Polar structures in binary mixtures of bent-core liquid crystals showing ferroelectric and antiferroelectric B2 phases.

We have studied the polar structures in the binary mixtures of bent-core liquid crystals P-n-O-PIMB(n-2)*-(n-4)O showing the ferroelectric smectic- CAPF (Sm-CAPF) (n=8 and 10) and antiferroelectric Sm-CSPA (n=9) B2 phase. Although the polar structure of the one-to-one mixture is governed by the compound with longer end chains, it is much more complicated in the mixtures with slightly less fraction of compounds with longer end chains. Even if the mixtures show the antiferroelectric phase before field application, the ferroelectric domains remain once the field is applied and coexist with the antiferroelectric domains. The coexistence structure was modeled by comparing the microscope real image with that of second-harmonic generation. The fraction of the ferroelectric domains is larger at higher temperature region of the B2 phase and decreases with decreasing temperature. The stable phase structures were discussed based on the phenomenological theory. We suggest that the coupling energy of the layer chirality is higher than the energies of the orientation correlations of tilts and polarizations between adjacent layers.