液晶SmC*相的螺距与手征中心分布

选用双轴性并含有多个手征中心的分子作用势，引入手征中心分布结构因子ζ_χ，采用格胞模型，求得液晶SmC^*相的螺距表达式和螺旋性反转条件.计算了TDOBAMBCC,FLC,PACMB和H6/10等4种典型SmC相液晶的螺距随温度的变化及其受ζ_χ的影响.结果与实验相符. 关键词：     

分子双轴性与向列型液晶在各向同性相的最低过冷温度

在分子场近似下,研究分子双轴性对向列型液晶在各向同性相最低过冷温度T^*的影响.理论结果表明,通过考虑分子双轴性,各向同性相最低过冷温度T^*与向列相—各向同性相相变温度T_c的比值向实验值靠近.当分子双轴性参数δ取到0.3时,T^*/T_c达到0.9817,已很接近0.99的实验值. 关键词：     

液晶SmC*相螺旋结构的统计理论

采用格胞模型,定义在格胞中心上的分子取向序参量、质心位置序参量和两者耦合序参量。选用双轴性并有横向电偶极矩的分子的作用两体势,求出系统的自由能,对自由能求变分得到指向矢满足的微分方程,由此方程得到SmC^*相指向矢与分子层面法线倾斜,并沿该法线作螺旋式变化,求得倾斜角和螺距的近似表示式,计算了DOBAMBC等三种典型SmC^*相液晶的倾斜角和螺距随温度的变化,结果与实验相符。     

液晶SmC■相的螺距与手征中心分布

选用双轴性并含有多个手征中心的分子作用势，引入手征中心分布结构因子ζχ，采用格胞模型，求得液晶ＳｍＣ相的螺距表达式和螺旋性反转条件．计算了ＴＤＯＢＡＭＢＣＣ，ＦＬＣ，ＰＡＣＭＢ和Ｈ６／１０等４种典型ＳｍＣ相液晶的螺距随温度的变化及其受ζχ的影响．结果与实验相符．     

外延PbZr0.4Ti0.6O3薄膜厚度对其铁电性能的影响

从Landau-Devonshire唯象理论出发，考虑到晶格失配导致的NFDA3位错应力场与极化场的耦合，研究了在SrTiO₃衬底上外延生长的PbZr_0.4Ti_0.6O₃薄膜厚度对其自发极化强度、电滞回线的影响. 结果表明，产生刃位错的PbZr_0.4Ti_0.6O₃薄膜临界厚度为～1.27nm，当薄膜厚度大于临界厚度时，在所形成的位错附近，极化强度出现急剧变化，形成自发极化强度明显减弱的“死层”；随着薄膜厚度的减小，位错间距增大，“死层”厚度与薄膜总厚度之比增加. 由薄膜电滞回线的变化情况可知，其剩余极化强度随着薄膜厚度的减小而逐渐减小. 关键词： 铁电薄膜 自发极化强度 电滞回线 位错     

液晶胆甾相的统计理论

胆甾相螺旋性结构的形成,分子间位置关联起十分重要的作用。采用格胞模型,导出了指向矢满足的二阶微分方程。有胆甾相解,并确定了螺距。位置关联可导致螺距随温度变化。这一理论可推广用来讨论蓝相的分子理论。 关键词：     

静电场对SmC^＊相液晶作用的计算研究

采用自洽模拟方法,计算研究了分子层面内的静电场对ＳｍＣ＾＊相液晶螺旋结构和铁电特性的影响。结果表明,ＳｍＣ＾＊相液晶的螺距和极化强度平均值随场强增大而增大,且越接近阈值电场增大真快;指向矢倾斜角和分子取向序参量随场强增大而缓慢增大;阈值电场与液晶的本征螺距和自发极化强度有关,并随温度升高而减小。     

液晶近晶C相双轴特性的统计理论

采用格胞模型,定义了分子取向序参量、双轴序参量、质心位置序参量以及取向与质心位置耦合序参量.由自由能极小求得指向矢倾斜角与双轴序参量间的关系.对典型Sc相液晶物质TBBA,TBSA和NOBA的双轴序参量随温度的变化以及取向序参量和倾斜角对双轴序参量的影响作了数值计算,并与实验结果进行比较.表明这种双轴特性主要产生于Sc相的指向矢倾斜结构和分子取向的有序性. 关键词：     

横向电场对平面织构胆甾相液晶螺距及反射特性的影响

阐述一种理论模型,讨论共面转换（IPS）模式下横向电场对平面织构胆甾相液晶螺距及扭曲角的影响．利用琼斯矩阵法计算了光正入射情况下未加电场时胆甾相液晶的反射谱．在上述模型基础上,计算了施加电场后胆甾相液晶的反射峰值波长以及反射带宽随场强的变化关系．探讨了忽略锚定与强锚定两种边界条件下横向电场对平面织构胆甾相液晶螺距及反射特性的影响．所得结论在理论上证实：共面转换模式下电场可以调谐胆甾相液晶的反射光颜色,从而为基于电控螺距原理的胆甾相液晶反射式彩色显示方案提供了理论上的依据． 关键词： 胆甾相液晶 电控螺距 共面转换 琼斯矩阵法     

SrBi4Ti4O15的化学键性质和铁电性研究

通过原子环境计算方法分析了正交相SrBi₄Ti₄O₁₅晶体内的键络结构、各原子的空间配位数及局域团簇结构. 在此基础上, 结合晶体分解理论将SrBi₄Ti₄O₁₅晶体分解为多个二元赝晶体, 根据化学键介电理论计算得到各赝晶体所对应化学键的有效价电子密度、离子性等化学键性质. 通过键偶极矩建立了铁电体自发极化强度与化学键性质之间的关系, 求得正交相SrBi₄Ti₄O₁₅沿a轴方向的自发极化强度为28.03 μC/cm², 与实验结果和其他理论计算值符合较好.     

