Precision electron diffraction structure analysis and its use in physics and chemistry of solids

The present level of the precision electron diffraction provides the quantitative analysis of the electrostatic potential and electron density in crystals and allows us to approach the direct study of properties of solids by electron diffraction. This is demonstrated by examples of ionic compounds with an NaCl structure and a covalent Ge crystal. Using the analytical structure models of crystals, one can quantitatively characterize chemical bonding and study topological characteristics of the electrostatic potential by electron diffraction data. It is established that the internal crystal field is well structurized, whereas the topological analysis revealed some important characteristics of its structure. These data considerably enrich our knowledge on atomic interactions in crystals.     

The effects of microgravity on protein crystallization: evidence for concentration gradients around growing crystals

Atomic force microscopy (AFM) investigations have revealed that macromolecular crystals, during their growth, incorporate an extensive array of impurities. These vary from individual molecules to large particles, and microcrystals in the micron size range. AFM, along with X-ray topology, has further shown that the density of defects and faults in most macromolecular crystals is very high in comparison with conventional crystals. The high defect density is a consequence of the incorporation of impurities, misoriented nutrient molecules, and aggregates of molecules. High defect and impurity density, contributes to a deterioration of both the mechanical and the diffraction properties of crystals. In microgravity, access by impurities and aggregates to growing crystal surfaces is restricted due to altered fluid transport properties. We designed, and have now constructed an instrument, the observable protein crystal growth apparatus (OPCGA) that employs a fused optics, phase shift, Mach–Zehnder interferometer to analyze the fluid environment around growing crystals. Using this device, which will ultimately be employed on the international space station, we have, in thin cells on earth, succeeded in directly visualizing concentration gradients around growing protein crystals. This provides the first direct evidence that quasi-stable depletion zones formed around growing crystals in space may explain the improved quality of macromolecular crystals grown in microgravity. Further application of the interferometric technique will allow us to quantitatively describe the shapes, extent, and magnitudes of the concentration gradients and to evaluate their degree of stability.     

Effects of melt composition on structural and electrical characteristics of LEC Si-doped gallium arsenide

The aim of this paper is that of briefly reviewing the experimental observations carried out on Si-doped gallium arsenide grown from off-stoichiometry melts (either Ga- or Asrich) in the author's laboratory. It will be shown that the melt composition strongly affect the electrical (electron mobility, density of deep levels) and structural (dislocation density, precipitates) properties of the bulk crystals which proves the existence of a link between point defects and physical and crystallographic characteristics. The experimental results will be discussed considering recent literature reports on point defects in bulk GaAs.     

不同位置Mg掺杂ZnO电荷转移和光学性质的第一性原理研究

基于密度泛函理论计算了本征氧化锌、6.25％Mg以及同位、邻位、间位12.5％Mg原子掺杂氧化锌晶体的几何结构、原子轨道电子布居、静电势和电子结构特征;探究了Mg原子掺杂对氧化锌能带结构、态密度以及对应的光学性质和电学性质的影响.结果表明Mg掺杂会导致氧化锌晶体的晶格体积变大,载流子迁移率降低和吸收边蓝移;邻位双原子掺...     

Properties of Doped GaSb Single Crystals Grown by the Czochralski Method

The specific electrical properties and average dislocation density of GaSb crystals are shown and discussed regarding various elements presented as dopant. The single crystals were grown by the Czochralski method without encapsulant in a flowing atmosphere of molecular hydrogen, on the one hand, and of atomic hydrogen, on the other hand. The results are summarized in the Table II.     

Vacuum-evaporated ferroelectric films and heterostructures of vinylidene fluoride/trifluoroethylene copolymer

The potential of the vacuum method for preparing ferroelectric films and photonic heterostructures from organic materials is studied. Vacuum-evaporated films of fluoropolymers and heterostructures on their basis are obtained and their ferroelectric and spectral properties are studied. In particular, homogeneous films of the well-known piezoelectric polymer polyvinylidene fluoride and ferroelectric material vinylidene fluoride/trifluoroethylene copolymer (P(VDF/TFE)) are produced. Experimental studies of vacuum-evaporated P(VDF/TFE) films confirmed their ferroelectric properties. The heterostructures composed of alternating layers of P(VDF/TFE) copolymer molecules and azodye molecules are fabricated by vacuum evaporation. Owing to the controlled layer thickness and a significant difference in the refractive indices of the P(VDF/TFE) copolymer and azodyes, these heterostructures exhibit properties of photonic crystals. This finding is confirmed by the occurrence of a photonic band in the absorption spectra of the heterostructures.     

Crystal structure of 2-mercaptobenzimidazole and bis[2-mercaptobenzimidazole]dichlorocobalt(II)

Delocalization of the imidazole electron density is observed in both structures. In the dichlorocobalt complex structure, the two crystallographically independent mercaptobenzimidazole molecules are attached together through the direct bonding of the Co(II) ion with two S atoms. The cohesion of the molecules in the crystals is due to intermolecular N–H...S, and; intra and intermolecular N–H...Cl hydrogen bonds, respectively.     

