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-VIIt 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.

Automated summary

X-ray diffraction and Raman spectroscopy were used to study the structures of ice under static compression and heat treatment. The transition from cubic ice-VII to a tetragonal structure, ice-VIIt, was observed at a pressure of 5.1 +/- 0.5 GPa. At a pressure of 30.9 +/- 3 GPa, a transition to H-bond symmetrization was observed. The experimental observations were supported by simulated Raman spectra from density-functional theory calculations.