Raman spectroscopy of dense H2O and the transition to symmetric hydrogen bonds

High-pressure Raman measurements of H2O ice using synthetic diamond anvils reveal major changes associated with the transition to the nonmolecular, sy mmetric hydrogen-bonded state. At 60 GPa the strongly pressure-dependent O- H symmetric stretching mode disappears, and the translational modes exhibit frequency and damping anomalies. With further increase in pressure, a sing le peak appears and becomes the dominant feature in the spectrum in the meg abar range. The band is assigned to the predicted Raman-active O-O mode of the nonmolecular phase, consistent with the formation of cuprite-type ice X with static, symmetric hydrogen bonds.