First-order smectic-hexatic phase transition in thick liquid crystal films*

Synchrotron studies with a focused x-ray beam of the first-order smectic-A – hexatic-B (Sm-A – Hex-B) phase transition in 2-10 μm thick free standing films of 54COOBC compound are presented. The Hex-B phase is distinguished from Sm-A phase by long-range bond-orientational (BO) order, which can be conveniently studied by the angular X-ray cross-correlation analysis (XCCA). Discontinuity in temperature dependence of positional correlation length and the BO order parameter as well as direct observation of phase coexistence clearly indicate the first-order character of the phase transition. Experimental data unambiguously show that the width of temperature region of two phases coexistence increases with film thickness. This effect is explained on the basis of Landau mean-field theory in the vicinity of a tricritical point by anomalous penetration of the Hex-B order parameter into the film interior.