Infrared observation of the phase transitions of ice at low temperatures and pressures up to 50 GPa and the metastability of low-temperature ice VII

The phase transitions of H₂O ice were investigated by infrared-absorption measurements in the P, T ranges of 0.2--50 GPa and 20--298 K. Under compression at temperatures below 150 K, ices II and VI were found to transform directly into orientationally disordered low-temperature ice VII due to their mechanical instability, with the transition pressure increasing approximately from 2.8 GPa at 120 K to 7.9 GPa at 40 K. The infrared spectral features of low-temperature ice VII are identical to those of ice VII at ambient temperature. Under isobaric heating in the range of 100 to 180 K at pressures below 30 GPa, low-temperature ice VII transforms into fully ordered antiferroelectric ice VIII; during this transition, the water molecules reorient by rotation. The kinetic phase boundary between low-temperature ice VII and ice VIII at temperatures below 100 K was determined to be a vertical line at about 30 GPa. The low-temperature ice VII to ice VIII transition is interpreted here as occurring as a result of thermally activated rotational disordering below 30 GPa and as a result of rotational tunneling coupled with bending vibrations above 30 GPa; the orientational disordering of ice VII can be explained accordingly.