Crystal-like high frequency phonons in the amorphous phases of solid water

The high frequency dynamics of low-density (LDA) and high-density (HDA) amo rphous ice and of cubic ice (I,) has been measured by inelastic x-ray scatt ering in the 1-15 nm(-1) momentum transfer (Q) range. Sharp phononlike exci tations are observed, and the longitudinal acoustic branch is identified up to Q = 8 nm(-1) in LDA and I-c and up to 5 nm(-1) in HDA. The narrow width of these excitations is in sharp contrast to the broad features observed i n all amorphous systems studied so far. The "crystal-like" behavior of amor phous ices, therefore, implies a considerable reduction in the number of de cay channels available to soundlike excitations which is interpreted as a s ign of low local disorder.