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

I A Zaluzhnyy,R P Kurta,M Sprung,E A Sulyanova,O Yu Gorobtsov,A G Shabalin,B I Ostrovskii,I A Vartanyants,E Górecka,A Krówczyński,A V Zozulya,A P Menushenkov

doi:10.1039/c7sm00343a

Abstract

We report on the X-ray studies of freely suspended hexatic films of three different liquid crystal compounds. By applying angular X-ray cross-correlation analysis (XCCA) to the measured diffraction patterns the parameters of the bond-orientational (BO) order in the hexatic phase were directly determined. The temperature evolution of the BO order parameters was analyzed on the basis of the multicritical scaling theory (MCST). Our results confirmed the validity of the MCST in the whole temperature range of the existence of the hexatic phase for all three compounds. The temperature dependence of the BO order parameters in the vicinity of the hexatic-smectic transition was fitted by a conventional power law with a critical exponent β ≈ 0.1 of extremely small value. We found that the temperature dependence of higher order harmonics of the BO order scales as the powers of the first harmonic, with an exponent equal to the harmonic number. This indicates a nonlinear coupling of the BO order parameters of different order. We demonstrate that compounds of various compositions, possessing different phase sequences at low temperatures, display the same thermodynamic behavior in the hexatic phase and in the vicinity of the smectic-hexatic phase transition.

Highlights

Warsaw, Poland f Landau Institute for Theoretical Physics, Russian Academy of Sciences, prospect akademika Semenova 1-A, 142432 Chernogolovka, Russia † Present address: Department of Physics, University of California-San Diego, La Jolla, California 92093-0319, USA. ‡ Present address: European XFEL GmbH, Holzkoppel 4, D-22869 Schenefeld, Germany.the hexatic phase was experimentally observed in a threedimensional (3D) stack of parallel molecular layers.[9]
The hexatic phase in liquid crystals9–11 (LCs) can be fully described on the basis of symmetry considerations, which does not imply any specific melting mechanism
In the present work we investigated the structure of three different LC compounds, prepared as thick freely suspended hexatic films, which are truly 3D systems

Summary

Introduction

Poland f Landau Institute for Theoretical Physics, Russian Academy of Sciences, prospect akademika Semenova 1-A, 142432 Chernogolovka, Russia † Present address: Department of Physics, University of California-San Diego, La Jolla, California 92093-0319, USA. The transition to an isotropic liquid occurs only after a subsequent unbinding of the disclination pairs. Such a defectmediated mechanism clearly does not work in 3D crystals due to the difference in defect formation energies in the 2D and 3D cases. The hexatic phase in LCs can be fully described on the basis of symmetry considerations, which does not imply any specific melting mechanism. The quantitative characterization of the BO order in 2D and 3D hexatic LCs can be made using the multicritical scaling theory (MCST) developed by Aharony et al.[12]

Methods

Results

Conclusion