LANDAU-EXPANSION COEFFICIENTS RELATED TO 2ND-ORDER SMA-SMC-ASTERISK TRANSITION FOR FERROELECTRIC LIQUID-CRYSTAL MIXTURES COMPOSED FROM ACHIRAL MATRIX AND CHIRAL DOPANTS .3. DEPENDENCE ON MOLECULAR-STRUCTURE OFCHIRAL 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 di fference between them is the different lengths of chiral molecules. Th e Landau coefficients a and b have been found to be appreciably increa sed with an increase of chiral dopand molecular length. It is accounte d for by an increase in orientational ordering P-2 of the mixture with the longer dopant, which, in turn, results in an increase in the smec tic wave amplitude psi. The increase in P-2 has been confirmed by X-ra y measurements of the interlayer distance in the smectic A and C phase s of the two mixtures.