Evidence for Transition of Metastable Ice to Hexagonal Ice in Bulk Solutions at Relatively High Temperatures

There have been many reports that a metastable form of ice can exist in the atmosphere and that it transitions rapidly to stable, hexagonal ice at temperatures above about 200 K. Although this often-called cubic ice has also been created in laboratories over the years, we present here a method for the simple formation of this metastable phase in the laboratory, at one atmosphere, in relatively large volumes and at higher temperatures than previously reported. Evidence for this phase is found during the monitoring of optical transmission through bulk samples of quenched aqueous solutions. In our experiments, frozen samples were created by quenching 0.2 ml aqueous volumes in glass tubes to 195 K which are then warmed to and held at 267 K. Results show an unusual drop in optical transmission occurring in the first few minutes. Such a change is best explained by the transition of a metastable phase to hexagonal ice, rather than by any freeze concentration effects. In the minutes following nucleation and freezing of the sample, the average size of the poly-crystals forming the frozen matrix would be typically expected to increase due to recrystallization, causing lower side and back-scatter of the traversing light and so a subsequent increased optical transmission. However, the drop in transmission we see with samples nucleated at such a low temperature cannot be explained by recrystallisation but rather by a re-ordering of the ice, the grain boundaries, and the interstitial water.