Critical behavior at the isotropic to nematic, nematic to smectic-A and smectic-A to smectic-C phase transitions in a pyrimidine liquid crystal compound

Abstract

High-resolution optical birefringence (∆n) measurement of a pyrimidine liquid crystal compound having nematic, smectic-A and smectic-C phases are reported. The high-resolution ∆n data are rather successful in assessing the critical anomaly at different phase transitions in the investigated compound with a reasonably good accuracy. The critical exponent β, describing the limiting behavior of the nematic order parameter close to the isotropic–nematic (I–N) phase transition, is found to be in good agreement with the tricritical hypothesis. The critical behavior at the nematic–smectic-A (N–Sm-A) and the smectic-A–smectic-C (Sm-A–Sm-C) phase transitions has been explored with the aid of a differential quotient extracted from the ∆n values. The yielded effective critical exponent α′ is appeared to be nearly tricritical in nature for the N–Sm-A phase transition. For the Sm-A–Sm-C phase transition, α′ exhibits a weak dependence on the fit range and assumes tricritical value for large temperature range considered, which again is found to be diminished slightly with reduction in the temperature range. Related critical amplitude quotient and corrections-to-scaling quotient are found to display deviations from the theoretical models. Such behavior signals the appearance of a non-Landau character for the orthogonal to tilted smectic phase transition in the investigated compound.