Anisotropy in structural transitions between basal and prismatic faces of ice studied by molecular dynamics simulation

Molecular-scale structures of the basal and prismatic faces of an ice crystal are investigated using a molecular dynamics simulation. Simulations are carried out over a temperature range, from 170 to 250 K. The simulation results clearly indicate that structural transitions occur, at least twice, on both faces as the temperature increases to the melting point. The transitions are (1) a transition from a molecularly flat surface to a surface with admolecules, and (2) a transition from a surface with admolecules to a melted surface (i.e. surface melting). The temperatures of these structural transitions, however, are completely different in the case of basal and prismatic faces. These anisotropic structural transitions are explained by the difference between the energetic states of surfaces and between the molecular arrangements of these faces. Moreover, the anisotropy in surface melting between the faces, which agrees with the experimental results, is obtained. The anisotropic surface melting is discussed on the basis of the interface structures between the surface-melted layer and the underlying ice.