A Density Model for Multicomponent Iron‐Rich Alloys Under Earth's Outer Core Conditions

Abstract Understanding how light elements (LEs: C, H, O, Si, S) and Ni influence the density of iron‐rich alloys under core‐relevant conditions is crucial for constraining Earth's core composition. Earth's core is possibly an iron‐nickel alloy with multiple LEs (Fe‐Ni‐multi‐LE). However, existing reports on the density of iron‐rich alloys are limited and often overlook or simplify non‐ideal mixing effects in estimating the density of multicomponent alloys under core conditions by interpolation and/or extrapolation. Ignoring these effects oversimplifies interactions at the atomistic level, leading to inaccuracies in property estimates and unreliable constraints on core composition. First‐principles molecular dynamics simulations were employed to compute the thermodynamic mixing properties—particularly volume and enthalpy—of binary iron‐rich alloy systems as a function of concentration along Earth's core isentrope. Results show that the volume and enthalpy of Fe‐S and Fe‐Si alloys increase non‐linearly with S and Si concentration, respectively, whereas those of Fe‐H, Fe‐O, and Fe‐C alloys decrease non‐linearly. For Fe‐Ni alloys, the volume change is also non‐linear but smaller in magnitude. The calculated mixing properties for the binary systems were then combined to establish a model to obtain the density of Fe‐Ni‐multi‐LE alloys. The resulting density model achieves an adequate accuracy for density predictions of the multicomponent alloys within ±0.1% error compared with first principles calculations, and an average of 0.8% error relative to the broad literature. The calculated mixing properties and the proposed density model provide a basis for an efficient way to explore Earth's core composition scenarios.