Ab initio superionic-liquid phase diagram of Fe1-xOx under Earth's inner core conditions (2410.23557v2)

Abstract: The superionic state is a unique phase of matter in which liquid-like ion mobility coexists within a solid crystalline lattice. Recently discovered in Earth's inner core, this state has been extensively studied for its kinetic properties and geophysical impact. However, the equilibrium between the superionic phase and the liquid solution under core conditions remains unexplored. Here we present a thermodynamic method to calculate the Gibbs free energy and construct the ab initio superionic-liquid phase diagram for the Fe1-xOx system under inner core (IC) boundary conditions. We find that oxygen forms superionic states in both hcp and bcc iron in the IC, influencing iron's cooperative diffusion in the bcc phase. The stability fields of these superionic phases depend strongly on oxygen stoichiometry. Our results suggest that the oxygen concentration in the IC is higher than previously estimated due to the presence of superionic states. Our work provides a framework for studying superionic-liquid equilibria in planetary interiors.