Crystalline inelastic response of high-density amorphous ice

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=2.0-15.5 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 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.