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

From first-principles molecular dynamics, we 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-centered-cubic (bcc) H2O ice between 1300 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" hereinafter) is stable at low pressures. A first-order transition separates this phase from a superionic VII", characterized by a finite degree of localization of protons along the nonsymmetric 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.