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 th eory within the generalised gradient approximation (GGA) using ultrasoft no n-norm conserving Vanderbilt pseudopotentials, have been used to predict th e transport properties of liquid Fe-S. In order to compare our simulations with experimental data, the simulations were performed for the eutectic com position of Fe-S at the experimentally accessible conditions of 5 GPa and o f 1300 and 1500 K. Our results give values for Fe and S diffusion of a few times 10(-5) cm(2) s(-1). Our calculated viscosities, obtained directly fro m the simulations, are 11 +/- 5 and 4 +/- 1 mPa s at 1300 and 1500 K, respe ctively. Our calculated diffusion and viscosity coefficients agree well wit h recent experiments at similar pressures and temperatures, supporting a hi gh diffusivity and low viscosity in liquid Fe-S at temperatures up to a few hundred Kelvin above the eutectic temperature. Furthermore, an extensive s tudy of the liquid structure shows no evidence for sulphur polymerisation o r the existence of any large viscous flow units. These results are in direc t conflict with the previously reported experimental results of Le Blanc an d Secco (C) 2000 Elsevier Science B.V. All rights reserved.