Superionic - Superionic Phase Transitions in Body-Centered Cubic H2O Ice

We used first-principles molecular dynamics to investigate the relation between the superionic proton conduction and the behavior of the O-H...O bond (ice VII to ice X transition) in body-cubic centered (bcc) H2O ice between 1300 K and 2000 K and up to 300 GPa. We bring evidence that there are three distinct phases in the superionic bcc stability field. A first superionic phase characterized by extremely fast diffusion of highly delocalized protons (denoted VII'') is stable at low pressures. A first-order phase transition separates this phase from a superionic VII', characterized by a finite degree of localization of protons along the non-symmetric O-H...O bonds. The transition is identified in structural, energetic, and elastic analysis. Upon further compression a second-order phase transition leads to the superionic ice X with symmetric O-H-O bonds.