The effects of pressure on the phases in biphenyl

Abstract

The influence of pressure on the phase transitions in biphenyl is discussed in detail. It is shown that a simple model, containing a harmonic intermolecular as well as a double-well-shaped intramolecular potential, can account for the experimentally observed phase transitions and their temperature dependence. The vanishing of the phase transition I-II at higher pressure and the transition temperature T_I-II at atmospheric pressure are used to give an estimate of the parameters of the intramolecular potential. These two estimates will prove to be sufficient to explain all the characteristic features of the phase transitions. Furthermore it is shown that the incommensurate phase III in biphenyl at low pressure must be metastable but there is a higher-pressure range in which the stable structure at zero temperature is incommensurate, presumably the phase II as observed. Finally the dependence of the transition temperatures from phase I to II and from phase II to III on pressure is calculated and the results compared with experiment.