Partially Diffusive Structural Mechanism of the Transitions between Ices III and V

It is shown that the structures of ices III and V can be divided into nearly identical chiral layers. In achiral ice V, the right-handed enantiomorphic layers alternate with the left-handed ones. Chiral ice III consists of only right-handed or only left-handed layers. A structural mechanism preserving these layers is proposed for the transitions between ices III and V. This mechanism is partially diffusive. The possibility for the proposed structural mechanism to provide the experimentally observed high transition rate is discussed. Small structural units of crystal ices are shown to change regularly with increasing pressure in the sequence III, V, and VI. Such small structures formed by water molecules can be present in liquid near the crystallization curve.