Ferrielectricity in Chiral Smectic Cβ 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-beta phase (SmCbeta*) was discussed. Optically pure samples show two-step decrease in the real parr 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-alpha phase (SmCalpha*), corresponds to the suppression of a new relaxation mode induced by DC electric bias field. Three peaks of the switching current were observed in SmCbeta* and also in the lower-temperature region of SmCalpha*. Temperature dependence curves of the peak positions are continuously connected together at the phase transition temperature between SmCbeta* and SmCalpha*. It can be said that the antiferroelectricity of SmCalpha* is conserved in SmCbeta*, and that the ferrielectricity of SmCbeta* may consist of the ferroelectricity of SmC* and the antiferroelectricity of SmCalpha*. Striped domains observed in SmCbeta* after removing the DC electric bias field can he considered to be the SmCalpha* phase.