Cation diffusion in magnesium aluminate spinel

Abstract

The mechanisms by which Mg(2+) and Al(3+) ions are transported through the MgAl(2)O(4) spinel lattice are investigated using atomic scale computer simulation. Both vacancy and interstitial cation processes are considered. Stable vacancies can be generated on either the magnesium or aluminium sublattices but the Mg(2+) and Al(3+) cation interstitials are most stable when located in split form with another Mg(2+) ion about a vacant Mg(2+) site. The pathways for diffusion of defects both via vacancy and interstitial mechanisms are analysed in detail with calculation of the energy barriers and the associated exponential prefactors. The results show that vacancies can be exchanged between the two sublattices resulting in the formation of antisite defects (though these processes have a high activation energy); that the Mg(2+) ions are more mobile than the Al(3+) ions and that the preferred mechanism for Al(3+) ion diffusion is via a vacancy mechanism on the magnesium sublattice. Although the calculated values of the prefactors can differ in size by an order of magnitude, in this system it is the relative size of the energy barriers that dominate the diffusion rates. (C) 2008 Elsevier B.V. All rights reserved.