Effect of optical purity on phase sequence in antiferroelectric liquid crystals.

We use the discrete phenomenological model to study theoretically the phase diagrams in antiferroelectric liquid crystals (AFLCs) as a function of optical purity and temperature. High sensitivity of the phase sequence in the AFLCs to optical purity is attributed to the piezoelectric coupling which is reduced if optical purity is reduced. We limit our study to three topologically equal smetic (Sm)phases: Sm-C(*), Sm-C(*)(alpha), and Sm-C(*)(A) and show that the reduction of optical purity forces the system from the antiferroelectric to the ferroelectric phase with a possible Sm- C(*)(alpha) between them. The effect of the flexoelectric and the quadratic coupling is considered as well. If the phase diagram includes only two phases, Sm-C* and Sm- C*(A), the flexoelectric coupling is very small. The materials which exhibit the Sm-C(*)(alpha) in a certain range of optical purity and temperature, can be expected to have a significant flexoelectric coupling that is comparable with the piezoelectric coupling. Upon lowering the temperature the phase sequence Sm-A-->Sm-C(*)(alpha)-->Sm-C*-->Sm-C(*)(A) is possible in systems where quadratic coupling is very strong.