Character of the smectic-A–chiral-smectic-Cphase transition near a chiral-nematic–smectic-A–chiral-smectic-Cpoint

Binary mixtures of two compounds with phase sequences isotropic (I)–chiral-nematic (N)–chiral-smectic-C (${\mathit{C}}^{\mathrm{*}}$) and I–N–smectic-A (A)–${\mathit{C}}^{\mathrm{*}}$ have been investigated. Spontaneous-polarization and tilt-angle measurements near the A-${\mathit{C}}^{\mathrm{*}}$ transition indicate a mean-field-like to tricritical crossover behavior near the N-A-${\mathit{C}}^{\mathrm{*}}$ point of the phase diagram, the critical exponent β 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-${\mathit{C}}^{\mathrm{*}}$ and N-A transitions of three mixtures. It is found that the A-${\mathit{C}}^{\mathrm{*}}$ 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-${\mathit{C}}^{\mathrm{*}}$ transitions are briefly discussed.