The Structure and Unusual Optical Textures of Smectic C2 Phases of New Tail-to-Tail Twins

New tail-to-tail twins with alkyl or perfluorinated spacers were synthesized. The mesomorphic properties were studied by polarizing microscopy and X-ray diffraction measurements. It was found that the smectic C phase is a Sc₂ phase with alternating tilt and an intercalated structure. The alternating tilt gives rise to an unusual schlieren texture which exhibits not only singularities with the strength s = ±1 but also those with s = ±1/2 and s = ±3/2.     

Tilt Angle Determination of a Smectic C Phase by Field-Induced Freedericksz Transition and x ray Investigations

A smectic C liquid crystal exhibiting positive dielectric anisotropy was oriented with the director parallel to the glass substrates. When an electric field was applied normal to the director direction a dielectric reorientation (Freedericksz transition) occurred. Dissolving a dichroic dye in the smectic C phase and applying an electric field, a guest host effect similar to nematic phases was observed. From the dielectric reorientation the tilt angle of the smectic C phase was determined at different temperatures. The tilt angles obtained by the electrooptical measurements agree sufficiently with the values measured by x ray investigations.     

Quasielastic Neutron Scattering Study of Reorienting di-heptyloxyazoxybenzene (HOAB) Molecules in the Nematic and Smectic C Phases

Quasielastic neutron scattering (QNS) measurements carried out for heptyloxyazoxybenzene (HOAB) revealed the existence of fast molecular reorientation in the nematic and smectic C phases. The correlation time of this process, derived by fitting to a uniaxial rotational diffusion model, is of order of 10–20 ps in both liquid crystal phases. An explanation of the reasons for the disagreement between the correlation time values obtained by methods based on QNS and dielectric relaxation in the microwave region is suggested.     

The Smectic A and the Smectic C Phase: A Coherent Molecular Picture

A molecular field theory of the smectic A and smectic C phase is presented which is based on an interaction derived in terms of the molecular polarizability, its anisotropy and the components of the static molecular quadrupole tensor. It is shown that this attractive interaction strongly depends on the anisotropy of the excluded molecular volume. The results obtained explain the stability of the A phase, the appearance of the tilt in the C phase and the A-C phase transition, without resource to adjustable parameters which indeed have no molecular significance.     

Twist Grain Boundary Phases in the Binary Mixtures of Smectic and Cholesteryl Compounds

Abstract

The binary mixture of Terephthal-bis-p-n-hexadecylaniline (TB16A) and cholesteryl nonanoate (CN) exhibits I - N* - TGB _A - TGB _c*- Sm A - Sm I - Sm F - Sm G - K phases in the concentration region between 95 to 98% of TB16A at higher temperature range. The 90% of TB16A exhibits I - N* - TGB _A - TGB _c - Sm A - Sm I - Sm F - K phases. The mixtures of concentration between 50 to 80% of TB16A exhibits I - Sm A - Sm I - K phases. The frustrated blue phase is also observed in the mixtures of lower concentration of TB16A in CN at higher temperature. Optical, DSC and X-ray studies were carried out for the identification of the different phases. The phase diagram of the Chen-Lubensky model in this context is discussed.     

Structure and Packing in Smectic E and Smectic A Phases in the Series of 4-n-Alkyloxy-4′-alkanoylbiphenyls

By means of X-ray investigations the structure of the S_E and S_A-phases of the title compounds has been studied. The lattice parameters of the orthorhombic S_E structure are given. The thickness of the smectic layers is nearly the same in the S_E and S_A phases and agrees well with the length of the molecules in their most stretched form. That suggests a similar conformation of the alkyl chains in both phases. A model of the packing in the S_A phase is discussed which is based on a dense packing with antiparallel orientation of molecules with a non-symmetrical shape.     

Studies on Induced Smectic Phases (II) Transition Enthalpies,X-ray Patterns and Layer Spacings of Orthogonal Phases of Binary Mixtures of Nematogens with Contrasting Dielectric Anisotropies

Binary mixtures of two nematogens, 4,4′-di-n-heptyl-azobenzene with 4-n-pentyl-4′ -cyano biphenyl were studied by optical, calorimetric and X-ray measurements. At a given concentration induced phases with a sequence S_A, S_B, S_E were detected, the properties of which were discussed.     

