Infrared Absorption Study of Fermi Resonance and Hydrogen-Bond Symmetrization of Ice Up to 141 GPa

Infrared absorption spectra of H₂O and D₂O ices have been measured up to 141 GPa at room temperature. A sequence of pressure-tuned Fermi resonances was observed between the stretching modes and the combinations of rotational and bending modes below 50 GPa as reported previously. At 53 �} 2 GPa for H2O ice and 68 �} 3 GPa for D2O ice, one rotational mode indicted a discrete jump in frequency. This can be attributed to a vibrational mode conversion from the molecular rotation in ice VII to a distortion of hydrogen tetrahedron in disordered ice VII with translational proton tunneling. Further transition into proton-disordered ice X was inferred from the observed peak shape change in disordered ice VII. The pressures were roughly estimated to be 60 GPa for H₂O and 78 GPa for D₂O. Proton-ordered symmetric ice X was expected to be realized above 100 GPa in H₂O ice and above 120 GPa in D2O ice.