Predicting the phase behaviors of superionic water at planetary conditions

Abstract

Most water in the universe may be superionic, and its thermodynamic and transport properties are crucial for planetary science but difficult to probe experimentally or theoretically. We use machine learning and free energy methods to overcome the limitations of quantum mechanical simulations, and characterize hydrogen diffusion, superionic transitions, and phase behaviors of water at extreme conditions. We predict that a close-packed superionic phase with mixed stacking is stable over a wide temperature and pressure range, while a body-centered cubic phase is only thermodynamically stable in a small window but is kinetically favored. Our phase boundaries, which are consistent with the existing-albeit scarce-experimental observations, help resolve the fractions of insulating ice, different superionic phases, and liquid water inside of ice giants.

References in corpus (5)

• Growth Model Interpretation of Planet Size Distribution
• Quantum-mechanical exploration of the phase diagram of water
• Thermal evolution of Uranus and Neptune I: adiabatic models
• Polymorphism of superionic ice
• Thermal evolution of Uranus and Neptune II -- Deep thermal boundary layer

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• Structure and Melting of Fe, MgO, SiO2, and MgSiO3 in Planets: Database, Inversion, and Phase Diagram
• Observation of hexagonal close-packed water ice at conditions in ice giant planetary interiors