Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, D-22607 Hamburg, Germany.
European XFEL GmbH, Holzkoppel 4, D-22869 Schenefeld, Germany.
Soft Matter. 2017 May 3;13(17):3240-3252. doi: 10.1039/c7sm00343a.
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.
我们报告了三种不同液晶化合物的自由悬挂六方膜的 X 射线研究。通过对测量的衍射图案应用角度 X 射线互相关分析（XCCA），直接确定了六方相中键取向（BO）有序的参数。基于多重临界标度理论（MCST），分析了 BO 有序参数随温度的演化。我们的结果证实了 MCST 在所有三种化合物的六方相存在的整个温度范围内的有效性。在六方-向列相转变附近，BO 有序参数的温度依赖性通过具有非常小的临界指数β≈0.1 的常规幂律拟合。我们发现，BO 有序参数的更高阶谐波的温度依赖性作为第一谐波的幂次缩放，指数等于谐波数。这表明不同阶的 BO 有序参数之间存在非线性耦合。我们证明，具有不同低温相序列的各种组成的化合物在六方相和近向列-六方相转变附近表现出相同的热力学行为。
