Ultralow Melting Temperature of High-Pressure Face-Centered Cubic Superionic Ice

Superionic ice with oxygen in a face-centered cubic (fcc) sublattice is ascribed to the origin of magnetic fields of Uranus and Neptune, since the melting temperature (T) of fcc-superionic ice is believed to be higher than the isentropes of ice giants. However, precisely measuring the fcc-superionic phase experimentally remains a difficult task. The majority of the systematic investigations of its T were performed using perfect oxygen fcc-sublattice computations, which could result in superheating and overestimation of T. On the basis of the ab initio molecular dynamics method and the model with H₂O vacancy, we avoid superheating and obtain a much lower T than previous reports, indicating that fcc-superionic ice cannot exist in the interiors of Uranus and Neptune. Further simulations with the two-phase method justify the conclusion. The results suggest that superheating should be seriously treated when simulating the phase diagram of other hydrogen-related superionic states, which are widely used to understand the properties of ice giants, Earth, and Venus.