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On heating the recovered high-pressure phase ice VIII at ambient pressure, the transitions to high-density amorphous (hda) and low-density amorphous ices, to cubic ice Ic, and finally to hexagonal ice Ih were observed using real-time neutron diffraction. A new form of ice, "defective ice VI", was prepared for the fist time by pressurizing ice Ih to 30 kbar at liquid nitrogen temperature. Vibrational spectra of ices Ih, hda, VI, VI′, and VIII prepared by different thermobaric treatments were studied by inelastic neutron scattering. The translational band of the intermolecular part of ice VI′ vibrational spectrum was shown to be similar to that of hda ice, whereas its librational band was similar to that of ice VI. We conclude that ices VI, VI′, and hda have essentially the same atomic correlations and force constants, though with the different degree of disorder in the case of ices VI′ and hda.

Vibrational spectra of high-pressure phase ice VIII have been studied by inelastic neutron scattering over a wide energy transfer range, including external intermolecular and internal intramolecular modes up to 500 meV. It was clearly shown that the stretching intramolecular modes in ice VIII exhibit a narrow peak at an energy of 426 meV ͑or 3436 cm Ϫ1 ͒, which is higher than observed in all other known ice phases. The width of the peak ͑ϳ20 meV or 161 cm Ϫ1 ͒ is much narrower than for other forms of ice ͑e.g., ϳ40 meV or 323 cm Ϫ1 for ice Ih͒, indicating a weak coupling between lattice and intramolecular stretching modes.

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