Thermodynamic Studies on Incommensurate Biphenyl and Related Substances

Abstract

As the temperature decreases, the heat capacity of biphenyl decreases less steeply than those of p-terphenyl and p-quaterphenyl, and crosses them at about 12 K, and at about 6 K, respectively. A simple model calculation of lattice vibration spectra shows that the large heat capacity of biphenyl at low temperatures is attributed to the twisting mode of low frequency of the phenyl rings in biphenyl. However, the heat capacity measurements on 4,4'-difluorobiphenyl which Should have rather large heat capacity at low temperatures according to this explanation reveals that the crossover occurs at about 12 K. An attempt is described to account for the relative magnitude of the heat capacities of biphenyl and 4,4'-difluorobiphenyl by calculating the crystal energies for the planar and the twisted molecular conformations.