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 ω² 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.