Crystal-like High Frequency Phonons in the Amorphous Phases of Solid Water

The high frequency dynamics of low-density (LDA) and high-density (HDA) amorphous ice and of cubic ice ( Ic) has been measured by inelastic x-ray scattering in the 1--15nm^-1 momentum transfer ( Q) range. Sharp phononlike excitations are observed, and the longitudinal acoustic branch is identified up to Q0ex0ex=0ex0ex8nm^-1 in LDA and Ic and up to 5nm^-1 in HDA. The narrow width of these excitations is in sharp contrast to the broad features observed in all amorphous systems studied so far. The ``crystal-like'' behavior of amorphous ices, therefore, implies a considerable reduction in the number of decay channels available to soundlike excitations which is interpreted as a sign of low local disorder.