Raman Spectroscopy of Dense H2O and the Transition to Symmetric Hydrogen Bonds

High-pressure Raman measurements of H{sub 2}O ice using synthetic diamond anvils reveal major changes associated with the transition to the nonmolecular, symmetric hydrogen-bonded state. At 60thinspthinspGPa 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 single peak appears and becomes the dominant feature in the spectrum in the megabar 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. {copyright} {ital 1999} {ital The American Physical Society}