On the coexistence of cubic and hexagonal ice between 160 and 240 K

The observation that the cubic ---> hexagonal ice transformation begins at 160 K and ends at 240 K, and that the two coexist over this temperature range, has been considered in terms of the grain-boundary energy and the strain energies of the coherent and incoherent interphase boundaries. It is shown, qualitatively, that the broad transformation range and coexistence of the two solids result from the bulk and interfacial energy changes as hexagonal ice crystals grow in the bulk of cubic ice. Conditions for the triple point of very small crystals of cubic and hexagonal ices sharing a common vapour are derived and it is shown that depending on their relative sizes, the triple point of small crystals may be below the triple point of the bulk ices, or above it. According to an extension of the Gibbs-Thomson effect to solid-solid-phase transformations, small cubic ice crystals would transform to hexagonal ice at a lower temperature than large crystals. Thus, a distribution in the nanometre-sized particles of cubic ice formed by fracture of the films formed on a substrate, or deposited as clusters, may lead to a broad temperature range for the phase transformation. Recrystallization, rearrangements of hydrogen bonds at the interface, and deformation of the dispersed and matrix phases, or a change in the morphology of very small crystals with time would make the transformation irreversible and kinetically controlled in appearance.