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

Binary mixtures of octyloxyphenyl-nitrobenzoyloxy benzoate (DB₈{ONO}₂$) and decycloxyphenyl-nitrobenzoyloxy benzoate (${DB}₁₀ONO₂) 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-Ad--Nᵣ--Sm-A₁ point at X{}56. The sample with X=57 exhibits a first-order direct Sm-Ad--Sm-A₁ transition, while the sample with X=51.3 exhibits two transitions---a second-order Sm-Ad--Nᵣ transition with a very small excess heat-capacity peak and a tricritical Nᵣ--Sm-A₁ transition with a large heat-capacity peak. For samples with intermediate compositions, only weakly first-order Nᵣ--Sm-A₁ heat-capacity peaks are observed in the vicinity of the Sm-Ad--Nᵣ--Sm-A₁ point. All six investigated samples exhibit the Sm-A₁--Sm-C ̃ \~{}{}--Sm-C₂ phase sequence at lower temperatures. The Sm-A₁--Sm-C ̃ \~{}{} transition is a weakly first-order phase transition with pretransitional excess heat capacity below the transition temperature. In contrast, the Sm-C ̃ \~{}{}--Sm-C₂ transition is a weakly first-order inverted Landau transition with no pretransitional behavior in the Sm-C₂ phase below the transition but a large mean-field excess heat capacity observed in the Sm-C ̃ \~{}{} phase above the transition.