Vibrational and relaxational properties of crystalline and amorphous ices

Pure water forms 15 crystalline ices at different temperatures and pressures, and its solutions containing small molecules form three crystallographically distinct clathrates. Its vapours deposited on a substrate at T < 100 K produce a porous amorphous solid and pure water vitrifies (Tg = 136 K) when hyperquenched in micron-size droplets. At a temperature below 140 K, hexagonal and cubic ice collapse when pressure exceeds ∼1 GPa to a ∼30% denser amorphous solid, which on heating at ambient pressure transforms to an amorphous solid with density similar to that of hexagonal ice. In this essay, we describe (i) the thermal conductivity of the ices and clathrates and the thermal conductivity and heat capacity of waterʹs amorphous solids, their thermodynamic paths and their transformations, and (ii) the dielectric relaxation time of ultraviscous water formed on heating the amorphous solids. We also describe the characteristics of pressure collapse and subsequent amorphization of hexagonal and cubic ices that occurs over a period of several days according to a stretched exponential kinetics and a pressure-, and temperature-dependent rate constant. This process is attributed to the production of lattice faults during deformation of the ice and the consequent distribution of the Born instability pressures. This ultimately produces a kinetically unstable high-energy amorphs in the same manner as random deformation of crystals produces kinetically unstable high-energy amorphs, with density and properties depending upon their temperature–pressure–time history. On heating at 1 GPa pressure, the pressure-amorphized solid relaxes to a lower energy state, becoming ultraviscous water at 140 K. But on heating at ambient pressure, it irreversibly transforms slowly to a low-density amorph that differs from glassy water and vapour-deposited amorphous solid.