Polarization Reversal by Intramolecular Disordering in Organic Ferroelectrics Trichloroacetamide

Mechanism of successive ferroelectric-paraelectric phase transitions exhibited by an organic ferroelectrics, trichloroacetamide, is clarified through structural and calorimetric experiments. The transition mechanism, intramolecular disordering, reasonably explains the polarization reversal. This mechanism is only possible in molecular solids and applicable to other compounds as a possible strategy to design new molecular ferroelectrics. Also revealed is the incommensurate nature of the intermediate phase, the origin of which is briefly discussed at phenomenological and molecular levels.
(CrystEngComm, 13, 2693-2698 (2011))

Thermodynamic Studies on Order-Disorder Phase Transitions of p-Terphenyl and p-Terphenyl-d₁₄

The heat capacities of p-terphenyl and p-terphenyl-d₁₄ were measured between 3 and 300 K. The phase transition associated with a molecular conformation change was observed as a lambda-shaped anomaly at (193.5 +- 0.1) K and at (180.3 +- 0.1) K. The enthalpies of transition are (304 +- 20) J mol^-1 and (288 +- 20) J mol^-1 and the entropies of transition are (1.63 +- 0.10) J K^-1 mol^-1 and (1.63 +- 0.10) J K^-1 mol^-1, respectively. The anisotropy of the intermolecular interaction is discussed and the anomalous portion of the heat capacities in the trasnition region was compared favorably with a two-dimensional Ising model.
(Bull. Chem. Soc. Jpn., 61, 679-688 (1988))

Incommensurate Phase Transitions and Anomalous Lattice Heat Capacities of Biphenyl

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, delta_trsH = (5.02 +- 0.08) J mol^-1 and delta_trsS = (0.129 +- 0.003) J K^-1 mol^-1, and for the lock-in transition at (16.8 +- 0.1) K, delta_trsH = (0.15 +- 0.02) J mol^-1 and delta_trsS = (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 flexbility to the incommensurability is discussed through the comparison of the low temperature heat capacities of biphenyl, p-terphenyl, and 4,4'-difluorobiphenyl.
(Bull. Chem. Soc. Jpn., 61, 679-688 (1988))

See also

• Displacive Transition or Order-Disorder Transition?
• Lattice Instability Coupled with Intramolecular Motion
• Glass Transition due to Freezing of Intramolecular Motion

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