Molecular structure and physical parameters of some ferroelectric liquid crystal systems

We investigate thermodynamic properties and spontaneous polarization of ferroelectric liquid crystal (SmC^*LC) systems: smectic C matrix + chiral adding (ChA). The fact of existence of an optimum concentration of ChA, for which the studied SmC^*LC systems have the maximum value of spontaneous polarization, and a certain concentration, at which the phase transition of the chiral tilted smectic C phase to the smectic A phase occurs, is established. Temperature dependence curves for the free energy, configuration energy, specific heat, and the orientational order parameter are calculated in the temperature range of existence of liquid crystal mesophases. The curves are in good agreement with the experimentally observed trends of the SmC^*LC properties.     

Smectic islands in antiferroelectric nanofilms

Heterochiral islands, in which topological dipoles are oppositely directed, are observed in freestanding antiferroelectric (SmC _A ^*) films. The topological dipoles in films with a transverse electric polarization and a planar molecule orientation at island boundaries are coplanar with an electric field. The topological dipoles in films with a longitudinal polarization and a planar orientation at island boundaries are perpendicular to an electric field. For a radial director orientation at island boundaries, the topological dipoles in films with a longitudinal polarization are coplanar with a field. Changing the orientation of an electric field, we can control the position of a topological defect at an island boundary and the orientation of a topological dipole. Heterochiral islands can form dimers with an anomalously small interisland distance.     

X-ray scattering study of the electric-field-induced layer deformations of an antiferroelectric Liquid Crystal

The effect of an external electric field on the local layer structure is investigated in the SmC^* _A, SmC^* _FI and SmC^* phases using X-ray diffraction. X-ray scattering and spontaneous polarization measurements are performed under several increasing (and decreasing) steps of AC applied voltage. The effects of the electric field differ significantly from one phase to the other. The chevron-to-bookshelf transition occurs abruptly at a high field value in the SmC^* _A phase whereas layer deformations are more progressive and occur at lower field values in the other phases. Comparison of the different behaviours suggests that the destruction of the chevron structure is mainly affected by the local molecular order. Received 16 July 2001 and Received in final form 7 December 2001     

Electrooptic study of antiferroelectric mixtures for display application

The aim of this paper is to study the influence of electric field on alignment of para-, ferro- and antiferroelectric phases in the vicinity of SmA^* — SmC^* or SmC^* — SmC _A ^* phase transitions as to obtain mono-domain cells. Four mixtures studied (W-193B, W-193B-1, W-201, W-204D) show the SmC _A ^* phase in a wide room temperature range. Measurements of the spontaneous polarization versus temperature by using reversal current method give an answer to the question, what kind of the transitions take place between para-, ferro- or antiferroelectric phases using the Landau mean field theory. Optimal electrooptic parameters for different compositions of the mixtures such as tilt angle, spontaneous polarization and saturation voltage have been measured to compare parameters of the mixtures studied.     

Spontaneous polarization and repolarization in crystals of the perovskite type

Based on molecular kinetic concepts, we have theoretically studied the processes of polarization and repolarization in ABO₃ crystals with a perovskite structure. We have calculated the free energy taking into account the possible displacements of the A atoms of the transition element from the equilibrium positions and the ordering of the dipole moments; we have found the dependence of the free energy for different phases of the crystal on temperature, the dipole order parameter, and the energy constants. We have obtained the thermodynamic equilibrium equation determining the temperature dependence of the degree of dipole order; we have found the Curie-Weiss transition temperature from the paraelectric to the ferroelectric state. Analysis of the equations obtained allowed us to provide a basis for the polarization and repolarization processes observed experimentally in BaTiO₃ and KNbO₃ crystals and to estimate the phase transition temperatures.Dnepropetrovsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 8–14, May, 1995.     

Broadening of IR Absorption Bands of p-Methoxybenzylidene p,n-Butylaniline (MBBA) Under the Influence of an A.C. Electric Field

Abstract

Infrared and Raman spectroscopy has been widely used at last decade to elucidate the properties of liquid crystals, the structure of the liquid crystal molecules and the molecular motion in various types of mesophase¹. Among more than 50 papers dealing with liquid crystals only a few consider the influence of electric field on mesophase vibrational spectrum. First Nelf² has used the electric field to orient nematic sam ples of p-methoxybenzylidene p′-cyanoaniline for the investigations of IR dichroism. Assuming that a large liquid crystal line domain acts like a rigid dipole and taking into account the effects of the walls an expression for the dichroism ratio was derived to evaluate the degree of orientational order. Later, Ohnishi³ has applied a d.c. electric field to a cell filled with MBBA and studied the changes in the relative transmittance of some bands in the region 4000–1000 cm^?1 of the dynamic scattering mode. Pan and Wang⁴ found that in the presence of turbulence due to the application of electric field, the intensity of all Raman bands decreases drastically but without any shift in the peak positions.