Calorimetric study reveals successive phase transitions and anomalous heat capacities in crystalline biphenyl, suggesting torsional bond flexibility drives incommensurate instability.
Heat capacities of crystalline biphenyl were measured between 3 and 300 K by adiabatic calorimetry and some thermodynamic functions including the calorimetric standard entropy were tabulated. Thermodynamic properties of the successive phase transitions were determined; for the twist transition at (40.4±0.2) K, ΔtrsH=(5.02±0.08) J·mol−1 and ΔtrsS=(0.129±0.003) J·K−1·mol−1, and for the lock-in transition at (16.8±0.1) K, ΔtrsH=(0.15±0.02) J·mol−1 and ΔtrsS=(0.009±0.001) J·K−1·mol−1. Anomalously large heat capacities at low temperatures were analyzed based on lattice dynamics calculation, and the crossover of the low temperature heat capacities of biphenyl and p-terphenyl was attributed to the greater activity of the twisting mode in biphenyl. The relation of the bond flexibility to the incommensurability is discussed through the comparison of the low temperature heat capacities of biphenyl, p-terphenyl, and 4,4′-difluorobiphenyl.
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Saito et al. (1988) studied this question.
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