Structural studies of the bond-orientational order and hexatic-smectic transition in liquid crystals of various compositions

摘要

We report on X-ray studies of freely suspended hexatic films of threedifferent liquid crystal compounds. By applying angular X-ray cross-correlationanalysis (XCCA) to the measured diffraction patterns the parameters of thebond-orientational (BO) order in the hexatic phase were directly determined.The temperature evolution of the BO order parameters was analyzed on the basisof the multicritical scaling theory (MCST). Our results confirmed the validityof the MCST in the whole temperature range of existence of the hexatic phasefor all three compounds. The temperature dependence of the BO order parametersin the vicinity of the hexatic-smectic transition was fitted by a conventionalpower law with a critical exponent $\beta\approx0.1$ of extremely small value.We found that the temperature dependence of higher order harmonics of the BOorder scales as the powers of the first harmonic, with exponent equal toharmonic number. This indicates a nonlinear coupling of the BO order parametersof different order. It is shown that compounds of various composition,possessing different phase sequences, display the same thermodynamic behaviorin the hexatic phase and in the vicinity of the smectic-hexatic phasetransition.