Calorimetric Study of the Phase Transition of Para-Terphenyl

At 300 K crystalline para-terphenyl (PTP) may be considered to be planar, but the librational amplitude of the central ring is unusually high (16°). This behaviour has been explained by the occurrence of a double well potential for the central cycle which occupies at random one of the two possible positions and jumps continuously from one minimum to the other one.1 When the temperature is lowered, a superstructure appears: an antiferro-reorientational ordering results from the stabilization of the central ring in one of the two possible wells.2 A recent treatment of the diffraction data (neutron as well as x-ray) collected in the high temperature phase has confirmed this hypothesis: a double-peaked distribution function was obtained for the central ring librations.3