Vol 64, No 2 (2019)

A successive approach to the solution of the inverse problem of diffraction X-ray topo-tomography is proposed. It is based on the semikinematic solution to the Takagi–Taupin equations for the amplitude of diffracted σ-polarized wave. An example of a point defect of Coulomb type in a Si(111) single-crystal plate under conditions of symmetric Laue diffraction and a set of oblique two-dimensional topographic projections corresponding to the rotation of plane-parallel sample crystal around the diffraction vector [\(\bar {2}20\)] is considered. Iterative algorithms for simulated annealing (SA) and quasi-Newton-type algorithms are used for computer reconstruction of the three-dimensional function of the displacement field around a point defect. The results of computer simulation of the displacement field function based on the data for one 2D projection, corresponding to the point defect image in an X-ray topogram in classical X-ray diffraction topography, are presented.

Minimization of generating sets (sets of generators) indicated in the International Tables for Crystallography has been performed for tetragonal, trigonal, and hexagonal space groups. It is established that generating sets of some groups cannot be minimized to a set of minimally possible power (minimal set) for a given group, which suggests that the generating set used in the International Tables for Crystallography should be improved.

The structural characteristics of a new structure for photoconductive antennas have been investigated. This structure is a multilayered epitaxial film grown on a GaAs(111)A substrate; it consists of alternating undoped low-temperature grown GaAs (LTG-GaAs) layers and GaAs layers synthesized in the standard high-temperature regime and doped with silicon (GaAs:Si). The As₄/Ga flow ratio γ is chosen such as to produce p-type GaAs:Si layers. LTG-GaAs layers were grown at an enlarged γ value. The samples grown on GaAs(100) substrates were single-crystal, whereas the single-crystal growth on GaAs(111)A substrates changed to polycrystalline when the film thickness reached 320–340 nm. The sizes of As precipitates in annealed samples, their distribution over the film thickness, and specific features of their crystal structure have been analyzed.

The defect structure of as grown Sr_1 – xNd_xF_2 + x (x = 0.10–0.50) crystals grown from a melt by directional solidification under identical conditions has been studied by X-ray diffraction. All crystals belong to the CaF₂-type structure (sp. gr. \(Fm\bar {3}m\)). The change in the type of structural defect cluster with a change in the NdF₃ content, which occurs in some range of x rather than stepwise, was traced for the first time in the entire homogeneity range of fluorite nonstoichiometric phase Sr_1 – xNd_xF_2 + x. Clusters of two types coexist in crystals with х = 0.10 and 0.25: tetrahedral [Sr_4 – nNd_nF₂₆] and cuboctahedral [Sr_14 – nNd_nF_64 + n]. With a further increase in the NdF₃ content in the Sr_1 – xNd_xF_2 + x structure, a transition from two types of clusters to only one type occurs, and only tetrahedral clusters are formed in crystals with х > 0.40.

A novel sodium and chromium borophosphate, obtained by hydrothermal synthesis in the Na₂O–P₂O₅–B₂O₃–Cr₂O₃–Н₂О system, has been investigated using X-ray diffraction and IR spectroscopy. The monoclinic cell parameters are found to be a = 10.4220(3) Å, b = 8.2468(2) Å, c = 9.2053(3) Å, β = 116.568(4)°, and V = 707.63(4) ų; sp. gr. С2/с. The structure is interpreted and refined to the final reliability factor R₁ = 2.23% in the anisotropic approximation of atomic displacements using 1311 reflections, I > 2σ(I). The novel compound is relative to the family of alkaline borophosphates with the general formula A{M[BP₂O₇(OH)₃]} (A is an alkaline or alkaline-earth element; M is a transition метал) and is described by the crystallochemical formula (Z = 4) Na{Cr[BP₂O₇(OH)₃]}, where the compositions of borophosphate anion and microporous framework are in square brackets and braces, respectively. The presence of alternative systems of strong symmetric hydrogen bonds is established; the localization of all hydrogen atomic sites has made it possible to analyze them in detail. An additional electron density peak is found, which can be interpreted as a statistically distributed proton H⁺. The IR spectrum is characteristic of compounds with strong hydrogen bonding; it confirms the statistical proton distribution over subsites around the inversion center. It is suggested that representatives of this family may possess proton conductivity.

