Binary mixtures of 4-n-undecyloxy-(4’-isothiocyanatophenyl)-benzoate (11.O.NCS) and 4-n-decyloxyphenyl-4’-(4’’-cyanobenzyloxy)-benzoate (10OPCBOB) exhibit a closed reentrant nematic (${\mathit{N}}_{\mathit{r}}$) region—a ‘‘nematic lake’’—between X=51 and 79, where X is the mole percent of 10OPCBOB. A high-resolution ac calorimetric study has been carried out on six mixtures with this composition range to characterize the smectic-${\mathit{A}}_{\mathit{d}}$ (Sm-${\mathit{A}}_{\mathit{d}}$)–${\mathit{N}}_{\mathit{r}}$ and ${\mathit{N}}_{\mathit{r}–}$smectic-${\mathit{A}}_1$ (Sm-${\mathit{A}}_1$) transitions plus one mixture (X=80.7) that undergoes a direct Sm-${\mathit{A}}_{\mathit{d}–}$Sm-${\mathit{A}}_1$ transition. Of special interest is the ${\mathit{N}}_{\mathit{r}–}$Sm-${\mathit{A}}_1$ transition, which exhibits three-dimensional XY critical heat-capacity behavior at X=53.8 and 64.5, shows crossover behavior with nonuniversal effective exponents α≃0.26 for X=74.5 and α≃0.31 for X=76.4, and seems to approach tricritical behavior near the entry to the lake. The presence of such a tricritical point is consistent with theoretical predictions of the multicritical topology near a Sm-${\mathit{A}}_{\mathit{d}–}$${\mathit{N}}_{\mathit{r}–}$Sm-${\mathit{A}}_1$ point.

• Received 17 January 1992

DOI:https://doi.org/10.1103/PhysRevA.46.973