Unified rotational dynamics of molecular crystals with orientational phase transition

A unified theory for the rotational dynamics of molecular crystals with orientational phase transitions is given. As basic secular variables one takes symmetry adapted functions, which describe the molecular orientations, and the angular momenta of the molecules. Using Mori’s projection operator technique, one obtains a coupled set of two dynamic matrix equations for the corresponding relaxation functions. The coupling is proportional to the order parameter and accounts for the reactive coefficients. The corresponding collective excitations are librons. The damping of librons is described by two transport coefficients that account for orientational relaxation and angular momentum relaxation, respectively. By approaching the phase transition, both matrix equations decouple, the orientational relaxation describes the critical dynamics. In the disordered phase, this equation describes collective hindered rotations. In the same framework one describes phases of partial order as CD4 II. Comparison is made with neutron scattering experiments.