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.

中文翻译：

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Fe-Ni-S 液体在高温高压下的结构和非理想混合及其对地球外核成分的启示


在地球外核条件下，硫等轻元素 （LE） 对液态铁镍合金物理性能的影响对于理解核心组成和动力学至关重要。采用第一性原理分子动力学模拟对 Fe-Ni-S 液体进行建模，其中 S 浓度在 4050 K 时为 （0-25） 原子百分比 （at%） 范围内，在 5530 K 时为 （0-33.33） at%，压力分别与核-地幔边界 （CMB） 和内核边界 （ICB） 相关。计算了 Fe-Ni-S 液体的热力学混合特性，表明 Fe-Ni-S 合金的超量体积与理想混合负偏差为 -0.33%，在 CMB 为 12.5 at% S 时为 -0.33%，在 ICB 为 17 at% S 时为 -0.35%。同样，在 CMB 和 ICB 的相似 S 浓度下，超载焓分别与理想混合负偏差 -3.4 kJ/mol 和 -13 kJ/mole，表明整个外核混合不理想。在等温体积模量 （KT_{） 和}地震速度方面也观察到类似的行为。分析了短程和中程结构，并用于解释非理想的混合行为。结果表明，使用理想混合的外推低估了 CMB 附近约 0.14 km/s 和 ICB 附近约 0.10 km/s 的声速，这对于限制核心成分非常重要。如果 S 是唯一的 LE，则 10-12 wt% S 处的密度与初步参考地球模型 （PREM） 相匹配。12-15 wt% S 时的地震速度与 PREM 相匹配。这些结果表明外核中存在其他 LE。