Landau expansion coefficients related to second order SmA-SmC* transition for ferroelectric liquid crystal mixtures composed from achiral matrix and chiral dopants: Dependence on molecular structure of chiral dopants

Landau expansion coefficients have been compared for the two mixtures composed from the same achiral smectic matrix and two different chiral impurities. Coefficients were determined from temperature dependences of capacitance, tilt angle and polarization. Mixtures have relatively small, but the same concentration of chiral dopants, and the basic difference between them is the different lengths of chiral molecules. The Landau coefficients α and b have been found to be appreciably increased with an increase of chiral dopand molecular length. It is accounted for by an increase in orientational ordering P2 of the mixture with the longer dopant, which, in turn, results in an increase in the smectic wave amplitude ψ. The increase in P2 has been confirmed by X-ray measurements of the interlayer distance in the smectic A and C phases of the two mixtures.