Amorphous polymorphism in ice investigated by inelastic neutron scattering

High-density I-hda and low-density I-lda amorphous ice phases have been investigated by inelastic neutron scattering (INS) with emphasis on the energy window from 0.5 to 20 meV. At variance with earlier measurements 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 harmonic. I-hda converts with a strongly temperature-dependent rate towards I-lda ice. We have investigated in detail the time evolution of both the static and dynamic response functions at several temperatures. Elastic small-angle signals indicate the presence of strong heterogeneities at the early stages of the conversion process. At least two different time scales are present in the transition. The structural changes 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 than both I-hda and crystalline cubic ice I-c. No signs of a fast ps process related to melting can be detected in the dynamics. (C) 1998 Elsevier Science B.V. All rights reserved.