Heat-capacity study of tricritical behavior at the smectic-A–chiral-smectic-Cliquid-crystal transition

The smectic-A (Sm-A) to chiral-smectic-C transition has been studied by high-resolution ac calorimetry in seven mixtures of two chiral esters of 2,5-diphenyl pyrimidine (2f+3f) over the range 38.72≤${\mathit{X}}_{2\mathit{f}}$≤65.30, where ${\mathit{X}}_{2\mathit{f}}$ is the mole percent of 2f. The heat-capacity results have been fit below the transition temperature using an extended mean-field model and show that the transition is quite close to a tricritical point over the entire range of composition. In all seven mixtures, the excess heat capacity above the transition temperature is observed to scale in the same manner as the Landau contribution does below the transition. This behavior is consistent with the presence of thermal fluctuations in the Sm-A phase.