MICROSCOPIC MODEL WITH TEMPERATURE-DEPENDENT INTERACTIONS FOR THE FREE-MOLECULE AND FOR THE TRIGONAL PHASE OF BENZIL

The molecule of benzil (diphenylethanedione, C14H10O2) has been approx imated by a system of rigid segments to model the lowest-frequency par t of its vibrational spectrum. The interactions of internal degrees of freedom have been described with the use of phenomenological force co nstants. The structure of the trigonal (P3(1)21) phase has then been m odelled by means of a temperature-dependent atom-atom potential based on thermal motions of atoms. The potential gives the correct account o f the softening of an E-symmetry, zone-center mode which underlies the phase transition to the low-temperature monoclinic phase (P2(1)). The low-frequency modes at the zone center, supposed until now to be diff erence overtones, have been shown to result from a coupling between in ternal and external degrees of freedom. A low-frequency soft mode at t he point M of the zone border has been found, which explains the behav ior of observed peaks in diffuse x-ray scattering experiments. The val ues and the temperature evolution of the effective elastic constants c alculated within the model are in a very good agreement with the resul ts of ultrasonic and Brillouin scattering data. The model has been sho wn insufficient in the description of dielectric and piezoelectric pro perties of benzil. (C) 1996 American Institute of Physics.