The transition to a symmetric ice has been observed at about 60 GPa by infrared absorption spectroscopy. The hydrogen-bond symmetrization, which is accompanied by a subsequent deformation of the hydrogen-bond potential from a double to single minimum shape, is well recognized as a turn in the pressure dependence of the OH stretching frequencies from a decrease to increase at the transition point. In the intermediate state, the stretching vibrational modes is shown to form a continuous energy band as a result of quantum-mechanical manybody effects. This effect leads to an interference phenomenum between the proton-related vibrational states.
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