VARIATION OF INTERATOMIC DISTANCES IN ICE-VIII TO 10 GPA

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 VII I, as well as those of the internal atomic coordinates of oxygen and d euterium 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 struc tural 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 A1g vibron frequency in ice VIII, which is shown not to be caused by an increase of the covalently bonded O-D distance .