New transformations between crystalline and amorphous ice

AMONG the unusual physical properties of H₂O, its behaviour at low temperatures and high pressures is particularly anomalous and incompletely understood^1-3. Mishima et al.^1,2 have shown that crystalline ice-I_h transforms at 1 GPa and 77 K to a high-density amorphous (hda) form, which exhibits unusual thermal properties. With the exception of volume displacement measurements, however, no in situ studies of this high-pressure polymorphism have yet been reported³. Using high-pressure optical and spectro-scopic techniques, we have observed directly the ice-I_h -> hda-ice transformation in a diamond-anvil cell, and have examined the stability of the amorphous form as functions of pressure and temperature. We demonstrate that hda-ice transforms abruptly at 4 GPa and 77 K to a crystalline phase close in structure to orienta-tionally disordered ice-VII and to a more highly ordered (ice-VIII-like) structure at higher temperatures. This is the first time that an amorphous solid is observed to convert to a crystalline solid at low temperatures by compression alone. The results are in excellent agreement with recent theoretical predictions⁴ for H₂O and support the accuracy of recently developed interatomic potentials for its ordered and disordered hydrogen-bonded phases. Phase transitions of this type may be relevant on icy planetary satellites, and there may also be implications for the high-pressure behaviour of silica.