Phase transitions involving superionic ice XVIII

Following the experimental verification in 2019 of the existence of a face-centered-cubic superionic form of water in extreme settings of pressure and temperature, known as ice XVIII, there has been an increased interest in understanding the phase behavior of water under such extraordinary circumstances, in particular due to their potential role in the anomalous magnetic fields of Neptune and Uranus. Given the experimental challenges associated with the creation and maintenance of such conditions, atomic-level modeling techniques have played a central role in making progress towards this goal. In this talk, we will describe a recent application of such methods to investigate the thermodynamics and kinetics of phase transitions between superionic ice XVIII and the insulating ice X phase as described by a machine-learned interaction model derived from density-functional-theory calculations. The results highlight the dual displacive/martensitic nature of the transition and provide insight into its kinetics.