Static and dynamic critical behavior at orientational phase transitions in molecular crystals

Critical behavior at orientational order-disorder transitions in plastic molecular crystals is discussed. Primary emphasis is placed on an analysis of the coherent elastic and inelastic neutron cross sections. It is shown that in molecular crystals, where the multipolar operators describing the orientational interaction between pairs of molecules have several components, it is necessary to take into account the full vector character of the order parameter when calculating static and dynamic scattering laws. In the case of the I-II transition in C${\mathrm{D}}_4$, the recent experimental neutron-scattering results of Press et al. are well understood on the basis of a molecular-field-type analysis of the James and Keenan Hamiltonian.