X-ray diffraction analysis of modulated crystals: Review

The state of the art of X-ray diffraction analysis of modulated crystals is reviewed. The review begins with a brief historical overview followed by the consideration of the main concepts and notations used in this field. Then, methods of structural analysis of modulated crystals are considered with emphasis on recent achievements. The most interesting objects are listed, and the directions of investigation are outlined. Examples of analysis of both individual structures and families of modulated and incommensurate composite structures are given in terms of superspace symmetry.     

Neutron diffraction and X-ray diffraction investigations of langasite crystals

Crystals of langasite La₃Ga₄(GaSi)O₁₄ grown by the Czochralski method are studied using neutron diffraction for the first time. It is established that the compositions of the upper and lower parts of an orange crystal grown from the La₃Ga_5.14Si_0.86O₁₄ seed in an (Ar + O₂) atmosphere (the 〈 0001 〉 growth direction) can be written as(La_2.85(2)□_0.15)(Ga_0.95(2)□_0.05) Ga₃(Ga_1.15 Si_0.85(5))(O_13.72□_0.28(7)) and(La_2.89(1)□_0.11)·(Ga_0.98(1)□_0.02) Ga₃(Ga_1.06Si_0.94(4))(O_13.81□_0.19(9)), respectively. The La content in the upper and lower parts of this crystal is lower and higher than the Ga content, respectively, and the Ga content exceeds the Si content in the (GaSi) position. By contrast, in a colorless crystal of the composition(La _2.97(4)□_0.03) Ga(1)(Ga_2.94(9)□_0.06)(Ga_0.7(1) Si_1.3)(O_13.9□_0.1(1)), which is grown from the La₃ Ga₅SiO₁₄ seed in an argon atmosphere (the 〈 01\(\bar 1\)1 ?growth direction), the Ga content in the (GaSi) position is lower than the Si content. A relation between the Ga: Si ratio and the (Ga,Si)-3O interatomic distances is found.     

Low temperature X-ray diffraction study of CdI2 crystals

Solid state structural transformations in polytypic crystals of cadmium iodide have been investigated at low temperatures using X-ray diffraction techniques. The crystals were cooled to liquid nitrogen and liquid helium temperatures employing three different methods. The results obtained are discussed on the basis of bond dissociation energies of interlayer and intralayer bonds and the possible mode of layer displacements in MX₂-type polytypic crystals.     

Development of methods of X-ray diffraction analysis for determining the composition and structure of sillenite-family crystals

A methodology for refining the crystal structure of sillenites of nominal composition Bi₂₄ M ₂O₄₀ based on the choice of the correct initial model and thermal atomic parameters is reported. The validity of the approach proposed is demonstrated by examples of crystals with M = Si, Fe, or V, for which the real composition is found with allowance for the composition of each structural site. Individual structural details are confirmed by IR and Raman spectroscopy data.     

Broadening of X-ray diffraction lines of crystals containing dislocation distributions

The first part of the paper deals with a brief review over the present stage of the (kinematical) theory of the broadening of X-ray diffraction lines of crystals containing dislocation distributions. In the second part experiments of X-ray line broadening of plastically deformed Cu and NaCl single crystals are reported by which the validity of the theoretical approaches are tested. It is shown that values of the dislocation density and the elastically stored energy derived from broadened X-ray diffraction lines agree well with corresponding data obtained by other experimental techniques.     

X-ray diffraction study of crystals under a static electric field

This paper gives an overview of the effect of electric field on the diffracted intensity and contrast on the X-ray topographs recorded from crystals of non-linear and electro-optic materials, ionic conductors, semiconductors, insulators and ferro-electric materials. Application of a dc electric field gives rise to a change in the diffracted intensity by several orders of magnitude and contrast on the topographs. The effect is illustrated with examples of -LiIO₃, KTiOPO₄, LiN₂H₅SO₄, KNbO₃, LiNH₄SO₄, quartz, silicon etc. and the mechanism for it is discussed.     

