A new description of the coupling between translational and rotational degrees of freedom in plastic crystals is presented. Using the lattice-gas variable { t i } for translations and the q -state Potts variable { r i } for rotations a tetralinear coupling t i r i t j r j is introduced between nearest neighbours. Moreover, ionic crystals and molecular crystals are found to be described by different Hamiltonians. Random fields which couple solely to the { r i } are involved in the molecular case. The corresponding phase diagrams are obtained. The results are discussed with respect to experiments.
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Galam et al. (1989) studied this question.
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