SMECTIC-A NEMATIC PHASE-TRANSITION IN A SYSTEM OF BIAXIAL MOLECULES

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(SAN), noticeable for the tricritical point position T(tcp) on the line of S(A)-N transitions and strong for the entropy of the transition DELTAS(SAN). With the McMillan parameters alpha and delta being constant the entropy change DELTAS(SAN) decreases with increasing biaxiality parameter lambda. For sufficiently high values of delta and for alpha corresponding to a narrow nematic range increasing lambda can result in changing the S(A)-N transition from first to second order. The influence of lambda 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.