Reorientations and random fields in plastic crystals

A simple Ising-like model is presented to describe the phase diagram of plastic crystals. In addition to a bilinear coupling between translations and rotations a reorientation-translation coupling is introduced via random fields which couple solely to rotational degrees of freedom. The physical basis of the reorientation-translation coupling is discussed using the rotational state hypothesis. The present approach is found to be consistent with some recent neutron experiments.