The pressure dependence of the structural parameters of deuterated ice VIII has been studied by time-of-flight neutron powder diffraction up to 10 GPa. Variations of the a and c parameters of tetragonal ice VIII, as well as those of the internal atomic coordinates of oxygen and deuterium are reported and discussed. Isothermal equations of state for ice VIII between 0 and 300 K are established. A periodic Hartree-Fock calculation performed at the all-electron level is shown to reproduce the equation of state at 0 K and the pressure dependence of the structural parameters, including the unexpectedly small variation of the O-D bond length up to 10 GPa. It also exactly accounts for the decrease under pressure of the ${\mathit{A}}_{1\mathit{g}}$ vibron frequency in ice VIII, which is shown not to be caused by an increase of the covalently bonded O-D distance.

• Received 6 December 1993

DOI:https://doi.org/10.1103/PhysRevB.49.12540