The problem of the first- or second-order nature of the nematic-smectic-$A$ phase change is explored. It is found that couplings between various order parameters, especially the first and second translational ones, are important. General Landau theory for coupled orientational and translational order parameters is developed. Numerical methods and results for translational ordering are presented. The effects of molecular polarity are considered. The relative importance of different couplings is discussed, and some experiments are proposed.

• Received 9 August 1976

DOI:https://doi.org/10.1103/PhysRevA.14.2307