Model for phase transitions of the crystalline polyphenyls

Classical and quantum mechanical models for the phase transitions in the polyphenyls are developed. Starting with the high symmetry phases of these crystals, the models are able to predict several key features of the polyphenyl transitions. First, the nature of the phase transitions, order-disorder in p-terphenyl and displacive in biphenyl, is confirmed. Second, the transition is predicted to occur at a substantially lower temperature for biphenyl than for p-terphenyl. Third, the models predict a decrease of the p-terphenyl transition temperature with pressure, in agreement with experiment. The trasition to an incommensurate phase for biphenyl is discussed, and the origin of the observed isotope effect is explored.