The Fe-Ni phase diagram and the Earth's inner core structure.

The Fe-Ni alloy is believed to be the main component of Earth's core. Yet, a comprehensive understanding of phase equilibria near the melting point of this alloy under core conditions is still lacking, leaving the effect of nickel inconclusive. Using ab initio simulations, we computed Gibbs free energy and phase diagram for liquid and solid solutions of the Fe-Ni alloy under conditions close to the inner core. The Fe-Ni phase diagram provides crucial insights for understanding previous experimental observations and crystallization simulations of the Fe-Ni alloy under core conditions. It also presents complex scenarios for inner core structures, suggesting body-centered cubic (bcc)-liquid coexistence at the inner core boundary and the possibility of multilayer structures consisting of bcc-hexagonal close-packed (hcp) composites within the inner core. Our work clarifies nickel's substantial impact on the inner core structure, providing additional constraints for studying the core's composition and formation.