On the amorphization of hexagonal ice, the nature of water's low-density amorph, and the continuity of molecular kinetics in supercooled water. Invited Lecture

Abstract

Three studies are reported here: (i) dilatometric experiments on pressure amorphization of polycrystalline hexagonal ice (ice Ih) of different grain-size; (ii) interpretation of the stability of water-B against its transformation to water-A (Johari et al., Science, 1996, 273, 90) in terms of the time required for the liquid water's structure to go through all its configurations; and (iii) scrutiny of a conjecture that supercooled water's structure changes at ∽228 K. The first study showed that amorphization pressure decreases with decrease in the crystal grain-size. This indicates that either the stress concentrations at the grain boundaries and grain junctions are important in amorphizing ice Ih, or the conditions for the onset of the Born instability, which is presumed to amorphize ice Ih, are crystal-size-dependent, i.e. that the lattice dynamics and acoustic phonons become crystal-size-dependent, when the surface-to-volume ratio is very large. On the basis of the known (a) lack of a characteristic glass-softening feature for hda and (b) irreversible transformation of high-pressure ices to low-density amorph on heating at 1 bar, it is proposed that, instead of being a homogeneously random structure, high-density amorph may be a mixture of highly strained microcrystalline high-pressure phases of ice. The second study suggested that, in the configurational space, the temporal distance between the energy minimum for local O–H···O bonding in water-A is far from the energy minimum for local O–H···O bonding in water-B. An analogous situation was observed when supercooled liquid glycerol coexisted with millimetre-size glycerol crystals for many months without adding visibly to the crystal growth. The third study showed no evidence for the premise that molecular diffusivity and dielectric relaxation time would follow an Arrhenius kinetics at temperatures below 228 K. A description in terms of the Adam and Gibbs entropy theory led to unrealistic parameters.