Dielectric Studies of the Successive Phase Transitions in Cholesteryl 2,2,3,3,-Tetrafluoropropionate (CTFP)

Abstract

The successive phase transitions and the thermal hysteresis in CTFP crystals were studied by measuring the dielectric constant, c', and the loss, c, as a function of temperature along the three principal axes of a monoclinic single crystal. The two anisotropic peaks of c’ were observed near - 120 and - 94°C. The value of c’ indicated a maximum of about 21 at - 94 °C along the c*-axis. The former peak was explained by a dielectric relaxation through modulation wave motion including polar groups in an incommensurate (INC) phase (Phase II), and the latter peak was related to a Phase I (normal phase)-Phase II transition. In Phase III/III* (INC phase) and Phase IV (commensurate phase)/IV* (INC phase) below - 130°C, the molecular motion associated with the phase transition was dielectrically inactive, and one dielectric relaxation was observed near - 150°C, which is not related to the phase transition phenomenon. In this work, two transitions near - 80 and 0°C were found in the normal phase (Phase I) as peaks of c'. The peak near - 80°C appeared only in the crystals which had undergone a cooling run from room temperature. From these dielectric results, the mechanism for the thermal hysteresis of the successive phase transitions was discussed and compared with our previous X-ray diffraction work.