X-ray and calorimetric study of smectic [Ctilde]-smectic A₂-smectic C₂ transitions in a liquid crystal mixture

Abstract

The phase diagram has been established for binary mixtures of nonyloxy-biphenyl cyanobenzoate (9OBCB) and pentylphenylcyanobenzoyloxy benzoate (DB₅CN). These frustrated liquid crystals exhibit a rich variety of polymorphic smectic A (S_A) and smectic C (S_C) phases. X-ray and calorimetric studies have been made of S_Ad -S_[Ctilde]-S_C2 and S_Ad -S_A1 -S_[Ctilde]-S_A2 S_C2 phase sequences. Major emphasis is given to the tilted ribbon smectic (S_[Ctilde])-bilayer smectic A (S_A2 ) transition and the S_A2 -S_C2 transition. The former is first order, but there is substantial pretransitional heat capacity in the S_[Ctilde] phase that can be described by an inverted Landau model. The latter transition is second order and exhibits a classical Landau step in C _p, in contrast to the behaviour of S_A-S_C transitions in non-polar systems where an extended Landau model with a large sixth order term is required.