Explore the fluid-to-superionic phase transition of water at giant planetary interior conditions

Superionic water, combining solid and liquid properties, plays a crucial role in thedynamics and magnetic fields of giant planets. However, its phase boundary and whether the fluid-to-superionic transition involves an insulator-to-metal transition remain uncertain. We performed extensive ab-initio molecular dynamics simulations to investigate the dynamic properties of water. Oxygen self-diffusion shows a sharp decline between 5000 and 10000 K, while hydrogen exhibits atomic fluid behavior. Structural analyses indicate that the fluid-to-superionic transition is marked by the formation of an ordered oxygen sublattice with a fcc arrangement, which enhances ionic interactions and restricts ion mobility. Electronic analyses demonstrate significant peak merging and reduced conductivity during the transition, confirming the absence of a metallization transition. Finally, a comprehensive phase boundary for water under giant planetary interior conditions is constructed, offering a clear picture of water's phase transitions in deep planetary interiors and enhancing our understanding of the interior structure of these planets.