PHONON-DISPERSION IN POLYCRYSTALLINE ICE - IMPLICATIONS FOR THE COLLECTIVE BEHAVIOR OF LIQUID WATER

The steep phonon-dispersion curves found in computer-simulation studies on normal and super-cooled water which have been interpreted as evidence of the existence of ''fast-sound modes'' of kinetic origin are considered. Rom the analysis of the dynamical structure factor of polycrystalline ice it is shown that such features are present in the solid calculated within the harmonic approximation, and the anomalous steep increase in frequency is shown to be originated by optical phonon branches, which cross the acoustical ones at wave vectors below the zone center, and result in a pull-out up to a maximum frequency at Q(p)/2, which roughly coincides with that of the sharp translational edge.