Protein crystal nucleation: Recent notions

The nucleation of protein crystals is reconsidered taking into account the specificity of the protein molecules. In contrast to the homogeneous surface properties of small molecules, the protein molecule surface is highly inhomogeneous. Over their surfaces proteins exhibit high anisotropic distribution of patches, which are able to form crystalline bonds, the crystallization patch representing only a small fraction of the total surface of the protein molecule. Therefore, an appropriate spatial orientation of the colliding protein molecules is required in order to create a crystalline cluster. This scenario decreases considerably the success ratio of the attempt frequency for crystal nucleation. On the other hand a heterogeneous nucleation of (protein) crystals may be accelerated due to the arrival on some support of under‐critical clusters that are formed in bulk solution; when arriving there they may acquire the property of critical nuclei. Thus, a plausible explanation of important peculiarities of protein crystal nucleation, as inferred from the experimental data, is suggested. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of calcium carbonate in polyacrylamide hydrogel: Investigation of the influence of polymer content

The growth of calcium carbonate crystals has attracted growing attention as a model system for biomineralisation processes. Organic molecules and gelatinous matrices are known to play an essential role in the formation of hard tissues. For the investigation of the function of specific influence factors, a model experiment is necessary. Several hydrogels were previously tested as growth matrices for calcium carbonate.

For laboratory experiments, a double diffusion set-up for the growth of crystals in gels was established earlier. Calcium carbonate crystals were grown in polyacrylamide hydrogels.

Here the influence of the polymer content in the hydrogels on the crystallisation behaviour is reported. Time-resolved and spatially resolved crystallisation experiments were conducted. The collected calcium carbonate precipitates were analysed by light microscopy, scanning electron microscopy and X-ray diffraction.

The morphology of the developing crystals was found to be dependent on the polymer content of the hydrogels.     

In situ data collection and structure refinement from microcapillary protein crystallization

In situ X-ray data collection has the potential to eliminate the challenging task of mounting and cryocooling often fragile protein crystals, reducing a major bottleneck in the structure determination process. An apparatus used to grow protein crystals in capillaries and to compare the background X-ray scattering of the components, including thin-walled glass capillaries against Teflon, and various fluorocarbon oils against each other, is described. Using thaumatin as a test case at 1.8 ? resolution, this study demonstrates that high-resolution electron density maps and refined models can be obtained from in situ diffraction of crystals grown in microcapillaries.     

Structural reasons for the nonlinear optical properties of KTi<Subscript>0.96</Subscript>Zr<Subscript>0.04</Subscript>OPO<Subscript>4</Subscript> single crystals

This paper reports on the results of the precision X-ray structural investigations of KTi_0.96Zr_0.04OPO₄ single crystals at room temperature. It is established that the incorporation of zirconium atoms into the structure of KTiOPO₄ (KTP) crystals does not lead to substantial changes in the framework structure and results only in an insignificant decrease in the scatter of the distances in the PO₄ tetrahedra and the formation of more symmetric (TiZr)O₆ octahedra as compared to the TiO₆ octahedra. However, the incorporation of zirconium atoms into the KTP structure is accompanied by the redistribution of the electron density in the crystal as a whole, so that the electron density increases in the region of the positions occupied by the potassium atoms. This changes the nonlinear optical properties of the given series of crystals, which are estimated from the intensity of the second harmonic generation signals.     

Linear optical properties and Raman spectroscopy of natural fluorapatite

Precision refractive indices and their dispersion in the visible and near IR range (430 – 2400 nm) and thermal expansion (including its anisotropy and temperature dependence between 144 K and 673 K) were determined for gem‐quality crystals of fluorapatite from Durango, Mexico. In addition, results of a polarized Raman spectroscopy study on these crystals are given. Fluorapatite crystals show moderate values of thermal expansion with small anisotropy. The linear optical properties of the crystals allow no phase matching for third harmonic generation, but signalize, together with the Raman characteristics, a potential suitability of fluorapatite as nonlinear optical material for χ⁽³⁾‐based stimulated Raman scattering.     

Terrestrial and space-grown HAP and OCP crystals: effect of growth conditions on perfection and morphology

This paper reports comparative characterizations of calcium phosphate crystals grown on earth and in space. At the CaCl2 and KH2PO4 + K2HPO4 solution concentrations and the pH used, only hydroxyapatite (HAP) crystals grow under terrestrial condition while both HAP and octacalcium phosphate (OCP) crystals grew during the space experiment. The space-grown OCP crystals reach 3 mm in size, the space-grown HAP crystals reach sizes up to 100 times larger than the earth-grown crystallites. It was found also that the space-grown crystallites are more perfect than the terrestrial ones, being more stable under electron beam during HRTEM examination. Spherolites of hydroxyapatite consist of small and thin HAP crystals with different orientations. Space-grown OCP crystals containing almost pure OCP phase show strong striations along the c direction due to thickness variations. Terrestrial OCP crystals grown at lowest supersaturation on earth may be almost as large as the space-grown ones, possess a regular habit and are homogeneous in thickness. However, they always contain substantial regions of HAP structure. Also, in these crystals electron irradiation induces phase transformation from crystalline to amorphous (disordered) state during transmission electron microscopy observations. In the space-grown crystals, such transformation needs longer radiation time. We believe that the differences described above come from much lower supersaturation and different pH for crystals nucleating and growing in space compared to those formed on earth.     

