CROSSOVER FROM 3-DIMENSIONAL XY TO TRICRITICAL BEHAVIOR FOR THE NEMATIC SMECTIC-A1 PHASE-TRANSITION

A high-resolution ac calorimetric technique has been employed to study the nematic-smectic-A1 (N-Sm-A1) phase transition in liquid-crystal binary mixtures of octyloxyphenyl-nitrobenzoyloxy benzoate (DB8ONO2) and decyloxyphenyl-nitrobenzoyloxy benzoate (DB10ONO2). The existence of a tricritical point on the N-Sm-A1 transition line has been reconfirmed in this study. This tricritical point occurs at mole percent X congruent-to 51 of the decyl homolog in the mixture. Heat-capacity measurements characterizing the N-Sm-A1 transition have been carried out for three samples in the vicinity of the tricritical point (X = 47.0, 48.7, and 50.0) and for one sample with X = 31.5. The results reveal a complete crossover behavior for the critical exponent alpha, which changes from the three-dimensional XY value (alpha-XY = -0.007) for X = 31.5 to a value alpha = 0.422 close to tricritical (alpha-t = 0.50) for x = 50.0. The amplitude ratio A- / A+ for the X = 31.5 sample conforms to that expected from the noninverted XY model. In addition, heat-capacity measurements on one sample with X = 53.7 indicate that near the termination point of the Sm-A(d)-Sm-A1 transition line the phase sequence Sm-A(d)-N(d)-N1-Sm-A1 occurs, in accordance with theoretical predictions.