Molecular dynamics simulation of vacancy-solute binding free energy in periclase

Abstract

The free energy of binding between a cation vacancy and charge-balancing tetravalent cation in MgO is calculated from molecular dynamics simulations, at temperatures of 1200–2400 K and pressures of 0–25 GPa. The entropy of binding, obtained from the temperature dependence of the binding free energy, is found to be independent of pressure and to have a value of 13(5) J/mol K (2σ). The binding volume, obtained from the pressure dependence of the binding energy, is independent of temperature and has a value of 1.7(3) cm3/mol (2σ). These results are in excellent agreement with experimental estimates for several different trivalent cations, showing that the binding entropy and volume are not strongly dependent on the identity or charge of the cation. The binding energy is predicted to increase with depth along a mantle adiabat, leading to lower concentrations of unbound vacancies, and correspondingly slower diffusion rates for unbound cations.