AMORPHOUS POLYMORPHISM IN ICE INVESTIGATED BY INELASTIC NEUTRON-SCATTERING

High-density I-hda and low-density I-lda amorphous ice phases have bee n investigated by inelastic neutron scattering (INS) with emphasis on the energy window from 0.5 to 20 meV. At variance with earlier measure ments the spectra in the I-hda phase show a simple omega(2) behaviour in the acoustic region and the temperature dependence is found to be h armonic. I-hda converts with a strongly temperature-dependent rate tow ards I-lda ice. We have investigated in detail the time evolution of b oth the static and dynamic response functions at several temperatures. Elastic small-angle signals indicate the presence of strong heterogen eities at the early stages of the conversion process. At least two dif ferent time scales are present in the transition. The structural chang es are reflected in the frequency distribution. The first peak in the phonon density-of-states softens appreciably when going from I-hda to I-lda. In the acoustic region there is evidence that I-lda is softer t han both I-hda and crystalline cubic ice I-c. No signs of a fast ps pr ocess related to melting can be detected in the dynamics. (C) 1998 Els evier Science B.V. All rights reserved.