High-pressure Raman measurements of ${\mathrm{H}}_2\mathrm{O}$ ice using synthetic diamond anvils reveal major changes associated with the transition to the nonmolecular, symmetric 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 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.

• Received 8 December 1998

DOI:https://doi.org/10.1103/PhysRevLett.83.1998