Computer simulation techniques were used to investigate the effect of pressure

upon magnesium diffusion in forsterite between 0 and 10GPa. The diffusion path

along the c crystallographic axis (Pbnm system) via a vacancy mechanism was

studied. Using a Mott–Littleton approach, it was possible to map precisely the

diffusion path of a Mg vacancy and an activation energy of E = 3.97eV was found

at 0GPa (with Ef = 3.35eV for formation and Em = 0.62eV for migration) with E =

4.46eV at 10GPa (Ef = 3.81eV and Em = 0.65eV). Preliminary results, obtained

using the super-cell technique, gave the same saddle-point coordinates and

energies. This saddle-point in Mg vacancy diffusion was then introduced into an ab

initio study; confirming the migration energy values found for 0 and 10GPa. It was

therefore possible to estimate that the activation volume (ΔV) was around

5cm3/mol, with d(ΔV)/dP ≈ 0. The pressure acted mainly upon defect formation

and hardly upon migration.

A Computer Simulation Study of the Effect of Pressure on Mg Diffusion in

Forsterite. F.Béjina, M.Blanchard, K.Wright, G.D.Price: Physics of the Earth and

Planetary Interiors, 2009, 172[1-2], 13-9