The equilibrium low-temperature structure of icea)

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 Å in dimension.