Calorimetric study of the smectic-A–smectic-${\mathit{C}}^{\mathrm{*}}$ transition and the nematic–smectic-A–smectic-${\mathit{C}}^{\mathrm{*}}$ point

The phase behavior near the nematic–smectic-A–smectic-${\mathit{C}}^{\mathrm{*}}$ (N–Sm-A–Sm-${\mathit{C}}^{\mathrm{*}}$) 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-${\mathit{C}}^{\mathrm{*}}$(Sm-${\mathit{C}}^{\mathrm{*}}$) phase transitions are continuous for all the mixtures studied. The Sm-A–Sm-${\mathit{C}}^{\mathrm{*}}$ 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-${\mathit{C}}^{\mathrm{*}}$ composition. As the N–Sm-A–Sm-${\mathit{C}}^{\mathrm{*}}$ point is approached, the Sm-A–Sm-${\mathit{C}}^{\mathrm{*}}$ transition became more tricritical-like, indicating that the tricritical point (TCP) is located at or very near the N–Sm-A–Sm-${\mathit{C}}^{\mathrm{*}}$ point. These observations contradict a previous report that the Sm-A–Sm-${\mathit{C}}^{\mathrm{*}}$ TCP is displaced from the N–Sm-A–Sm-${\mathit{C}}^{\mathrm{*}}$ point in this system. Anomalous ${\mathit{C}}_{\mathit{p}}$ variations in the N range suggest a restructuring that might be associated with biaxial nematic fluctuations.