Symmetry properties, normal modes, and free energy of orientationally disordered crystals

Symmetry-adapted functions are the appropriate coordinates for the description of orientational disorder in crystals. Here this approach is extended and orientational fluctuations are classified according to the irreducible representations of the space group of the crystal. As a counterpart to the theory of lattice vibrations, normal-mode coordinates are introduced for orientational fluctuations. The coupling of translational and rotational motion is studied. The nonequilibrium free energy for molecular crystals with translation-rotation coupling is derived. The results of the present work allow a systematic description of the symmetry reduction at phase transitions which involve orientational degrees of freedom.