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 at 74 GPa and 3400 K is 8.03(35) g/cm3, 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 KTPr = 110.5(250) GPa and KTPr′ = 7.2(25), respectively, with a fixed density of rPr = 6.43 g/cm3 at 24.6 GPa. The change in the atomic volume of Fe46.5Ni28.5S25 upon melting was found to be ~10% at the melting temperature, a significantly larger value than that of pure Fe (~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.

中文翻译：

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高压液态铁镍硫的密度测定

通过同步加速器 X 射线衍射 (XRD) 数据的对分布函数 (PDF) 分析，在高达 74 GPa 的压力和 3400 K 的平均温度下测定液态铁镍硫 (Fe46.5Ni28.5S25) 合金的密度使用激光加热的金刚石砧细胞获得。液态 Fe46.5Ni28.5S25 在 74 GPa 和 3400 K 下测得的密度为 8.03(35) g/cm3，比纯液态 Fe 低 15%。将获得的密度数据拟合为三阶 Vinet 状态方程 (EoS)，确定的等温体积模量及其在 24.6 GPa 时的压力导数分别为 KTPr = 110.5(250) GPa 和 KTPr' = 7.2(25) ，在 24.6 GPa 时，rPr = 6.43 g/cm3 的固定密度。在熔化温度下发现 Fe46.5Ni28.5S25 的原子体积变化约为 10%，明显大于纯铁（~3%）的值。结合上述 EoS 参数和文献中报道的热依赖性，我们的数据被外推到地球的外核条件。假设 S 是唯一的轻元素并考虑到建议的 Ni 含量范围，我们估计地球外核中 S 含量为 5.3-6.6 wt%。