THE STRUCTURE OF A NEW PHASE OF ICE

Ice has eleven known crystalline phases (Fig. 1), in which the water m olecules are linked through hydrogen bonds into tetrahedral frameworks (1). This uncommonly large number of different solid phases attests to the structural versatility of the water molecule, Here we report the identification of a new, twelfth phase of crystalline ice in the press ure range 0.2-0.6 GPa. The topology of this phase is unlike that of an y of the known phases, and contains a mixture of five-and seven-member ed rings of water molecules. It has a density similar to that of ice I V, which also occurs in this pressure range within the stability regio n of ice V. Both phases are likely to be metastable with respect to th e less-dense ice V. This region of the water phase diagram thus provid es a potential model system for experimental and theoretical studies o f metastability.