Crystallization of water in a dynamic diamond-anvil cell: Evidence for ice VII-like local order in supercompressed water

We report the observation of ice VII, directly crystallized from metastably supercompressed liquid water at pressure of up to 1.80.3em0exGPa, well within the stability field of ice VI. This result is achieved by making time-resolved measurements of pressure-induced crystallization using a unique instrument, a dynamic diamond anvil cell (dDAC), which permits the measurement of pressure/time-dependent phase transformation pathways. We are able to evaluate the interfacial free energy, and find that the value for supercompressed water (SW)/ice VII is smaller than that of SW/ice VI, indicating that the local order of the SW is more similar to ice VII than ice VI. This result is consistent with recent studies, which suggest that the local order of high density water is bcc-like, as in ice VII.