How Cubic Can Ice I be?

Barbara Wyslouzil, A Amaya, V Modak, H Pathak, M Bogan, H Laksmono, C Stan, D Loh, A Nilsson, J Sellberg, R Sierra, S Boutet, G Williams, M Messerschmidt

Recent experimental1,2 and theoretical3,4 work both suggest that ice I produceded by freezing supercooled water forms faulty cubic ice, also referred to as stacking disordered ice Isd, rather than purely cubic ice Ic. The ratio of cubic to hexagonal sequences present in Isd depends on the temperature at which freezing occurred2 but is generally less than 1:1. Simulations, in contrast suggest that at short times this ratio should be closer to 2:1, especially in the confined conditions characteristic of nanopores3 or nanodroplets.4 We recently conducted in situ Wide Angle X-ray Scattering experiments on nanodroplets with radii of ~10 nm, formed in a supersonic nozzle, that froze at temperatures close to 224 K. Fitting the strongest diffraction peak, while accounting for the particles size distribution and finite width of the particle stream, suggests the ratio of cubic to hexagonal ice can be as high as 4:1. On the short timescales (10 – 100 µs) of the experiment we do not observe any transformation from Isd to the more stable hexagonal ice, Ih.

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