ON THE NATURE OF THE ORIENTATIONAL TRANSITION IN CRYSTALLINE A-TCNB

The orientational properties of crystalline A-TCNB are calculated using a Monte Carlo procedure, based upon a previously reported site-site expression for the interactions between molecules. The nearly static TCNB complexes are orientationally quiescent. The A molecules respond to an orientational double well potential provided by the TCNB, and by their mutual interactions. As a consequence, the dominant part of the orientational Hamiltonian depends on only a single degree of freedom, from which the results are determined. Properties calculated include the orientational structures, internal energy, specific heat, and the phase transition temperature. Various order parameters are calculated to understand the microscopic nature of the transition, which seems to show features of both an order-disorder and displacive character, much like experimental interpretations. © 1990 American Institute of Physics.