High-pressure elastic properties of the VI and VII phase of ice in dense H2O and D2O

High-pressure Brillouin spectroscopy has been used to determine acoustic velocities in any direction, refractive index, adiabatic elastic constants, and adiabatic bulk modulus of H2O and D2O ices at 300 K and at pressures between 1.05 and 2.1 GPa for tetragonal ice VI and above pressures up to 8 GPa for cubic ice VII. Typical values of elastic constants for H2O ice VI are C-11=32.8, C-12=11.8, C-13=14.7, C-33=27.8, C-44=6.3, and C-66=5.9 GPa at the pressure of 1.23 GPa. Refractive index and elastic constants show no distinction between H2O and D2O over the observed pressure ranges. At ice VI-->VII transition at 2.1 GPa, acoustic velocities, refractive index, and elastic constants present drastic changes, but the adiabatic bulk modulus shows almost continuous change which is most likely to relate the common features of the self-clathrate structure and the potential function in both VI and VII ices.