Aluminum diffusion and Al-vacancy association in periclase

Trivalent cations in periclase are attracted to cation vacancies and these defects tend to associate on adjacent sites to form mobile pairs. A theory has been developed to describe the concentration-dependent diffusion of Al in the presence of Al-vacancy pairs, and has been applied to experiments conducted at 1577-2273 K and 1 atm to 25 GPa. In all but one experiment, the Gibbs free energy of binding, inferred from the diffusion profiles, is between -44 and -60 kJ/mol, with an average value of -50 kJ/mol. The absolute value of the entropy of binding is constrained to be less than 50 J mol^-1 K^-1, and the volume of binding is constrained to be between -1.8 and 0 cm³/mol. The diffusion coefficient of the Al-vacancy pair can be described by the equation D₂ = D_2,0 exp(-(E + PV)/RT), with E = 213 ± 32 kJ/mol, V = 3.22 ± 0.25 cm³/mol, and log₁₀D_2,0 = -6.17 ± 0.99 (in m²/s; all uncertainties 2σ). Calculations are presented for the diffusion of Al and Mg as functions of Al concentration, temperature and pressure.