First principles calculations on the diffusivity and viscosity of liquid Fe–S at experimentally accessible conditions

Ab initio molecular dynamics calculations, based upon density functional theory within the generalised gradient approximation (GGA) using ultrasoft non-norm conserving Vanderbilt pseudopotentials, have been used to predict the transport properties of liquid Fe–S. In order to compare our simulations with experimental data, the simulations were performed for the eutectic composition of Fe–S at the experimentally accessible conditions of 5 GPa and of 1300 and 1500 K. Our results give values for Fe and S diffusion of a few times 10−5 cm2 s−1. Our calculated viscosities, obtained directly from the simulations, are 11±5 and 4±1 mPa s at 1300 and 1500 K, respectively. Our calculated diffusion and viscosity coefficients agree well with recent experiments at similar pressures and temperatures, supporting a high diffusivity and low viscosity in liquid Fe–S at temperatures up to a few hundred Kelvin above the eutectic temperature. Furthermore, an extensive study of the liquid structure shows no evidence for sulphur polymerisation or the existence of any large viscous flow units. These results are in direct conflict with the previously reported experimental results of Le Blanc and Secco [LeBlanc, G.E., Secco, R.A., 1996. Viscosity of an Fe–S liquid up to 1300°C and 5 GPa, Geophys. Res. Lett., 23, 213–216.].

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