Experimental study of high-temperature smectic-C*F12 phase in chiral smectic liquid crystals that exhibit phase-sequence reversal

Abstract

We report the results of an experimental study of a recently observed phase sequence reversal of smectic-C_FI2^* [SmC*(qT=1/2); a four layer antiferroelectric] phase appearing in the temperature range above the smectic-C* (SmC*) phase from the results of optical birefringence, spontaneous polarization, selective reflection, conoscopy, and dielectric spectroscopy. The SmC_FI2^* phase is observed in an antiferroelectric liquid crystalline compound, 10OHF, in a temperature range above that of SmC* phase and is found to be thermodynamically monotropic, i.e., it appears only upon cooling from SmC_alpha^* phase. This is also unstable as if it is once transformed to SmC* by the application of the bias, it does not return to its original state unless the sample is heated and cooled again in the absence of the bias. Nevertheless this phase is stabilized by the addition of a chiral smectic compound 9OTBBB1M7 (abbreviated as C9), having a wide temperature range of the SmC_FI2^* phase. The temperature range of the low temperature SmC* decreases with increase in the concentration of C9 and for a concentration of 55 wt. %, SmC* disappears and the transition takes place directly from SmC_FI2^* to the crystalline phase on cooling. The existence of such a high-temperature SmC_FI2^* phase is also supported by a phenomenological model.