Phase transition in KOH-doped hexagonal ice

A few crystals exist which have residual entropy1,2. The most notable of these is hexagonal ice, Ih, the ordinary form of solid H2O (ref. 3). The structural interpretation of the residual entropy proposed by Pauling and widely accepted ascribes it to positional disorder of the protons in the ice conditions4. As such a disordered arrangement of the constituent atom cannot be an equilibrium structure of the crystal at the lowest temperature, there has been great interest in the search for a possible ordering phenomenon in ice crystals. We report here a calorimetric experiment which shows that a first order phase transition takes place in KOH-doped ice crystals and that the transition removes most of the residual entropy of the ice crystal.