Double-well potentials and structural phase transitions in polyphenyls

Abstract

In their crystalilne phases, the conformation of non-rigid polyphenyl molecules results from a delicate balance between competing intramolecular and intermolecular forces. At low temperature, the polyphenyl crystals undergo structural phase transitions associated with stablization of a non-planar conformation, with respect to a torsional angle between the phenyl rings. An interesting feature is the different natures of these phase transitions: displacive in biphenyl and order–disorder in p-terphenyl. In order to illustrate this point we consider the following features: the thermal motion in the high-temperature phases, the influence of hydrostatic pressure on the phase transition in p-terphenyl, the transitional heat capacities, the observation of the critical slowing down of fluctuations in p-terphenyl and the existence of incommensurate phases in biphenyl.