Pseudosymmetry and some characteristics of pyroelectric properties of crystals

The relation between pseudosymmetry characteristics of the atomic structure and the values of the pyroelectric coefficients of model and real pyroelectric crystals has been studied. It is established that the electron-density projection onto the polar axis of crystals with the highest values of pyroelectric coefficients possesses the property of pseudosymmetry, i.e., that the considerable part of the electron density is invariant with respect to an additional center of symmetry.     

Macroscopic Phenomena in Polymer Liquid Crystals

Rigid and semirigid backbone polymers based on either helical structures or conjugated aromatic bonding often exhibit nematic ordering in solution or as melts. Because of the large axial ratio of these molecules, the resulting nematic phases exhibit hiahly anisotropic elastic and viscous properties. The study of macroscopic phenomena in these liquid crystals serves two purposes. First is the elucidation of the relationship between macroscopic material parameters and molecular properties. Second is the understanding of the consequence of the highly anisotropic nature of these materials, namely new phenomena not observed in low molecular weight liquid crystals. Several new phenomena will be described related to the Frederiks transition and flow instabilities.     

Precipitates of MnSi cubic phase in tetragonal Mn4Si7 crystal

Higher manganese silicides (HMSs) exhibit interesting thermoelectric and optoelectronic properties. Development of HMS-based thermoelements and microthermopiles of different designs may meet a number of problems, which can be solved only when the real structure of crystals and thin layers on which they are based is established. We have applied scanning and transmission electron microscopy and electron diffraction to investigate HMS crystals of two types: single crystals grown from melt by the Bridgman method and microislands formed by reactive diffusion during manganese vapor deposition on silicon substrates. The exact phase composition of these materials is established: matrix HMS crystal belonging to tetragonal system (Mn₄Si₇ composition) and precipitates of cubic manganese monosilicide MnSi. The shape and sizes of precipitates are determined, the crystallographic relationships between the tetragonal and cubic phases are found, and the interface is investigated.     

Undoped semi-insulating GaAs crystals grown by a modified low pressure LEC method

A modified low pressure in-situ synthesis LEC method of growing undoped SI (semi-insulating) GaAs crystals has been established. The key points for controlling melt composition and As evaporation during synthesis and growth have been described. Using this novel approach, crystals are able to be grown from the nominal melt composition in the range of 0.491–0.499 As fraction with high reproducibility. Some characteristics of the undoped SI crystals grown by the present work including electrical properties, dislocation density, carbon and EL2 concentrations and thermal annealing effects have been studied.     

Nonlinear optical response of fullerenes

A critical review of nonlinear optical properties of fullerenes is presented. We explore structure property relationships for optical nonlinearities of these unusual π electron systems from two complementary angles: (i) to explore the possibility of obtaining structural information from nonlinear optical experiments and (ii) to assess the potential of fullerenes, fullerites and their derivatives for the practical nonlinear optical devices. In particular we show that substituted fullerenes form an interesting class of molecules for nonlinear optics. We will also show that third harmonic generation of circularly polarised light in the solid fullerites should yield some information on the intermolecular interactions in these solids.

We critically examine the various calculations indicating that the screening of the optical fields due to electron-electron interaction may reduce the optical susceptibilities of these molecules by a large factor and suggest experiments that may clarify the issue.     

Structural and mechanical properties of KН2PО4 single crystals with embedded nanoparticles and organic molecules

Single crystals of KDP crystals with embedded Urea molecules and TiO₂ nanoparticles have been grown from aqueous solution by the temperature lowering method. The effect of the organic molecules and nanoparticles on the structural and mechanical properties has been studied. It has been observed that addition of Urea molecules improves laser induced damage threshold and mechanical strength of the crystal, while TiO₂ nanoparticles have the opposite effect. The structure and composition of KDP:Urea crystal are studied by three‐crystal X‐ray diffraction analysis, which reveals the existence of a correlation between the increase of the microhardness value and the change of the crystal lattice parameter. The surface features of KDP:TiO₂ crystals are analyzed by scanning electron microscopy that reveals the presence of quasi‐equidistant growth bands caused by capture of the nanoparticles. It is shown that the rise of TiO₂ nanoparticles concentration up to 10⁻⁴ wt.% and higher resulted in 3‐fold reduction of the laser damage threshold of KDP:TiO₂ relative to pure KDP in [001] and [100] crystallographic directions. It is found that microhardness and fracture toughness decrease at the nanoparticles concentration of 10⁻³ wt.% due to crack formation at crystal lattice discontinuities. The grown crystals also have been subjected to dielectric studies.     

铌酸锂晶体的生长研究

近年来,铌酸锂晶体由于其自身所具有的多种优异性能和巨大的应用前景而受到了人们的广泛关注,但生长出满足不同市场要求的高质量铌酸锂单晶体比较困难.本文从晶体生长技术的角度综述了铌酸锂单晶体不同的生长方法以及各自的特点,并分析了在生长铌酸锂晶体时出现的一些问题.