CHARACTER OF THE SMECTIC-A CHIRAL-SMECTIC-C PHASE-TRANSITION NEAR A CHIRAL-NEMATIC SMECTIC-A CHIRAL-SMECTIC-C POINT

Binary mixtures of two compounds with phase sequences isotropic (I)-chiral-nematic (N)-chiral-smectic-C (C*) and I-N-smectic-A (A)-C* have been investigated. Spontaneous-polarization and tilt-angle measurements near the A-C* transition indicate a mean-field-like to tricritical crossover behavior near the N-A-C* point of the phase diagram, the critical exponent beta varying continuously from values near 0.5 to almost 0.25. The heat capacity has also been measured with a high-resolution ac calorimetric technique at the A-C* and N-A transitions of three mixtures. It is found that the A-C* transition is well described by an extended mean-field model. The fits show that, in accordance with polarization and tilt-angle measurements, the transition evolves from a second-order regime towards tricriticality as the A range decreases. Based on these results and other analogous studies published by other authors, some considerations about the material requirements necessary to obtain first-order A-C* transitions are briefly discussed.