Pressure-driven symmetry transitions in dense H2O ice

X-ray diffraction and Raman spectroscopy of H₂O (ice) structures are measured under static compression in combination with grain normalizing heat treatment via direct laser heating. We report the transition from cubic ice-VII to a structure of tetragonal symmetry, ice-VII_t at 5.1 ±0.5 GPa . This is succeeded by the H-bond symmetrization transition occurring at a pressure of 30.9 ±3 GPa . Both experimental observations are supported by simulated Raman spectra from density-functional theory quantum calculations. The transition to H-bond symmetrization is evidenced by the reversible emergence of its characteristic Raman mode and a 2.5-fold increase in bulk modulus, implying a significant increase in bonding strength.