A model for the phase transition from ice VII to ice VIII: rigorous results

A statistical mechanical model for the phase transition from disordered ice VII to ordered ice VIII is introduced. The model includes steric repulsions between neighboring atoms on different hydrogen-bonded structures. These repulsions can be reduced by small displacements of neighboring molecules, hydrogen-bond distortion energies being associated with such displacements. A phase transition is proved to occur as the temperature is lowered, and molecules in the low temperature phase are shown to be ordered in the same way as in ice VIII.