The structure of a new phase of ice

Ice has eleven known crystalline phases (Fig. 1), in which the watermolecules are linked through hydrogen bonds into tetrahedralframeworks1. Thisuncommonly large number of different solid phases attests to thestructural versatility of the water molecule. Here we reportthe identification of a new, twelfth phase of crystalline ice inthe pressure range 0.2–0.6 GPa. Thetopology of this phase is unlike that of any of the knownphases, and contains a mixture of five- and seven-membered ringsof water molecules. It has a density similar to that of iceIV, which also occurs in this pressure range within thestability region of ice V. Both phases are likely to bemetastable with respect to the less-dense ice V. This region ofthe water phase diagram thus provides a potential model system forexperimental and theoretical studies ofmetastability. Figure 1 The phase diagram of ice.