Molecular symmetry and translation-rotation coupling in orientationally disordered crystals

The theory of coupling between rotational and translational degrees of freedom in orientationally disordered crystals is studied in detail, with careful attention to the requirements of symmetry. An essential feature of this coupling is the presence or absence of a center of symmetry in the molecule or molecular ion, which determines the nature of the coupling to optic- and acoustic-phonon modes. The present analysis is relevant for the understanding of ferroelastic phase transitions, of incommensurate transitions in insulators, and of structural transitions and related properties in synthetic organic conductors.