RECONSTRUCTION OF ICE CRYSTAL-SURFACES AT LOW-TEMPERATURES

It is generally accepted that the faces of ice crystals in equilibrium with the vapour undergo reconstruction to a quasi-liquid transition film at temperatures above about -20-degrees-C, though experimental evidence is equivocal. At much lower temperatures it appears that the free energy can be reduced by a less drastic surface reconstruction involving the orientation of surface dipoles to a superlattice. Calculations suggest that this superlattice should be energetically preferred on (0001) basal faces below about 30 K and on (0110BAR) prism faces below about 70 K. On (0001) faces, twin boundaries have very low energy and may exist in profusion. The rate of such a reconstruction is probably slow because of the low temperature involved, though molecular interchange with the vapour may facilitate it. It is possible that this structure could be observed by electron diffraction or by fine-focus infrared spectroscopy. A more radical reconstruction is also discussed, but considered to be energetically unlikely.