Infrared absorption study of Fermi resonance and hydrogen-bond symmetrization of ice up to 141 GPa

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The infrared absorption spectra of H₂O and D₂O ices were measured with a diamond-anvil cell up to 141 GPa at 298 K. The high-pressure spectra corrected for diamond absorption provided the substantial absorption profile of ices over the whole pressure range measured. A sequence of pressure-induced Fermi resonance was observed between the softening stretch mode and the combinations of rotational and bending modes below 55 GPa as reported previously. One rotational mode showed an abrupt increase in the peak frequency at 58±3GPa in H₂O ice and 68±3GPa in D₂O ice. This can be attributed to a transition from ice VII to proton-tunneling-disordered ice VII. A further transition into proton-disordered ice X was inferred from the spectral change to occur roughly at 65--75 GPa for H₂O and 80--90 GPa for D₂O.