The equilibrium low‐temperature structure of ice

Following the discovery of a first‐order phase transition in annealed KOH‐doped ice at 72 K, which was identified as the order–disorder transition associated with the proton positions, a structural study has now been made. Powder neutron diffraction measurements on KOD‐doped D2O above and below the phase transition, together with x‐ray diffraction measurements, reveal a partial ordering of the hydrogen atoms at low temperatures. The equilibrium structure of ice at low tempertures has orthorhombic symmetry, space group Cmc21, with the same lattice as the high‐temperature Ih (P63/mmc) modification. The structure is polar and there is evidence that the ordered domains are less than about 40 A in dimension.