The superionic state is a phase of matter in which liquidlike ionic mobility coexists with a solid crystalline lattice. Recently identified in Earth’s inner core (IC), this state has attracted considerable attention for its unique kinetic behavior and geophysical implications. However, the ab initio phase diagram describing the equilibrium between the superionic phase and the liquid solution under core conditions remains largely unexplored. Here, we present a thermodynamic approach to compute the Gibbs free energy and construct the ab initio superionic-liquid phase diagram for the ${\mathrm{Fe}}_{1\text{−}x}{\mathrm{O}}_x$ system under IC conditions. We find that oxygen forms superionic states in both hcp and bcc Fe phases, with a pronounced influence on cooperative diffusion of iron in the bcc lattice. The stability fields of these superionic phases are sensitive to oxygen stoichiometry. The presence of superionic states leads to a higher oxygen concentration in the IC than previously estimated. Our Letter establishes a framework for investigating superionic-liquid equilibria under extreme conditions.

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• Earth's interior
• Pressure effects
• Thermodynamics
• Alloys
• Liquid-solid interfaces
• Liquids
• First-principles calculations
• Molecular dynamics