Conoscopic study of the S_cα∗ phase and the Devil's staircase in an antiferroelectric liquid crystal

Abstract

The molecular orientational structure of the S_cα∗ phase has been studied by means of X-ray, dielectric and conoscopic measurements for 4-(1-methylheptyloxycarbonyl)phenyl 4′-octylcarbonyloxybiphenyl-4-carboxylate. It was found by X-ray scattering that the S_cα∗ phase is tilted. According to the conoscopic observation under a low field, the optic plane of S_cα∗ is parallel to the field direction similar to that of the S_cα∗ phase of 4-(1-methylheptyloxycarbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylate. It was also found that the molecular orientation of the S_cα∗ phase is antiferroelectric in the high temperature region, while in the low temperature region it is ferrielectric and changes with temperature, suggesting the so-called Devil's staircase. The critical temperature, where the biaxiality and the tilt angle of the average optic axis have maximum values, was assigned to the ferrielectric state with the largest spontaneous polarization.