Optical birefringence and its critical behavior in the vicinity of nematic–smectic A phase transition in a binary mixture

Optical birefringence (Δn) measurements as a function of temperature have been performed for binary mixtures of octyloxy cyanobiphenyl (8OCB) and octyl cyanobiphenyl (8CB) liquid crystals by means of a high resolution temperature scanning technique. The temperature dependence of the birefringence (Δn) was determined from the transmitted intensity data for wavelengths λ=532 nm. Using a fitting procedure consistent with the mean field theory and the first order nature of nematic–isotropic (N–I) phase transition, the order parameter critical exponent β has been determined. The critical behavior of the nematic–smectic A (N–SmA) phase transition has been investigated from the high resolution birefringence data and the nature of this transition in the mixtures has been assessed. From the evolution of the critical exponent α, it has been possible to predict the limiting value of the McMillan ratio for the tricritical point (TCP) as well as the 3D-XY universality class.