Inelastic incoherent neutron scattering study of the pressure dependence of ice VII and VIII

Incoherent inelastic neutron-scattering spectra of ice VII and VIII have been obtained at an in situ pressure of 2.2 ± 0.1 GPa on the TFXA spectrometer at the pulsed neutron source, ISIS. Comparisons with recovered phases (at ambient pressure) show large shifts of the phonon modes. For instance, the optic peak for ice VIII is shifted from 25 to 32 meV (dω/ω = 0.20 ± 0.01). The coefficient of increase of the frequency of the mode with pressure, dω/dp, is 2.3 meV/GPa (or 18.5 cm⁻¹/GPa). In ice VII, the proton-disordered form of ice VIII, the spectra, measured under the same pressure, show that: firstly, the sharp features of the librational band in the region 50–100 meV disappear, indicating that the protons are indeed disordered as is found in other proton-disordered forms of ice; secondly, a weak higher-energy optic mode at 49.5 meV appears due to the presence of the second proton configuration as seen in other proton-disordered ice Ic.