Very high-resolution inelastic x-ray scattering is employed to determine the dynamic response of the high-density crystalline water phases ice IX and ice XII in the $Q=2.0\mathrm{}–15.5{\mathrm{nm}}^{-1}$ momentum transfer range. The spectra in the first and second Brillouin zone of the crystalline samples possess distinctive $(Q,\omega )$ dependencies. In particular, the response of ice IX gives evidence of the presence of an ensemble of excitations already at lowest Q. Similar features are found in the high-density amorphous water phase (HDA). The acoustic mode in HDA is well defined and little damped up to $Q=9\mathrm{}{\mathrm{nm}}^{-1}.$ Its slope is close to that of ice IX. Despite the apparent structural disorder of HDA its dynamic response appears crystal-like with a close resemblance to ice IX pointing at an intriguing high degree of short-range order.

• Received 12 August 2003

DOI:https://doi.org/10.1103/PhysRevB.69.024204