First-principles calculations were made of the diffusivity of aluminium in periclase under pressures relevant to deep planetary interiors. Differences between experimental migration enthalpies and those obtained with previous theoretical studies were reconciled by finding a lower energy saddle-point for the aluminium atom migration. Previous studies did not recognise a bifurcation at the saddle point. Differences between experimental and theoretical binding enthalpies could be explained with a magnesium vacancy. It was found that binding enthalpies continuously increased with decreasing aluminium concentrations, such that the difference between experimental and theoretical binding energies can be attributed to differing concentrations. It was also found that binding energies increased with pressure as the permittivity decreased. Aluminium therefore not only causes extrinsic vacancy formation but also binds some of them, effectively removing them for magnesium diffusion. Ther were implications for how other 3+ ions affected diffusion in oxides and silicates.

Diffusion of Aluminium in MgO from First Principles. M.W.Ammann, J.P.Brodholt, D.P.Dobson: Physics and Chemistry of Minerals, 2012, 39[6], 503-14