Trivalent cations in periclase were attracted to cation vacancies and these defects tend to associate on adjacent sites to form mobile pairs. A theory was developed to describe the concentration-dependent diffusion of Al in the presence of Al-vacancy pairs, and was applied to experiments conducted at 1577 to 2273K and 1atm to 25GPa. In all but one experiment, the Gibbs free energy of binding, deduced from the diffusion profiles, was between −44 and −60kJ/mol, with an average value of −50kJ/mol. The absolute value of the entropy of binding was constrained to be less than 50J/molK, and the volume of binding was constrained to be between −1.8 and 0cm3/mol. The diffusion coefficient of the Al-vacancy pair could be described by:

D2(m2/s) = D2,0exp[−{213(kJ/mol)+ 3.22(cm3/mol)P}/RT]

and log10D2,0(m2/s) = −6.17. Calculations were presented for the diffusion of Al and Mg as functions of Al concentration, temperature and pressure.

J.A.Van Orman, C.Li, K.L.Crispin: Physics of the Earth and Planetary Interiors, 2009, 172[1-2], 34-42