The Dielectric Anomaly Near the Transition from the Smectic A* to Smectic C* Phase and Visco-Elastic Properties of Ferroelectric Liquid Crystals

A pyro-electric technique is developed which allows the measurement of the dielectric response near the A*-C* phase transition in ferroelectric liquid crystals. The temperature dependence of the elastic modulus K _θ(T) corresponding to the molecular tilt in smectic layers is calculated from the experimental data. Direct pyro-electric measurements of the relaxation time for the spontaneous polarization P _s and the data on K _θ(T) allow us to calculate the temperature behaviour of the twist-viscosity γ₁(T) for the smectic C* phase. The curves γ₁(T) are compared for the smectic C* and the nematic phase of the same compound and the dependence of the twist viscosity on the molecular tilt angle in the C* phase is investigated. The results of the dielectic measurements are discussed in terms of the mean-field approximation.     

Phase Diagram Involving the Thermal Stability of TGB and Induced Smectic Phases in Binary Mixture of Thermotropic Liquid Crystals

We report the results of our studies on the optical and thermal properties of the mixture of terephthalidene-bis-4, n-alkylaniline and cholesteryl nonanoate, which exhibits very interesting liquid crystalline mesophases such as cholesteric, twisted grain boundary, smectic-A, smectic-C^*, smectic-C, smectic-I, and smectic-G, sequentially when the specimen is cooled from its isotropic phase. These phases have been characterized by using microscopic techniques. The temperature variations of optical anisotropy, electrical conductivity, ultrasonic velocity, and molar and adiabatic compressibility have also been discussed.     

过饱和温度和降温速率对BBO晶体浓度边界层厚度的影响

浓度边界层是晶体生长过程中分析界面输运现象的重要元素.我们从实验角度研究了扩散机制下的BaB2O4单晶生长中过饱和温度和降温速率对浓度边界层厚度的影响.过饱和温度和浓度边界层厚度之间为抛物线关系,降温速率和浓度边界层厚度变化率之间为线性关系.     

不同膜厚对SiO2光子晶体形貌及光学性能的影响

利用改良的St6ber法制成了粒径均一的SiO2胶体颗粒,胶体微球颗粒平均粒径为200 nm.保持SiO2悬浮液体积分数相同,对双基片自组装垂直沉积进行改造,改变夹层厚度,沉积得到厚度不同的SiO2光子晶体薄膜.利用扫描电子显微镜观察了膜厚不同的SiO2光子晶体薄膜样品的表面以及断面微观形貌,测试了样品的光反射性能,此外还讨论了光子晶体薄膜厚度对样品表面的形貌以及样品光反射性能带来的影响.     

薄膜厚度对Ag纳米薄膜结构特性及光学性能的影响

采用密度泛函理论,研究了Ag(111)纳米薄膜的结构稳定性、电子特性及光学性能.结果表明,Ag (111)纳米薄膜原子层厚度增加到13层,即膜厚约为2.8nm时,纳米薄膜表面能趋于稳定,为薄膜能够稳定存在的临界厚度.薄膜表面处原子间为弱离子键作用,层间距变化及表面效应主要集中在表面附近几层.在可见光及红外波段,Ag(111)纳米薄膜的折射率明显高于块体材料而消光系数略高于块体材料;随着薄膜厚度的增加,在该波段,折射率减小,消光系数增加,吸收变大.     

