Highly Compressible Plastic and Superionic Ice

Water is a key constituent of icy planets and exhibits a remarkable diversity of crystalline ice phases that govern their internal structure and evolution. Recent discoveries of plastic and superionic ice before melting have greatly extended the stability range of solid ice, yet experimental observations remain limited. In particular, the relationships, physical properties, and transition mechanisms of these phases remain largely unexplored. Here, we combine in situ synchrotron X-ray diffraction and ab initio molecular dynamics at 8–80 GPa and 500–900 K to link hydrogen-bond dynamics to lattice response. Ice at 500 K follows the same ice-VII sequence as at 300 K, but at 700 K it transforms into a plastic phase at 13.7–37.4 GPa via rapid molecular reorientation. At 900 K, fast proton diffusion stabilizes a water-like superionic state at 14.3–25 GPa, which converts to an ice-like superionic state at 31.2 GPa. Disruption of hydrogen bonds and enhanced molecular freedom markedly increase compressibility from ice-VII to plastic and superionic ice, with plastic ice-VII at 13.7–37.4 GPa and 700 K even denser than ice-VII at 500 K. These findings redefine the high-pressure ice phase diagram and reveal how microscopic mechanisms govern both phase transitions and the exceptional compressibility of plastic and superionic ice, shedding light on the interiors and thermal evolution of icy planets.