LANDAU THEORY OF THE NEMATIC-SMECTIC A TRICRITICAL PHASE-TRANSITION IN A BINARY MIXTURE OF LIQUID-CRYSTALS

We have developed in this study a thermodynamic model for the nematic- smectic A (NA) tricritical phase transition of a binary mixture of liq uid crystals in analogy with the model for quasi fluid mixtures due to Griffiths. Using the Landau theory with a symmetrical TCP, we have pr edicted the critical behaviours of the nematic susceptibility, order p arameter and the specific heat discontinuity in the vicinity of the TC P. Our predictions for the tricritical exponents for the order paramet er and the specific heat discontinuity are those predicted by the mean field theory (beta=1/4 and alpha=1/2) with a symmetrical TCP. Our exp onent value of gamma=1/2 for the nematic susceptibility does not agree with the mean field value of gamma=1. Our predicted values agree with those obtained experimentally for some binary mixtures of liquid crys tals. This means that the mean field model with a symmetrical TCP is c apable of describing the nematic-smectic A tricritical phase transitio n for a symmetrical binary mixture of liquid crystals.