Calorimetric study of smectic polymorphism in octyloxyphenyl-nitrobenzoyloxy benzoate + decyloxyphenyl-nitrobenzoyloxy benzoate mixtures

Binary mixtures of octyloxyphenyl-nitrobenzoyloxy benzoate (${\mathrm{DB}}_8$${\mathrm{ONO}}_2$) and decycloxyphenyl-nitrobenzoyloxy benzoate (${\mathrm{DB}}_{10}$${\mathrm{ONO}}_2$) exhibit a rich variety of smectic polymorphism. An ac calorimetric study has been carried out on six mixtures with mole percent X of the decyl homolog between 51 and 57, a range that includes the Sm-$A_d$–$N_r$–Sm-$A_1$ point at X≃56. The sample with X=57 exhibits a first-order direct Sm-$A_d$–Sm-$A_1$ transition, while the sample with X=51.3 exhibits two transitions—a second-order Sm-$A_d$–$N_r$ transition with a very small excess heat-capacity peak and a tricritical $N_r$–Sm-$A_1$ transition with a large heat-capacity peak. For samples with intermediate compositions, only weakly first-order $N_r$–Sm-$A_1$ heat-capacity peaks are observed in the vicinity of the Sm-$A_d$–$N_r$–Sm-$A_1$ point. All six investigated samples exhibit the Sm-$A_1$–Sm-C̃–Sm-$C_2$ phase sequence at lower temperatures. The Sm-$A_1$–Sm-C̃ transition is a weakly first-order phase transition with pretransitional excess heat capacity below the transition temperature. In contrast, the Sm-C̃–Sm-$C_2$ transition is a weakly first-order inverted Landau transition with no pretransitional behavior in the Sm-$C_2$ phase below the transition but a large mean-field excess heat capacity observed in the Sm-C̃ phase above the transition.