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 nu(T), a low-frequency O-O stretching phonon mode, and of nu(1), 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 nu(tau) 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 nu(1) 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 nu(1) linewidth with increasing pressure up to 15 GPa. By contrast with the behavior of the nu(T) mode, no discontinuity occurs in the nu(1) 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.