Smectic A—nematic phase transition in a system of biaxial molecules

Abstract

The thermodynamics of the S_A‒N phase transition has been studied within the framework of the McMillan model generalized to biaxial molecules. The influence of the biaxiality of the molecular pseudopotential has been shown to be weak for the transition temperature T _{S A N}, noticeable for the tricritical point position T _{S A N}. on the line of S_A‒N transitions and strong for the entropy of the transition ΔS _{S A N}. With the McMillan parameters α and δ being constant the entropy change ΔS _{S A N} decreases with increasing biaxiality parameter λ For sufficiently high values of δ and for α corresponding to a narrow nematic range increasing λ can result in changing the S_A‒N transition from first to second order. The influence of λ on the change of the biaxiality G = S_xx — S_yy of the molecular order parameter in the smectic phase is in agreement with available experimental data.