Structure and Non-Ideal Mixing of Fe-Ni-S Liquid at High Temperature and Pressure and Its Implication for the Earth's Outer Core Composition

The effect of light elements (LEs) such as sulfur on the physical properties of liquid iron-nickel alloy under the earth's outer core conditions is critical for understanding the core composition and dynamics. First-principles molecular dynamics simulations were employed to model Fe-Ni-S liquid with S concentrations in the range of (0–25) atomic percent (at%) at 4050 K and (0–33.33) at% at 5530 K and pressures relevant to the core-mantle boundary (CMB) and inner core boundary (ICB), respectively. The thermodynamic mixing properties of Fe-Ni-S liquid were calculated, showing that the excess volume for Fe-Ni-S alloys deviates negatively from ideal mixing by −0.33% at 12.5 at% S at the CMB and −0.35% at 17 at% S at the ICB. Similarly, the excess enthalpy negatively deviated from the ideal mixing by −3.4 kJ/mole and −13 kJ/mole at the similar S concentrations at CMB and ICB, respectively, indicating non-ideal mixing throughout the outer core. Similar behaviors are observed for isothermal bulk modulus (K_T) and seismic velocity. The short- and intermediate-range structures were analyzed and used to explain the non-ideal mixing behaviors. The results suggest that extrapolations using ideal mixing underestimates the sound velocity by ∼0.14 km/s near CMB and ∼0.10 km/s near ICB, which is significant for constraining the core composition. If S is the only LE, the density at 10–12 wt% S matches the preliminary reference earth model (PREM). The seismic velocity at 12–15 wt% S matches PREM. These results suggest the presence of other LEs in the outer core.