On the spontaneous electro-optic effect in triglycine sulphate crystals with L-threonine admixture

The temperature and spectral dependences of the coefficients R _i2* of the quadratic electro-optic effect (EOE) in triglycine sulphate crystals with 5% L-threonine admixture have been calculated using the temperature and spectral dependences of the refractive indices n _i . It is established that the coefficients R _i2* of the admixture-containing triglycine sulphate crystals exhibit weak spectral and temperature dependences. In the polar direction, R _i2* are substantially different from the coefficients of admixture-free triglycine sulphate crystals. This indicates the occurrence of a bias field, the predominant orientation of the dipole moment of the polar defect, and a decrease in the spontaneous polarization in admixture-containing crystals.     

Influence of optical inhomogeneity of langasite crystals on the value of measured electro-optic coefficients

The electro-optic coefficients in langasite crystals have been determined by the polarization-optical method. It is established that the crystal inhomogeneity (regions with anomalous refraction and a gradient of the refractive index) affects the measured value r _ij . The influence of polarization of langasite elements on the electro-optic effect during measurements in a dc electric field is shown. A technique used to measure the electro-optic coefficients in a dc electric field on the basis of measurement of the shift of interference fringes in a weakly diverging laser beam is proposed.     

Piezo-optic properties of K2SO4 crystals

The effect of uniaxial stresses applied along the principal crystallographic directions on the dispersion and temperature dependences of birefringence of K₂SO₄ crystals is studied. The birefringence is found to be sensitive to uniaxial stresses. The spectral and temperature dependences of the combined and absolute piezo-optic coefficients of these crystals are investigated. Their significant anomalies are observed at the ferroelectric phase transition. They are caused by the change in the induced birefringence due to the occurrence of spontaneous deformation. The contributions of the electro-optic and elasto-optic effects to the spontaneous variations in the piezo-optic constants are separated.     

Distribution of aluminum and indium impurities in crystals of Ge-Si solid solutions grown from the melt

The problem regarding the distribution of aluminum and indium impurities in bulk crystals of solid solutions with a variable composition Ge_1−x Si _x (0 ≤ x ≤ 0.3) is solved in order to establish regularities of the changes in the segregation coefficients of impurities with variations in the composition of the host lattice in the germanium-silicon system. Aluminum-and indium-doped crystals of Ge_1−x Si _x (0 ≤ x ≤ 0.3) solid solutions with a silicon content decreasing along the crystallization axis are grown by a modified Bridgman method with the use of a silicon seed. The concentration distribution of impurities over the length of the crystals is determined from Hall measurements. It is demonstrated that the experimental data on the concentration distribution of impurities in the crystals are in good agreement with the results obtained from the theory according to which the equilibrium segregation coefficients of impurities vary linearly with a change in the composition of Ge-Si solid solution crystals.

     

Optical Properties of TGS Crystal with <Emphasis Type="Italic">L</Emphasis>-Valine Admixture

The thermal expansion and temperature and the spectral dependences of the refractive indices and birefringence of triglycine sulphate (TGS) crystals with a 5% L-valine admixture have been investigated. It is established that the introduction of L-valine weakens the temperature dependence of the refractive indices and the birefringence and thermal expansion of TGS crystals. The parameters of the Sellmeier formula, refractions, and electronic polarizabilities are calculated. The changes observed may be related to the increase in hardness of admixture-containing crystals, the decrease in the spontaneous polarization, the replacement of the refraction components of the valine bond, or the spontaneous electro-optic effect.     

On the orientation dependence of effective distribution coefficients of impurities

Germanium crystals doped by antimony are grown by the Czochralski technique along 12 different crystallographic directions. The effective distribution coefficients (K_ef) are determined here. It is shown that orientation dependence of k_ef could be explained by supercooling at growth surfaces correlating with number of layer systems {111} crossing that surfaces and with angles between that systems and direction of shift of the crystallization front.     

Bilayered Super-Structures of Antiferroelectric Mesogens

Abstract

Crystal structures of two antiferroelectric mesogens, TFMHPBC and MHPBC-10, were analysed by an X-ray diffraction method. In both crystals, mesogen molecules formed a herringbone structure which was essentially the same as that proposed for the antiferroelectric liquid crystal phase. Because of the crystallographic 2₁-symmetry along the b-axis, only the b-axis component of the dipole moment remains in a smectic layer. Since the dipole moment in the next layer has the same magnitude but the opposite direction, both crystals show no dipole moment as a whole. These structural features observed in their crystal states seem to be conserved in their antiferroelectric liquid crystal phases which are just above their crystal phases.     

Composition,morphology, and properties of sodium-bismuth tungstate crystals

A correlation has been revealed between the formation conditions, composition, morphology, and physicochemical properties of NaBi_{1 − x} (W_{1 + y} O₄)₂ crystals (0 ≤ x ≤ 0.16, 0 ≤ y ≤ 0.11). The effect of melt deviation from stoichiometry on the decomposition rate of grown crystals under electron beam irradiation is shown by transmission electron microscopy. A nonuniform dopant distribution over the crystal boule cross-section is found by microprobe analysis. The Raman spectra of the samples depend on their crystallographic orientation.

