Critical behavior at transitions from uniaxial to biaxial phases in a smectic liquid-crystal mixture

We report results of calorimetric and optical investigations of binary mixtures of rodlike and bent-shaped molecules. We find that the observed critical heat anomaly associated with the smectic-A(2) to biaxial smectic-A(2b) transition is well described with a Fisher-renormalized form of the usual scaling expression. The effect of renormalization is large in this system in part because of the moderately steep slope of the phase boundary (dT(c)/dX similar to 100 K, where X is the mole fraction of the bent-core molecules) and in part because of the proximity to the tricritical point. The magnitude of heat anomaly at the smectic-A(2)-smectic-A(2)b transition showed a drastic decrease as X becomes smaller. Moreover, the nematic-smectic-A(2) transitions investigated turned out to be always first order and the transition enthalpy showed only weak dependence on the concentration X. The results imply that the energy fluctuations around the smectic-A(2)-smectic-A(2)b transition are very sensitive to the underlying ordering of the smectic-A(2) background.