Abstract
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\ensuremath{\simeq}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-C\ifmmode \tilde{}\else \~{}\fi{}--Sm-${C}_{2}$ phase sequence at lower temperatures. The Sm-${A}_{1}$--Sm-C\ifmmode \tilde{}\else \~{}\fi{} transition is a weakly first-order phase transition with pretransitional excess heat capacity below the transition temperature. In contrast, the Sm-C\ifmmode \tilde{}\else \~{}\fi{}--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-C\ifmmode \tilde{}\else \~{}\fi{} phase above the transition.
Published Version
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