Pressure-driven symmetry transitions in dense H2O ice

X-ray diffraction and Raman spectroscopy of ${\mathrm{H}}_{2}\mathrm{O}$ (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-${\mathrm{VII}}_{\text{t}}$ at $5.1\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. This is succeeded by the H-bond symmetrization transition occurring at a pressure of $30.9\ifmmode\pm\else\textpm\fi{}3\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. 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.