The high frequency dynamics of low- (LDA) and high-density amorphous-ice (HDA) and of cubic ice (I_c) has been measured by inelastic X-ray Scattering (IXS) in the 1-15 nm^{-1} momentum transfer (Q) range. Sharp phonon-like excitations are observed, and the longitudinal acoustic branch is identified up to Q = 8nm^{-1} in LDA and I_c and up to 5nm^{-1} in HDA. The narrow width of these excitations is in sharp contrast with 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 sound-like excitations which is assimilated to low local disorder.
@article{arxiv.cond-mat/0002340,
title = {Crystal-like high frequency phonons in the amorphous phases of solid water},
author = {H. Schober and M. M. Koza and A. Toelle and C. Masciovecchio and F. Sette and F. Fujara},
journal= {arXiv preprint arXiv:cond-mat/0002340},
year = {2009}
}