A MODEL FOR THE PHASE-TRANSITION FROM ICE-VII TO ICE-VIII - RIGOROUS RESULTS

A statistical mechanical model for the phase transition from disordere d ice VII to ordered ice VIII is introduced. The model includes steric repulsions between neighboring atoms on different hydrogen-bonded str uctures. These repulsions can be reduced by small displacements of nei ghboring 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 ar e shown to be ordered in the same way as in ice VIII.