In Situ Observations of a High-Pressure Phase of H 2 O Ice

A previously unknown solid phase of H 2 O has been identified by its peculiar growth patterns, distinct pressure-temperature melting relations, and vibrational Raman spectra. Morphologies of ice crystals and their pressure-temperature melting relations were directly observed in a hydrothermal diamond-anvil cell for H 2 O bulk densities between 1203 and 1257 kilograms per cubic meter at temperatures between –10° and 50°C. Under these conditions, four different ice forms were observed to melt: two stable phases, ice V and ice VI, and two metastable phases, ice IV and the new ice phase. The Raman spectra and crystal morphology are consistent with a disordered anisotropic structure with some similarities to ice VI.