Ferrielectricity in Chiral Smectic C_β of an Antiferroelectric Liquid Crystal

The switching current behavior and the complex dielectric constant under DC electric bias field and domain structures of an antiferroelectric liquid crystal, 4-(1-methylheptyloxycarbonyl) phenyl 4'-octyloxybiphenyl-4-carboxylate (MHPOBC), were investigated as a function of optical purity and cell thickness, and the ferrielectricity of the chiral smectic C_β phase (SmC_β*) was discussed. Optically pure samples show two-step decrease in the real part of complex dielectric constant with increasing DC bias field. The first decrease corresponds to the suppression of the ferroelectric Goldstone mode, which is also suppressed by decreasing cell thickness. The second step, which is observed even in the chiral smectic C_α phase (SmC_α*), corresponds to the suppression of a new relaxation mode induced by DC electric bias field. Three peaks of the switching current were observed in SmC_β* and also in the lower-temperature region of SmC_α*. Temperature dependence curves of the peak positions are continuously connected together at the phase transition temperature between SmC_β* and SmC_α*. It can be said that the antiferroelectricity of SmC_α* is conserved in SmC_β*, and that the ferrielectricity of SmC_β* may consist of the ferroelectricity of SmC* and the antiferroelectricity of SmC_α*. Striped domains observed in SmC_β* after removing the DC electric bias field can be considered to be the SmC_α* phase.