Structural and Thermodynamic Studies on Ferroelectric Molecular Crystal Tricyclohexylmethanol

Department of Chemistry, University of Tsukuba* Molecular Analysis and Life Science Center, Saitama University**
â—‹Yasuhisa Yamamura* Hideki Saitoh** Masato Sumita* Kazuya Saito*

Tricyclohexylmethanol (TCHM) is a tertiary alcohol with three cyclohexyl groups, i.e. methanol substituted by three cyclohexyl groups. The crystal structure at room temperature is triclinic P-1 with two molecules in a unit cell [1]. The two molecules form a dimer through a hydrogen bond. Each hydroxyl group of the two molecules lies at the center of the dimer. The hydroxyl group of TCHM is, thus, not able to build a network structure like other ferroelectric crystals with hydrogen bond network, e.g. KH2PO4 (KDP). Crystalline TCHM undergoes a phase transition at 103 K and has a spontaneous polarization at low temperature phase [2]. That is, TCHM shows ferroelectricity at low temperature phase, though no hydrogen bond network exists. However, detail of the phase transition is not clear. To clarify the mechanism of the phase transition, we investigate the structural and thermodynamic properties on TCHM in this study.
An anomaly was detected at 103 K in the heat capacity of TCHM. This anomaly is due to the paraelectric-ferroelectric phase transition of TCHM. Our x-ray diffraction experiment on a single crystal suggests that the direction of tandem hydroxyl groups with hydrogen bond in dimer is disordered in two possible orientations at room temperature. In this case, entropy of transition is expected to be 1/2 Rln2 (= 2.9 J K-1 mol-1), because of the two orientations per one dimer. The expected entropy is comparable to the resultant excess entropy (1.9 J K-1 mol-1). We will discuss the mechanism of the paraelectric-ferroelectric phase transition of TCHM.

[1] P. Sgarabotto and F. Ugozzoli, Acta Cryst. C44, 674 (1988).
[2] P. Szklarz and G. Bator, J. Phys. Chem. Solid, 66, 121 (2005).