Manna U, Song Jang-Kun, Panarin Yu P, Fukuda Atsuo, Vij J K
Department of Electronic and Electrical Engineering, Trinity College, University of Dublin, Dublin 2, Ireland.
Phys Rev E Stat Nonlin Soft Matter Phys. 2008 Apr;77(4 Pt 1):041707. doi: 10.1103/PhysRevE.77.041707. Epub 2008 Apr 23.
Mixtures of different compositions of an antiferroelectric liquid crystal compound that exhibits direct smectic-A*(Sm-A*)-smectic-C*(A) (Sm-C*(A)) transition with a ferroelectric liquid crystal compound that exhibits Sm-A*-smectic-C*(Sm-C*) transition are studied using electro-optics and dielectric spectroscopy. The results of optical texture, birefringence, and the tilt angle suggest that a part of the Sm-A* phase is of de Vries type, since an increase in the tilt angle with decreasing temperature results in a reduction in the value of the birefringence in the Sm-A* phase, whereas the birefringence at Sm-A* to Sm-C* transition goes up by 12.7%. The soft mode relaxation strength, the Landau coefficient of the temperature dependent term, and the other related parameters of the de Vries-type Sm-A-Sm-C*(A) and Sm-A*-Sm-C* transitions are determined using the Landau theory of the second-order phase transition. For the Sm-A*-Sm-C* transition, we find that the soft mode relaxation strength decreases, the Landau coefficient increases, and the Curie-Weiss temperature range decreases with an increased ferroelectric composition in the mixture. These observations can be explained by assuming that with increased ferroelectric composition in the mixture, the layer shrinkage at the de Vries Sm-A*-Sm-C* transition increases. On comparing the results of de Vries-type Sm-A* to Sm-C*(A) and Sm-C* transitions, we find that the soft mode dielectric strength and the other related Landau parameters of the de Vries Sm-A* phase are of the same order of magnitude for transitions from Sm-A* to Sm-C* and to Sm-C*(A) except for the composition of the mixture where both Sm-C* and Sm-C*(A) transitions are stable and the phase diagram shows phase sequence Sm-A* to Sm-C* to Sm-C*(A).
研究了一种呈现直接近晶A*（Sm-A*）-近晶C*（A）（Sm-C*（A））转变的反铁电液晶化合物与一种呈现Sm-A*-近晶C*（Sm-C*）转变的铁电液晶化合物不同组成的混合物，采用了电光和介电谱技术。光学织构、双折射和倾斜角的结果表明，Sm-A相的一部分是德弗里斯型的，因为随着温度降低倾斜角增大，导致Sm-A相中的双折射值减小，而从Sm-A到Sm-C转变时双折射上升了12.7%。利用二阶相变的朗道理论确定了德弗里斯型Sm-A-Sm-C*（A）和Sm-A*-Sm-C转变的软模弛豫强度、温度相关项的朗道系数以及其他相关参数。对于Sm-A-Sm-C转变，我们发现随着混合物中铁电成分增加，软模弛豫强度减小，朗道系数增大，居里-外斯温度范围减小。这些观察结果可以通过假设随着混合物中铁电成分增加，德弗里斯Sm-A-Sm-C转变时的层收缩增加来解释。在比较德弗里斯型Sm-A到Sm-C*（A）和Sm-C转变的结果时，我们发现除了Sm-C和Sm-C*（A）转变都稳定且相图显示相序列为Sm-A到Sm-C到Sm-C*（A）的混合物组成外，德弗里斯Sm-A相从Sm-A到Sm-C以及到Sm-C（A）转变的软模介电强度和其他相关朗道参数具有相同的量级。