O、Se和Te掺杂对单层MoS2电子能带结构和光学性质的影响

利用密度泛函理论第一性原理,计算了本征单层MoS2以及O、Se和Te掺杂单层MoS2的结构参数、能带结构、氧化还原电势、态密度、光吸收特性和光催化潜力.结果表明,本征和掺杂单层MoS2的晶胞参数、键长随原子半径的增大而增大,键角随原子半径的增大而减小.掺杂后禁带宽度变小,导电性增强.本征和掺杂的单层MoS2的导带底电位负于水解制氢反应的还原势,其价带顶电位正于水解制氢反应的氧化势,均具有光催化水解制氢的潜力.但是,本征和O掺杂,由于价带顶与导带底电位相应于水的氧化还原势和电子空穴需求量均存在不平衡,使其光解水性能不理想.然而,Se和Te掺杂可以平衡其还原与氧化能力,改善单层MoS2的光催化的性能,其中Se掺杂改善效果最为明显.同时,掺杂后的单层MoS2对可见光吸收效率增强.此研究有助于筛选合适的单层MoS2掺杂元素,提高单层MoS2材料的光催化水解制氢性能.     

尿素晶体线性和非线性光学系数的计算

本文使用从头计算平面波赝势法计算了尿素晶体的电子能带结构、线性和非线性光学系数,折射率和倍频系数的计算结果与实验结果基本符合.计算发现尿素晶体中C、N和O元素的2p轨道电子态发生了明显杂化,而能量较高的2p轨道电子态的杂化是其非线性光学效应的主要来源.     

第一性原理研究氧化锌晶体的电子结构和光学性质

为了研究氧化锌晶体的光学性质,本文采用密度泛函理论(DFT)的广义梯度近似(GGA)下的平面波赝势法计算了氧化锌晶体的电子结构、复数折射率、介电函数、光电导谱和吸收光谱.介电函数的虚部、吸收光谱、折射率等它们的峰值位置存在一一对应关系,这表明了它们之间存在着内在的联系,它们都与电子从价带到导带的跃迁吸收有关,这为从物理本质上理解氧化锌晶体的光学性质提供了重要的依据.计算结果与实验结果吻合得很好.     

CsCdCl3:Ni2+晶体的光谱和EPR谱的理论研究

基于强场图像,采用点电荷-偶极子模型和双自旋-轨道耦合理论,建立了CsCdCl3:Ni2+晶体的局域结构与光吸收谱和电子顺磁共振(EPR)谱的关联公式.与以前的研究不同,我们在光吸收谱的计算中考虑了配体离子的自旋-轨道耦合作用的贡献,统一地解释了强场下CsCdCl3:Ni2+晶体的光学吸收谱、EPR谱及局域结构,所得理论结果与实验发现很好吻合.并讨论了所得结果的合理性.     

ThSiO4：V^4＋晶体的光谱及EPR参量的理论研究

基于晶体场理论,采用3d1离子在D2d对称中的晶场能级公式和EPR参量高阶微扰公式,计算了ThSiO4:V4 晶体的光谱和电子顺磁共振(EPR)参量g因子g//,g⊥和超精细结构常数A//,A⊥.计算结果与实验发现很好吻合.由于晶体中顺磁杂质中心的EPR参量与其缺陷结构密切相关,本计算还获得了V4 杂质中心缺陷结构的信息.对上述结果进行了讨论.     

Linear Optical Properties of the Nonlinear Optical Calcium and Strontium Tartrato-Antimonates (III), Ca[Sb2 {(+) - C4H2O6}2] x2H2O and Sr[Sb2{(+) - C4H2O6}2 ] x2H2O

The study of nonlinear optical properties of crystals requires a thorough knowledge of the linear optical properties: knowledge of the transparency range is necessary, some information about the optical activity is useful. Above all, high quality refractive indices (with errors less than 10^-4) and their dispersion are the most important basic data, as they enable a detailed analysis of phase-matching conditions of nonlinear optical processes and an estimation of the nonlinear optical potential of a new non-centrosymmetric crystal. Such high quality refractive indices are also necessary in other areas of nonlinear optics, for example for the evaluation of nonlinear optical measurements like measurements of Maker fringes. In this short communication we present the linear optical basis for nonlinear optical investigations of the title compounds. The Sr compound has been known for a long time; its morphological crystallography is already presented in the famous Chemische Krystallographie by Paul Groth (1910). The Ca salt was first synthesized 1983 by one of us (L.B.).