X-ray diffraction proof of noncentrosymmetricity of the HTCS crystals of the 1-2-3 family

X-ray diffraction and X-ray topography analysis of ReBa₂Cu₃O_x crystals (Re = Eu, Gd, Y) has demonstrated the noncentrosymmetricity of their atomic structure. It is shown that the centrosymmetricity of these crystals could not have been revealed in the previous structural studies because of their polydomain nature (the crystals consist of antiphase domains) and the use of hard MoK _α radiation.     

Preliminary X-ray diffraction study of crystals of photosystem II from Thermosynechococcus elongates

Photosystem II (PSII) is a multicomponent enzyme complex that catalyzes the light-induced water splitting to molecular oxygen, protons, and electrons. Photosystem II is located in the membranes of cyanobacteria, green algae, and plants. The crystallization of this complex from the thylakoid membranes poses great difficulties. The high sensitivity of photosystem II to light and radiation has an adverse effect on the crystal quality, as well as on the quality of X-ray diffraction data. This is the reason why the crystal structure of PSII from Thermosynechococcus elongates has as yet not been determined at high resolution. The optimization of the strategy for collecting X-ray diffraction data from PSII crystals has resulted in an increase in the resolution to 2.75 Å, which made it possible to determine the positions of ions and some water molecules playing an important role in the functioning of PSII.     

Growth of thiourea-doped TGS crystals and their characterisation

Thiourea-doped Triglycine sulphate (ThTGS) crystal with three different concentrations of thiourea was grown from aqueous solution by slow cooling technique. The cell parameters were determined from powder X-ray diffraction analysis. A qualitative analysis of the presence of thiourea in doped crystals was estimated by FTIR analysis. Microhardness studies were carried out using Leitz Weitzler hardness tester at room temperature. Dielectric properties of the crystals were studied which showed a shift in the T_c when compared to pure TGS crystal. Pyroelectric studies were carried out and the pyroelectric coefficient was found. Piezoelectric studies (d₃₃ coefficient) has also been carried out.     

State-of-the-art and problems of X-ray diffraction analysis of biomacromolecules

The state-of-the-art of X-ray diffraction studies of biomacromolecules is briefly characterized, and the challenge imposed by science is discussed. These studies are characterized by a wide scope and extensive use. This field of science is of great interest and is developed in many countries. The main purpose is to solve practical problems in medicine consisting in the design of drugs against various diseases. X-ray diffraction analysis of enzymes brought the pharmaceutical industry to a new level, thus allowing the rational design of drugs against formerly untreatable diseases. Modern X-ray diffraction studies of biomacromolecules laid the basis for a new science called structural biology. This method allows one to solve fundamental problems of physical chemistry for a new state of matter existing in living systems. Here, science poses numerous problems in analysis of X-ray diffraction data on biological macromolecules. Many of theses problems are in their infancy.     

Quantitative X-ray diffraction analysis of textured sheet metals

A disadvantage of the quantitative X-ray diffraction analysis of samples from sheet metals is the incorrectness of the measuring results owing to the presence of textures in the material. In considering several reflexes of every phase the influence of textures may be reduced. Such a method will be presented and tested. The yielded results show that the new method can be useful for textured sheet metal samples.     

单掺Rb+和Cs+的KTP晶体生长及其电导率

采用高温溶液降温法在掺质浓度均为5mol;的KTP-K4溶液中分别生长了单掺Rb+和Cs+的KTP晶体,发现掺质改变了晶体生长习性,在相应生长体系中掺质Rb+和Cs+的分配系数分别为O.646和0.08,掺质KTP晶体的晶胞参数a0和b0比纯KTP晶体者略有增长.通过掺Rb+或Cs+,KTP晶体的c向电导率明显降低,但晶体在350～1100nm范围内的光透过性质未受影响.     

