Compression of H2 O ice to 126 GPa and implications for hydrogen-bond symmetrization: Synchrotron x-ray diffraction measurements and density-functional calculations

We examined the volume compression and phase transformations of H2 O ice by a combination of synchrotron x-ray diffraction measurements and density-functional calculations up to 126 GPa. The experimental data demonstrate that ice changes its compressibility at 40 and 60 GPa at room temperature, corresponding to the phase transitions from ice VII to dynamically disordered ice VII and subsequently to dynamically disordered ice X. The intermediate phase, dynamically disordered ice VII, is highly compressible, possibly due to quantum effects of protons. In contrast, dynamically disordered ice X and ice X show much smaller compressibility.