Calorimetric study of the smectic-A–smectic-C^* transition and the nematic–smectic-A–smectic-C^* point

The phase behavior near the nematic–smectic-A–smectic-C^* (N–Sm-A–Sm-C^*) point has been studied with a high-resolution ac calorimetry technique in seven binary mixtures of two chiral compounds. The smectic-A (Sm-A)–smectic-C^*(Sm-C^*) phase transitions are continuous for all the mixtures studied. The Sm-A–Sm-C^* specific-heat variations are well described by the extended Landau model, with the possible exception of the mixture closest to the N–Sm-A–Sm-C^* composition. As the N–Sm-A–Sm-C^* point is approached, the Sm-A–Sm-C^* transition became more tricritical-like, indicating that the tricritical point (TCP) is located at or very near the N–Sm-A–Sm-C^* point. These observations contradict a previous report that the Sm-A–Sm-C^* TCP is displaced from the N–Sm-A–Sm-C^* point in this system. Anomalous C_p variations in the N range suggest a restructuring that might be associated with biaxial nematic fluctuations.