Amorphization of ice by collapse under pressure, vibrational properties, and ultraviscous water at 1 GPa

Abstract

When subjected to a uniaxial pressure of 0.7 to 1.5GPa, structures of hexagonal and cubic ices at a temperature below 140K collapse and the ordered arrangement of molecules is lost. Another well-known (tetrahedrally bonded and open structure) crystal, SiO, also collapses and become amorphous but at 25–30GPa and 300K. This is known as pressure-induced amorphization of crystals. Here we report, (i) how the vibrational properties, e.g., molar volume V, limiting high frequency permittivity ε _∞, ultrasonic sound velocity, and thermal conductivity κ change during the pressure-amorphization, and (ii) how the amorphized ice relaxes to a lower energy state on heating to 140K, and becomes ultraviscous water of dielectric relaxation time of ~1κs at 1κGPa pressure. As the extent of amorphization increases on increasing the pressure to 1.5κGPa, V and κ irreversibly decrease and ε _∞ and the ultrasound velocity increase. Amorphization begins at a lower pressure for micron-size ice crystals than for larger crystals. It also begins at a lower pressure at high temperatures of ice. At a fixed pressure and temperature, ice continues to amorphize up to a period of several days according to a stretched exponential kinetics and a pressure– and temperature-dependent rate constant. It is proposed that lattice faults, which are also produced during pressure-deformation of ice cause a distribution of the Born instability pressures, and the amorphization process becomes pressure– and time-dependent. Pressure-induced amorphization of ice at 77K produces kinetically unstable high energy amorphs in the same manner as mechanical deformation of other crystals produces kinetically unstable, high energy amorphs which, on heating, become an ultraviscous liquid. But, in contrast, the ice amorphs are denser than the parent ices, and bulkier than ice VI the stable phase, and ice XII the metastable phase.