     

Captured Orientational Order in Polymer Network Assemblies

Abstract

The “marriage” between low molecular weight liquid crystals and polymers burgeoned in the 1980s with the idea of dispersing liquid crystal droplets in a rigid polymer matrix to create an electrically controllable light scattering medium. The orientation of the liquid crystal droplets, and hence the refractive index match and scattering, can be systematically controlled with an electric field. Today, dispersions of liquid crystals and polymers are found in many forms depending on the concentration of polymer, which can be as large as 70% or as small as 1%. The systems most understood are those of larger concentration where the liquid crystal is segregated out in the form of droplets randomly distributed throughout the surrounding polymer (see page 2). Dispersions of liquid crystals and polymers differ from macroscopic bulk liquid crystals because of the large surface-to-volume ratio and symmetry breaking non-planar geometry imposed by the polymer. Their composite nature profoundly affects the ordering of the liquid crystal, and their susceptibility to external fields makes them suitable for many new electro-optic applications, as well as intellectually challenging to study from the basic science perspective.     

Dielectric properties of fine-grained PbTiO3 crystals precipitated in a glass

The spontaneous deformation of PbTiO₃ crystals precipitated in a PbOTiO₂Al₂O₃SiO₂ glass was found to decrease with decreasing grain size. This trend remains even after the glass matrix surrounding PbTiO₃ crystals is removed completely by HNO₃ acid immersion. Consequently, a linear electro-optic effect cannot be expected from transparent glass-ceramics which necessarily comprise very-fine-grained crystals. The dielectric constant of PbTiO₃ crystals surrounded by a glassy matrix shows two maxima at grain sizes 0.15 μm and 250 Å, but is still fairly high even at a grain size as small as 160 Å. This suggests that a material with a large quadratic electro-optic effect may be produced from a transparent glass-ceramic. The maxima of the dielectric constant at grain sizes 0.15 μm and 250 Å were interpreted in terms of the internal stress and internal electric field, respectively.     

Piezo-optic properties of TGS crystals doped with <Emphasis Type="Italic">D</Emphasis>-serine

The effect of uniaxial mechanical pressure along the main crystallophysical axes on the principal values of birefringence Δn _i in TGS crystals doped with 5% D-serine is investigated. It is ascertained that the values of Δn _i are rather sensitive to the action of uniaxial stresses. The temperature shift coefficients ?T _c /?σ_m are determined for the phase-transition points. The temperature and spectral dependences of the combined piezo-optic constants π _im ⁰ are calculated. The contribution of the secondary electro-optic effect to the changes in the birefringence and the piezo-optic constants of doped crystals is determined. It is found that the piezooptic constants π ₂₃ ⁰ and π ₁₂ ⁰ have the same values, which indicates a decrease in the anisotropy of the optical indicatrix of doped crystals under the action of uniaxial stress. The optical and deformation contributions to the relaxation effect of piezo-optical birefringence in doped TGS crystals are found.     

Static photoelasticity of gallium phosphide crystals

The piezo-optic effect (POE) in cubic GaP crystals (symmetry class [`4]\bar 43m) is studied in detail by interferometry. The relations for determining the absolute piezo-optic coefficients (POCs) π _im or their combinations on a sample of X/45° cut at all allowable geometries of the experiment are recorded. The determination of a specific coefficient π _im at different experimental geometries on samples of right cuts and a X/45° cut made it possible to find the π _im values with a high accuracy and reliability.     

Dependence of the Tilt Angle on External Forces for Smectic-C And Chiral Smectic-C Liquid Crystals—Measurement of the Heat Capacity of DOBAMBC

A measurement of the specific heat of DOBAMBC is presented, and it is shown that a Landau expansion of the free-energy density with a sixth-order term is adequate to describe the measurement. The coefficients in the Landau expansion are estimated from this measurement.

The dependence of the tilt angle on applied electric and magnetic fields is discussed for both smectic-C and chiral (ferroelectric) smectic-C liquid crystals. Numerical estimates of the Landau coefficients are used to show that the effect of external forces on the tilt angle is negligible except when being less than approximately 0.2 K from the transition to the smectic-A phase. The effect on the smectic-C-smectic-A phase-transition temperature is also discussed.

Boundary effects can also affect the tilt angle, and an expression for the penetration depth of a boundary-induced disturbance is calculated. For thin, planar samples, a coupling between the two boundaries can induce a transition to the smectic-A phase. It is demonstrated that this transition is analogous to the ordinary Freederickz transition in nematic liquid crystals.     

