Amorphization in quenched ice VIII: A first-principles study

Ice VIII is a high-density form of H₂O that amorphizes upon heating after being decompressed to 0 kbar. Here we investigate by first principles the structural and vibrational properties of ice VIII under decompression. We have reproduced the peculiar nonlinear behavior of some stretching and translational modes under decompression and relate this behavior to the eminent collapse of the hydrogen-bond networks. We also find that the transverse acoustic phonon branches almost collapse and are nearly unstable at 0 kbar. This is the sign of imminent amorphization, similar to that uncovered in α-quartz under pressure near the amorphous transition. By means of quasi-harmonic free energy calculations we also investigate its thermal equation of state and show, for the first time, the effect of zero point motion and temperature on its structure. The level of agreement between theoretical and experimental results is unprecedented in this class of materials. This is crucial to clarifying the relationship between amorphization and acoustic phonon collapse.