Comparative Analysis of Hydrogen Bond Vibrations in Ice VIII and VII

According to a report, the existence of two characteristic hydrogen bond (H-bond) peaks in the translational band of ice does not occur in high-pressure ice VIII. To test this, a comparative analysis of ice VII and VIII was conducted. We demonstrated that the two intrinsic H-bond vibration modes found in ice Ic are also fundamental in these two phases. However, the peaks are merged with non-H-bond modes due to a large red shift of the H-bonds. A 64-molecule supercell was constructed to mimic the hydrogen-disordered structure of ice VII. Using first-principles density functional theory, the phonon density of states and vibrational normal modes of ice VII and VIII were analyzed. The primitive cell of ice VIII contains only four molecules, resulting in a relatively small number of normal modes. This simplicity facilitates the analysis of ice VII, which is structurally similar to ice VIII. Interestingly, the characteristic ice modes reported by Whale et al., such as “rigid network modes” in the translational band and “isolated O–H” vibrational modes in the stretching band, were also found in this work. Examples were illustrated in detail, and specifically we attributed the isolated O–H vibrations to lattice deformation of the local tetrahedral structure.