Structural Mechanisms of the Ih-II and II → Ic Transitions between the Crystalline Phases of Aqueous Ice

Structural mechanisms are proposed for experimentally observed phase transitions between crystalline modifications of aqueous ice, Ih and II, as well as II and Ic. It is known that the Ih-II transition occurs with the conservation of large structural units (hexagonal channels) common for these ices. It is shown that the Ih -> II transition may occur with the conservation of 5/6 of all hydrogen bonds in crystal, including all hydrogen bonds in the retained channels (3/4 of the total number of bonds in crystal) and 1/3 of the bonds between these channels (1/12 of the total number). The transformation of other hydrogen bonds between the retained channels leads to the occurrence of proton order in ice II. A structural mechanism is proposed to explain the transformation of single crystals of ice Ih either into single crystals of ice II or into crystalline twins of ice II with c axes rotated by 180A degrees with respect to each other, which is often observed at the Ih -> II transition. It is established that up to 7/12 of all hydrogen bonds are retained at the irreversible cooperative II -> Ic transition.