MULTICRITICAL POINT INVOLVING HEXATIC SMECTIC PHASES

The triple point smectic-C-smectic-F-hexatic-B (C-F-B) has been found in binary mixtures of THI-n (thienyl-enaminoketone derivatives) compou nds with different length of molecular terminal chains n. Differential scanning calorimetry (DSC), x-ray scattering, and optical studies sho w that this point is a multicritical point, where two continuous phase transition lines, Sm-F-Hex-B and Sm-C-Hex-B, meet. The topology of th e phase diagram is different from that for the nematic-Sm-A -Sm-C (N-A -C) multicritical point. On the Sm-F-Hex-B line a tricritical point wa s observed for the system close to the Sm-F-Hex-B-Cry-B triple point. For mixtures of THI-n with some reference hexatic-B compounds, a phase diagram on which the Hex-B-Sm-C phase transition line extends between Sm-C-Sm-F-Hex-Band Sm-A-Sm-C-Hex-B multicritical points was found.