Micromorphology of pure and PbI2‐doped CdI2 dendritic single crystals

Undoped and PbI₂‐doped dendritic single crystals were grown by vapour growth technique. The basal surfaces of the as grown crystals were examined by optical and electron microscopy to observe wide variety of growth and defect features. Apart from typical features of dendritic growth, features of overgrowth, slip bands, growth steps and their bunching, etc. were observed. The basal surfaces of the crystals were then etched by controlled condensation of water vapour, after optimizing the etching condition, and the microscopic studies were repeated. Etch pits of hexagonal and triangular shape, both symmetric and asymmetric, and of different density, were observed in the case of undoped and doped crystals, respectively. In some cases, crystallographic hillocks were also observed. The crystals were also examined by X‐ray diffraction for their polytypism and related behaviour. The results are analyzed to elicit information on the correlation of structure, defects and surface features of the crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

OD Structures, — a Game and a Bit More

The notion of OD structures consisting of layers is explained, and visualization of its main features facilitated by a simple game: the player is asked to stack prefabricated layers periodic in 2 dimensions, which are all of the same kind or of a small number of different kinds, in accordance with some rules which correspond to the vicinity condition (VC). As shown, important features of the sequences of layers depend only on their symmetry (i.e. layer groups) and on geometrical features of any of the kinds of pairs, especially the layer group of any of the pairs. Table 1 shows that OD crystals consisting of layers occur amongst the most diverse chemical substances. In Tables 2 and 3 some examples of OD crystals with a variety of relations between crystallochemical entities and OD layers are listed. In the Appendix and Table 4 new and simplified formulae for the number Z_v'(v) of possible positions of a certain layer L_v′ relative to the fixed position of its predecessor (or successor) L_v are given.     

X-ray diffraction study of (TlInSe<Subscript>2</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>(TlGaTe<Subscript>2</Subscript>)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> crystal system

The crystallographic and dynamic characteristics of TlInSe₂ and TlGaTe₂ crystals have been studied by X-ray diffraction in the temperature range of 85–320 K. The temperature dependences of the unit-cell parameters a of TlInSe₂ and TlGaTe₂ crystals, as well as their coefficients of thermal expansion along the [100] direction, are determined. The concentration dependences of the unit-cell parameters a and c for (TlInSe₂)_{1 − x} (TlGaTe₂) _x crystals are measured. Anomalies are found in the temperature dependences of the unit-cell parameters a and, correspondingly, the coefficient of thermal expansion, indicating the existence of phase transitions in TlInSe₂ and TlGaTe₂ crystals.     

Electrooptical Properties of Triglycine Selenate

Materials having noticeable electrooptical properties can be used in different mechanisms for modulation and scanning of light. In this connection a detailed study of the electrooptical effect (EOE) nature and the detection of new materials having electrooptical properties are of great interest. In spite of the fact that the electrooptical properties of triglycine sulphate (TGS) were the subject of research in a number of papers, EOE in isomorphic compounds of TGS has not been studied up till now. The present paper deals with the investigation of the electrooptical effect features in triglycine selenate (TGSe) which represent one of TGS crystallochemical analogues. EOE in TGSe was observed at different directions of the applied electric field: along the ferroelectric axis and perpendicular to it — along OZ. Both longitudinal and transverse electrooptical effects occur in both cases. The features of the temperature variation of longitudinal EOE in TGSe are also considered in this paper.     

Shaped crystal growth of PbTe by the open tube technique

The possibility of growing shaped A^IVB^VI crystals from the vapour phase by the open tube technique was investigated on PbTe. The thermodynamic conditions to reproducible prepare several habits of PbTe crystals — whiskers, tetrahedral prisms and cubes, platelets (a), dendrites, cubic and octahedral skeletons (b) as well as their properties were examined. The crystalline habits (a) are characterized by perfectly mirror-like (100) surfaces and low dislocation densities. The type and concentration of charge carriers are mainly determined by the vapour composition within the crystallization zone and very from n = 3 × 10¹⁷ to p = 1 × 10¹⁹cm⁻³. These properties are the reason of the interest in shaped PbTe crystals for the aim of IR optoelectronic devices.     

Spatial distribution of piezoinduced change in the optical pathlength in lithium niobate crystals

For the first time, the “indicative” surfaces of the constants of the piezooptic effect and their stereographic projections have been constructed with due regard for crystal elasticity for lithium niobate crystals described by the symmetry class 3m. The comparative analysis of these surfaces and the corresponding surfaces of the true piezooptic effect has been performed.     

AFM in Organic Solid State Reactions

Abstract

Organic solid-state reactions are probed with the atomic force microscope (AFM). In all cases phase rebuilding gives rise to characteristic submicroscopic features which change in shape due to phase transformation in later stages of the chemical reaction. Photo-(E/Z)-isomerization of olefin 1 occurs in the crystal, photodimerization of 9-chloroanthracene 3 is used as a probe for characterizing the luminosity distribution of SNOM-tips. Gas/solid imbibition in chiral host 5 proceeds enantiospecifically. Histidine crystals form the dihydrochloride with HC1, ammonia and methylamine react face-selectively with crystalline adipic acid 8, furane-2-carboxylic acid 10 and 2-mercaptobenzothiazole 12. Crystals of olefin 14 add chlorine. Solid-state diazotations and subsequent transformations of the solid diazonium nitrates into triazenes occur quantitatively. Solid/solid pinacol- and benzilic acid rearrangements are probed with the AFM. The features formed by long range molecular movements relate to the crystal packing and are thus different on different faces. Correlations with X-ray structural data are demonstrated. All reactions proceed to completion on a preparative scale and do not produce wastes as do their less selective counterparts if performed in solution.