X-ray diffraction study of crystals of the organic luminophore Orlyum White 520T

The crystal and molecular structures of two crystal forms (pale yellow form 1 and yellow form 2) of N-[2-(4-oxo-4H-benzo[d][1,3]oxazin-2-yl)phenyl]naphthalene-2-sulfonamide (Orlyum White 520T), which is an organic luminophore with an anomalously high Stokes shift, were determined. Crystal 2 is a solvate with para-xylene. Crystal 1 is a solvent-free form. The molecular geometry in crystal 1 differs from that in 2 only in the orientation of the SO₂ Ar substituent. The bond-length distribution in the planar moiety of the molecule in crystal 1 is virtually identical to that in 2, but the bonds in the NH-SO₂ Ar-bearing benzene ring in crystal 1 are systematically longer than the corresponding bonds in crystal 2. This fact can be attributed to the crystal-packing effects. In 2 the molecules form stacked dimers with π-stacking interactions between two planar conjugated tricyclic systems. The charge transfer in this system accounts for the intensification of the color of these crystals and the observed difference in the optical properties of 1 and 2.     

Growth and analysis of barium-sodium niobate crystals

The conditions of growth, monodomenization, and detwinning are considered in order to obtain optically perfect barium–sodium niobate crystals (BSN). Optical homogeneity and that of chemical composition are investigated. It is shown that possible variations of composition in a crystal's volume are below 1%. Thermal E and optical E_g° widths of the forbidden zone are determined in the absorption spectra.     

Structural analysis of CaSiO3 glass by X-ray diffraction and Raman spectroscopy

The short-range structure of CaSiO₃ glass was studied by a combination of radial distribution analysis with an intensity comparison method on the basis of X-ray diffraction data. Three main peaks appeared at 1.64, 2.43 and 3.64 Å in the radial distribution function curve. The Raman bands were observed at 367, 642, 878 and 972 cm⁻¹ and the Raman spectrum was found to be similar to that observed in pyroxenes: single chains consisting of SiO₄ tetrahedra are linked by calcium ions laterally. The calcium ion is surrounded by six oxygen atoms at an average distance of 2.34 Å and the remainder Ca---O pairs at a distance of 2.54 Å. The calculated intensity curve based on this model was in good agreement with the observed one.     

Effect of the defect structure character in calcite single crystals on X-ray diffraction

The paper presents experimental results for the dependence of the integral intensity of anomalous transmission T_i and Laue reflection R_i of X-rays on the structural quality of calcite single crystals as obtained by a two-crystal spectrometer. The relation between X-ray dynamic and kinetic scattering as a function of the sample thickness, densities of perfect dislocations in perfect CaCO₃ crystals and of atom-vacancy complexes in calcite crystals with „background”︁ has been found. Distortions due to perfect grown-in dislocations in calcite are shown to exceed those produced in crystals with „background”︁. The latter appear to be more X-ray transparent. A comparison with results of twin layer thickening experiments on CaCO₃ crystals of different qualities again points to the different nature of these distortions. In CaCO₃ crystals with the dislocation density of up to 2 × × 10³ cm⁻², the X-ray scattering, in our experiments, was consistent with the dynamic diffraction theory, while at higher dislocation densities, the X-ray scattering markedly differs from it.     

Method for complex X-ray diffraction analysis of TiN coatings

A complex of X-ray diffraction methods for study of polycrystalline thin layers is presented. It is based on the approximation of measured profiles by suitable analytical functions. An application to TiN coatings prepared by PVD method on high-speed steel is presented. The necessity to investigate several diffraction parameters (position, integral intensity, breadth, asymmetry) of more reflections is emphasized.     

Growth and properties of (Pb0.90La0.10)TiO3 thick films prepared by RF magnetron sputtering with a PbO buffer layer

The (Pb_0.90La_0.10)TiO₃ [PLT] thick films (3.0 μm) with a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering method. The PLT thick films comprise five periodicities, the layer thicknesses of (Pb_0.90La_0.10)TiO₃ and PbO in one periodicity are fixed. The PbO buffer layer improves the phase purity and electrical properties of the PLT thick films. The microstructure and electrical properties of the PLT thick films with a PbO buffer layer were studied. The PLT thick films with a PbO buffer layer possess good electrical properties with the remnant polarization (P_r=2.40 μC cm⁻²), coercive field (E_c=18.2 kV cm⁻¹), dielectric constant (ε_r=139) and dielectric loss (tan δ=0.0206) at 1 kHz, and pyroelectric coefficient (9.20×10⁻⁹ C cm⁻² K⁻¹). The result shows the PLT thick film with a PbO buffer layer is a good candidate for pyroelectric detector.