On the role of cubic structure in ice nucleation

Abstract

To clarify the formation mechanism of snow polycrystals the possibility of formation of a cubic ice embryo is discussed on the basis of the homogeneous nucleation theory for supercooled water formed from ambient water molecules in the phase of supersaturated vapour. In this connection, attention is paid to a finding from a model of broken hydrogen bonds that the plane {111} of a cubic ice crystal has a smaller specific interfacial energy than each of the {0001} or {10ovbar|10} planes of a hexagonal ice crystal. Hence, it follows that a critical cubic embryo has a smaller activation energy than a critical hexagonal embryo below a critical temperature; namely, Ostwald's step rule (Stufenregel) holds for a change from cubic ice to hexagonal ice below a critical temperature. This discussion is reinforced by examining, from the viewpoint of this step rule, the observed misorientation of the c-axis of natural snow polycrystals and the results of experiments using frozen water droplets.