Mean field approach to ferroelastic transitions in molecular crystals

A mean field approach to the dynamics of structural phase transitions in molecular crystals is presented. The approach is based on a description of the rotational and translational molecular motions, and the coupling between them, in terms of generalized susceptibilities. Two models for the orientational susceptibility are used. One is a classical description in terms of two-dimensional rotors, the other a two-dimensional anharmonic oscillator model. The specific example considered is sym-triazine. In this crystal molecules experience a very strong orienting field which restricts the molecular rotational motion to libration. The coupling between the molecular rotations and translations is shown to lead to a softening of acoustic phonons. This softening has considerable anisotropy in reciprocal space. An approximate solution for the high temperature phase is shown to be in good agreement with experiments.