High-pressure x-ray diffraction and Raman spectroscopy of ice VIII

In situ high-pressure/low-temperature synchrotron x-ray diffraction and optical Raman spectroscopy were used to examine the structural properties, equation of state, and vibrational dynamics of ice VIII. The x-ray measurements show that the pressure-volume relations remain smooth up to 23 GPa at 80 K. Although there is no evidence for structural changes to at least 14 GPa, the unit-cell axial ratio $c∕a$ undergoes changes at 10–14 GPa. Raman measurements carried out at 80 K show that the $νTzA1g+νTx,yEg$ lattice modes for the Raman spectra of ice VIII in the lower-frequency regions $(50–800cm−1)$ disappear at around 10 GPa, and then a new peak of $∼150cm−1$ appears at 14 GPa. The combined data provide evidence for a transition beginning near 10 GPa. The results are consistent with recent synchrotron far-IR measurements and theoretical calculations. The decompressed phase recovered at ambient pressure transforms to low-density amorphous ice when heated to $∼125K$⁠.