Understanding the two-step nucleation of iron at Earth's inner core conditions: a comparative molecular dynamics study

TL;DR

This study uses molecular dynamics simulations with two different interatomic potentials to investigate the two-step nucleation process of iron under Earth's inner core conditions, revealing consistent mechanisms and nucleation rates.

Contribution

It demonstrates the robustness of the two-step nucleation mechanism of iron across different potentials under Earth's core conditions.

Findings

Metastable bcc phase has higher nucleation rate than hcp.

Both potentials predict similar relative Gibbs free energies.

Nucleation occurs at lower undercooling thresholds.

Abstract

Metastable phases can lead to multistep nucleation processes, influencing the liquid-to-solid transition in various systems. In this study, we investigate the homogeneous nucleation of iron's crystalline phases under Earth's inner core conditions, employing two previously developed interatomic potentials. We compare the thermodynamic and kinetic properties of iron relevant to the nucleation as predicted by these potentials. While the potentials differ in their predictions of melting temperature by a few hundred Kelvin, they show a consistent description of the relative Gibbs free energy between solid and liquid phases with respect to the undercooling. Both potentials also predict that the metastable bcc phase exhibits a significantly higher nucleation rate than the hcp phase over a wide range of undercooling temperatures below the melting point. This substantially lowers the undercooling thresholds required for the initial nucleation of Earth's inner core. The results validate the commonality of the two-step nucleation mechanism of iron under Earth's inner core conditions for two different potentials, providing a foundation for future studies about the influence of other elements on the nucleation of Earth's core.