Pressure dependence of Raman linewidths in ices VII and VIII

Vibrational modes of D₂O ices VII and VIII have been measured by Raman scattering in a diamond-anvil cell up to 25 GPa, between 80 and 300 K. The linewidths and frequency variation with pressure and temperature of νT, a low-frequency O-O stretching phonon mode, and of ν₁, the high-frequency totally symmetric O-H stretching vibron, are analyzed, and their behavior at the phase transition from ice VIII to ice VII is explained. The discontinuities in the linewidth and frequency of the ν_τ phonon, which occur when going from ordered ice VIII to site-disordered ice VII are shown to originate from the lifting of wave-vector selection rules. The temperature dependence of the ν₁ vibron linewidth shows its lifetime to be limited by two-phonon decay processes which involve a low-frequency compressional mode of the oxygen sublattice, in H₂O as well as in D₂O. This process quantitatively accounts for the decrease of the ν₁ linewidth with increasing pressure up to 15 GPa. By contrast with the behavior of the νT mode, no discontinuity occurs in the ν₁ vibron linewidth in D₂O and H₂O at the VIII-VII phase transition so that its lifetime is not affected by site disorder, as had previously been proposed. The increase of the vibron linewidth from 15 to 30 GPa is shown not to be related to proton or deuteron tunneling and is tentatively assigned to its decay into bending modes.