The six-fold symmetry of snowflakes is a never-ceasing source of marvel and wonder. For bulk ice, six is also the number of choice — the hexagonal crystal form provides the highest stability at ambient pressure. But nanoscopic ice breaks that pattern, as Laura Lupi and colleagues report. In numerical simulations, they found that for crystallites of up to at least 100,000 molecules, ice with randomly mixed layers of cubic and hexagonal symmetry is the most stable form.
The thermodynamic preference for such stacking disorder is important. Classical nucleation theory assumes that the initially formed 'ice embryo' consists of hexagonal ice. For stacking-disordered crystallites, however, nucleation rates can be three orders of magnitude faster, Lupi et al. found. Such accelerated nucleation, in which the crystallites continuously cross over to the hexagonal configuration, should occur in particular in conditions relevant to clouds. And as freezing typically initiates precipitation, the effect of stacking disorder in ice nucleation ought to be taken into account for accurate weather and climate forecasts.
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Trabesinger, A. Rain before six. Nature Phys 13, 1147 (2017). https://doi.org/10.1038/nphys4334