This study gives our calculation of the \(T-X\) phase diagram for the mainly smectic–hexatic phase transitions in the binary mixtures of THI14 + MPR6. By expanding the free energy in terms of the order parameters with the biquadratic coupling in the Landau mean field model, the phase line equations are obtained and they are fitted to the experimental \(T-X\) data from the literature. The first-order and the second-order transitions are considered between the smectic and hexatic phases in the existence of the triple and tricritical (TCP) points in those binary mixtures. Our study indicates that the observed behavior of the first-order and the second-order (smectic–hexatic) transitions can be characterized in the binary mixtures studied by the Landau models given here.
