HIGH-PRESSURE PHASE TRANSITIONS IN ICE OBSERVED BY REFRACTIVE INDEX AND X-RAY DIFFRACTION

We observe a phase transition in ice VII at 45 GPa by refractive index and X-ray diffraction measurements at high pressures. The presence of such a phase transition has been suggested in recent papers reporting discoveries of a triple point at 44 to 47 GPa along the melting curve of ice VII. Our refractive index measurements up to 120 GPa at room temperature show abnormal slope increases at 45 GPa and 90 GPa. The refractive index slope increase at 45 GPa may explain the abnormal longitudinal frequency shift in previously reported Brillouin scattering experiments. X-ray diffraction measurements to 54 GPa indicate that the abnormal increase in refractive index at 45 GPa is due to a cubic-to-tetragonal distortion structure change. Our results when combined with the melt studies suggest that the phase boundary between cubic ice VII and the tetragonal phase is nearly parallel to the temperature axis.