Crystals of new polar germanosilicate with a general formula K_1.46Pb_1.54Ca [(Ge_0.23Si_0.77)₃O₉](OH)_0.54 · 0.46H₂O were synthesized under hydrothermal conditions. The orthorhombic unit cell parameters are a = 14.0879(3) Å, b = 5.9202(1) Å, c = 13.0465(3) Å, sp. gr. Pmn2₁. The crystal structure was determined by direct methods using the SHELX program package and refined to R = 0.0388 based on 3643 reflections with I ≥ 1.96σ(I). The new compound is characterized by a wide isomorphism in the sites occupied by Ge and Si, K and Pb, (OH) and H₂O, only three of the ten cation sites in the structure being occupied by Pb, K, and Ca atoms without impurities. The anionic radical comprises wollastonite three-membered tetrahedral chains linked by Pb1 octahedra to form a mixed framework known for K,Sc silicate and the high-temperature modification of K,Ho silicate. The umbrella-like coordination of the lead atom in the site Pb2(Pb_0.64K_0.36) (the fourth coordination bond is formed with a water molecule or an OH group in the channel of the structure) reflects the fact that the lone pair of Pb atoms is inactive. Although the crystals are acentric, they apparently do not have good nonlinear-optical properties.

A novel hydrated cesium hydroxypentaborate is synthesized from boric acid flux at 180°С, and its crystal structure is determined by X-ray diffraction. The compound crystallizes in the triclinic system, sp. gr. P\(\bar {1}\), with the parameters a = 7.8107(5) Å, b = 9.1929(8) Å, c = 12.3553(11) Å, α = 98.98(1)°, β = 106.32(1)°, γ = 91.10(1)°, V = 839.1(1) ų, Z = 4. The structure is refined to R₁ = 5.8% with anisotropic displacement parameters for all nonhydrogen atoms, with allowance for the non-merohedral twinning by a twofold axis coinciding with the b axis. The twin components were 0.546(1) and 0.454(1). The features of the hydrogen bonds in Cs[B₅O₇(OH)₂] · 0.5H₂O are analyzed in comparison with isostructural rubidium analogue. The unit-cell parameters a and c of A[B₅O₇(OH)₂] · 0.5H₂O pentaborates are found to systematically increase with an increase in the radius of A cation in the NH₄ → Rb → Cs series; the small variation in the b parameter is due to the rigidity of the pentaborate chains elongated along this direction.

Synthesis, X-ray diffraction analysis, and IR spectroscopy of Sr₂[UO₂(CH₂C(CH₃)COO)₃]₄ · 26H₂O (I) and Ba₂[UO₂(CH₂C(CH₃)COO)₃]₄ · 26H₂O (II) (CH₂C(CH₃)COO⁻ is a methacrylate ion) crystals have been performed. The uranium-containing structural units I and II are one-core complexes [UO₂(CH₂C(CH₃)COO)₃]⁻, which belong to the crystallochemical group \(AB_{3}^{{0{\text{1}}}}\) (А = \({\text{UO}}_{2}^{{2 + }}\), B⁰¹ = CH₂C(CH₃)COO⁻) of uranyl complexes. Is established that the parallel orientation of methacrylate anions of neighboring uranium-containing complexes plays an important structure-forming role in crystals I and II. The bulk layers formed due to the interactions of methacrylate anions contain voids, which are occupied by Sr or Ba cations. Linking of structural fragments into a framework is due to the formation of a system of hydrogen bonds and electrostatic and van der Waals interactions.

A composite microfluidic chip (MFC), which comprises a technological system for protein crystal growth followed by X-ray crystallography, was developed. Crystallization tests were performed using chicken egg-white lysozyme, aminotransferase from Methanococcus vannielii, and esterase PMGL2 from the permafrost metagenomic library. Materials (borosilicate glass and poly(methyl methacrylate)) for MFC fabrication were compared. The use of poly(methyl methacrylate) was shown to allow comparative in situ X-ray crystallography of protein crystals directly in the chip. Investigations and experiments were carried out at the BELOK beamline at the Kurchatov synchrotron radiation source (National Research Centre “Kurchatov Institute”).

Raman scattering spectra have been investigated in La₃Ga₅SiO₁₄ langasite crystals with Cr and Fe impurities and La₃Ga_5.25Ta_0.25Si_0.5O₁₄ crystalline solid solution. It is shown that all spectra contain a low-frequency peak near 110 cm^–1. The La₃Ga_5.25Ta_0.25Si_0.5O₁₄ crystal exhibits high-frequency peaks in the range from 1000 to 2000 cm^–1, which are much stronger than the low-frequency peaks. The intensities of the Raman peaks recorded in the La₃Ga₅SiO₁₄ crystal are close in the entire spectral range. The differences in the spectra of the ideal single crystal and solid solutions can be explained by the manifestation of additional modes in the solid solutions.

