Density determination of liquid iron-nickel-sulfur at high pressure

The density of liquid iron-nickel-sulfur (Fe46.5Ni28.5S25) alloy was determined at pressures up to 74 GPa and an average temperature of 3400 K via pair distribution function (PDF) analysis of synchrotron X-ray diffraction (XRD) data obtained using laser-heated diamond-anvil cells. The determined density of liquid Fe46.5Ni28.5S25 GPa and 3400 K is 8.03(35) g/cm(3), 15% lower than that of pure liquid Fe. The obtained density data were fitted to a third-order Vinet equation of state (EoS), and the determined isothermal bulk modulus and its pressure derivative at 24.6 GPa are K-TPr = 110.5(250) GPa and K-TPr' = 7.2(25), respectively, with a fixed density of r(Pr) = 6.43 g/cm(3) at 24.6 GPa. The change in the atomic volume of Fe46.5Ni28.5S25 upon melting was found to be similar to 10% at the melting temperature, a significantly larger value than that of pure Fe (similar to 3%). Combined with the above EoS parameters and the thermal dependence reported in the literature, our data were extrapolated to the outer core conditions of the Earth. Assuming that S is the only light element and considering the range of suggested Ni content, we estimated a 5.3-6.6 wt% S content in the Earth's outer core.