgAl2O4 spinel, which shows a single Mg2+ ion (yellow), four Al3+ ions (green) and O2− ions (red). At the centre is a split oxygen interstitial cluster consisting of two O2− ions at interstitial sites, aligned along 〈111〉〈111〉, on either side of vacant oxygen lattice site (represented by a transparent cube).Figure optionsDownload as PowerPoint slide