The spectral and the total density of states are calculated for two-dimensional FeAs-clusters within the framework of the two-orbital model, which is widely used for modeling iron-based superconductors. An asymptotically accurate stochastic procedure is implemented that restores the kernel of the integral equation relating the Matsubara Green’s function and the spectral density of states. The data for the Matsubara Green’s function are obtained using the generalized Monte Carlo quantum algorithm. The data are presented for the momentum distribution of occupation numbers in the first Brillouin zone and along the main crystallographic directions.

The amplification of spontaneous fluorescence in a planar layer of nematic liquid crystal doped with DCM dye in the mode of waveguide light propagation has been studied. The gain reaches the value α = 0.0014 μm^–1 at a pump radiation intensity of 1.38 MW/cm². Numerical simulation of the structure imitating the experimental cell showed qualitative agreement of the calculation results with the experimental data.

The specific features of magnetron sputtering deposition of thin SmS films for thermoelectric generators that do not require any forced formation of temperature gradient have been investigated. An experimental design of a thermoelectric generator is developed in the form of a set of Ni–SmS–Ni films successively deposited on a Ti substrate. The generation of thermopower in a thin SmS film is demonstrated on an experimental sample.

The effect of electric charges and electrostatic fields on the growth of different films on the surface of dielectric crystals has been investigated. It has been established that the surface charge does not affect the nucleation but determines the diffusion processes. A negative surface charge facilitates high mobility of growing islands, which finally leads to the formation of a continuous film. The strain anisotropy is discussed from the viewpoint of anisotropic distribution of charges induced by the substrate electric field on an island.

The structure and magnetic properties of iron carbide nanoparticles encapsulated into carbon shells, obtained as a result of ferrocene transformations at a pressure of 8 GPa and different temperatures, have been investigated by powder X-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy. It is found that the main crystalline carbide states are hexagonal phase Fe₇C₃ and cementite Fe₃C, whose relative contents are determined by the treatment temperature. Mössbauer spectroscopy was used to identify three nonequivalent states of iron atoms in the structural sites of Fe₇C₃ hexagonal lattice. It is established that the occupancies of these sites differ from the known values for the bulk material, which is related to the specific features of structure formation under conditions of high pressure and temperature.

Chemical deposition from salt solutions (co-deposition) has been chosen and investigated as a method for synthesizing dysprosium hafnate complex oxide Dy₂HfO₅. Regimes for synthesis of low-aggregated single-phase Dy₂HfO₅ nanocrystalline powders using a microwave furnace have been developed. It is shown that the synthesized Dy₂HfO₅ powders subjected to high-energy grinding and mechanical activation consist of crystallites with a size of no larger than 10 nm and a specific surface of no smaller than10 m²/g. High-density ceramics has been prepared by uniaxial compacting of mechanically activated nanocrystalline powders with subsequent isothermal annealing of the compacts. Test batches of the Dy₂HfO₅ pellets with a density ranging from 8.25 to 9.10 g/cm³, a thermal conductivity varying from 1.21 to 1.63 W/(m K), a thermal expansion coefficient in the range from 7.1 to 8.7 × 10^–6 K^–1, and Young’s modulus in the range from 168 to 221 GPa have been fabricated and certified.

A mathematical model has been developed and numerical calculations of lysozyme protein crystallization from a homogeneous aqueous solution under temperature field control have been performed in order to develop a controlled method for growing high-quality protein crystals from solutions using a point temperature field effect. This mathematical model describes the formation of crystal nuclei and their growth in dependence of local values of supersaturation and temperature, as well as the heat and mass transfer in the entire volume of solution, including protein crystals.

K₂Co(SO₄)₂ · 6H₂O crystals have been grown by lowering temperature with the formation of controlled mass transfer by means of low-frequency harmonic oscillations of a chemically inert body immersed in a solution, i.e., by a technique of low-frequency axial vibration control technique. It is shown that application of this technique allows one to increase the volume growth rate by a factor of ~2 and improve the structural quality of crystals, i.e., to decrease the dislocation density on different faces by a factor of 10–100. An analysis of the transmission spectra in the range of 200–400 nm showed that the grown crystals are more transparent (by 26%) than the crystals grown in the absence of vibrations.

A new automated system for controlling crystallization setups, providing their reliable operation and remote control, has been developed. The principles of constructing a system controlling many setups for growing crystals from low-temperature solutions and its functioning are described.