BIREFRINGENCE AND TILT ANGLE IN THE ANTIFERROELECTRIC, FERROELECTRIC,AND INTERMEDIATE PHASES OF CHIRAL SMECTIC LIQUID-CRYSTALS

Using a high resolution optical polarimeter, we have measured the temp erature dependence of the birefringence and optical rotation in chiral smectic liquid crystals that exhibit antiferroelectric, ferroelectric , and intermediate phases. The temperature dependence of the magnitude of the tilt angle was determined from the birefringence of 4-(1-methy l-heptyloxycarbonyl-phenyl) 4'-octylbiphenyl-4-carboxylate (MHPOBC) an d of 4-(1-methylheptyloxycarbonyl-phenyl) 4'-octylcarbonyloxybiphenyl- 4-carboxylate (MHPOCBC). Both substances exhibit a crossover of the or der parameter exponent from the classical value of beta = 0.5 close to the transition to the tricritical value beta = 0.25 far away. This st resses the importance of the sixth order terms in the Landau free-ener gy expansion for ferroelectric and antiferroelectric liquid crystals. In addition, a discontinuous behavior in the magnitude of the tilt is observed when crossing the smectic-C-alpha-smectic-C* or smectic-C*- smectic-C-A transitions, whereas the smectic-A- smecric-C-alpha* tran sition is continuous. The simultaneously determined optical rotation i s used to elucidate the structures and the nature of phase transitions . The results are well explained within the framework of a discrete ph enomenological model with nearest and next nearest neighbor interactio ns between the smectic layers.