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

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Abstract
The molecular orientational structure of the Scα∗ 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 Scα∗ phase is tilted. According to the conoscopic observation under a low field, the optic plane of Scα∗ is parallel to the field direction similar to that of the Scα∗ phase of 4-(1-methylheptyloxycarbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylate. It was also found that the molecular orientation of the Scα∗ 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.