Vol 62, No 3 (2017)

Based on the assumption that crystal structures of a number of sulfides are the result of modulation of cationic lattices by anionic lattices, the versions of their conjugation in direct and reciprocal spaces have been analyzed using common translational lattices. The concept of this phenomenon, developed within the superspace formalism, is supplemented by a proposed interpretation of the real modulation of the structures.

In_0.52Al_0.48As/In_0.53Ga_0.47As/In_0.52Al_0.48As HEMT heterostructures on InP substrates with elastically strained InAs insert in combined quantum well (QW) have been investigated using a combination of X-ray methods: double-crystal X-ray diffraction, X-ray reflectivity, and reciprocal space mapping. This approach has provided detailed complementary information about the layered and real crystal structures of the samples. The data obtained have made it possible to perform structural analysis of the multilayer systems and compare their characteristics with specified technological parameters, due to which the HEMT growth technology can be corrected and improved.

Currently, bent silicon single crystals are used at large accelerators to extract and collimate proton beams. A device for multiple deflection of a proton beam based on several bent silicon strips operating in the volume reflection mode has recently been developed. In this device, the bending of silicon strips successively located on the surface of a thick plate is implemented due to the internal stress induced by grooves mechanically formed on the crystal surface (Twyman effect). Topography based on angular scanning and synchrotron radiation was applied to measure the bending of individual deflector strips and the crystal as a whole. The measurement results are compared with the data obtained with a proton beam.

The multicell model alternative to the model of mixed atomic sites used now is proposed for a single crystal of La₃Ga₅GeO₁₄ belonging to the langasite family. The multicell consists of four unit cells. In three identical cells of the structure, atoms adapt to the Ge atom occupying one of the two 2d positions on the threefold symmetry axis. In the fourth cell, atoms surround the Ge atom located at the 1a position. The multicell model allows one to study the short-range order of atoms by the methods of classical structure analysis based on Bragg scattering. Four high-resolution data sets measured at 295 and 111.5 K are used in the study. The results are obtained with high relative precision (space group P321, Z = 1; at 295 K a = 8.2020(6) Å and c = 5.1065(6) Å, R/wR = 0.81/0.73% for 3829 unique ref lections; at 111.5 K a = 8.1939(1) Å and c = 5.1022(4) Å, R/wR = 0.85/0.76% for 3880 reflections).

The crystal structure of the titanium-rich mineral wadeite K₂(Ti_0.55Zr_0.45)Si₃O₉ from rischorrites of the Khibiny Alkaline Massif (Kola Peninsula, Russia) is studied by X-ray diffraction (XCalibur-S diffractometer, R = 0.0459): a = 6.8611(6) Å and c = 10.0611(9) Å; space group P6₃/m, Z = 6, D_x = 3.03 g/cm³. It is shown that the unit-cell parameters and volume of the mineral of mixed (Ti/Zr) composition are naturally intermediate between those of the terminal members of the isomorphous wadeite-based K₂ZrSi₃O₉–K₂(Ti_0.55Zr_0.45)Si₃O₉–K₂TiSi₃O₉ series. The expected correlation is due to the ionic radii of Zr⁴⁺ and Ti⁴⁺ which determine the lengths of Zr/Ti–O bonds in octahedra. The data of field observations and microscopic studies show that the Ti-dominant wadeite is formed on the basis of primary zirconium mineral in the course of a late imposed process under unique geochemical conditions.

The product of the reaction of tetrasodium nitrilotris(methylenephosphonate) Na₄H(NH(CH₂PO₃)₃) with Ca(OH)₂ having the formula [CaNa₆{NH(CH₂PO₃)₃}₂(H₂O)₁₀][Na(H₂O)₆]₂ · 4H₂O (sp. gr. P2₁/c, Z = 2, a = 12.0579(4) Å, b = 10.4569(3) Å, c = 19.9346(4) Å, β = 95.355(2)°) was synthesized and characterized. The complex anion consists of a ring composed of six hydrate-bridged Na atoms. The cavity of the anion incorporates a Ca atom coordinated by two phosphonate ligands in a distorted octahedral geometry. Each of the two complex cations contains a sodium ion octahedrally coordinated by six water molecules. The crystal packing consists of infinite ribbons of the complex anions, which are linked by hydrate bridges to form layers alternating with layers of the complex cations and interlayers composed of disordered water molecules of solvation.

Bacterial phosphopantetheine adenylyltransferase from Mycobacterium tuberculosis (PPAT Mt) is a convenient target protein for the directed search for selective inhibitors as potent antituberculosis drugs. Four compounds suitable for the detailed investigation of their interactions with PPAT Mt were found by virtual screening. The active-site region of the enzyme was chosen as the ligand-binding site. The positions of the ligands found by the docking were refined by molecular dynamics simulation. The nearest environment of the ligands, the positions of which in the active site of the enzyme were found in a computational experiment, was analyzed. The compounds under consideration were shown to directly interact with functionally important active-site amino-acid residues and block access of substrates to the active site. Therefore, these compounds can be used for the design of selective inhibitors of PPAT Mt as potent antituberculosis drugs.

Crystals of new fluorite materials: ternary nonstoichiometric Sr_0.9Eu_0.086Eu_0.014F_2.014 and Sr_0.5Eu_0.432Eu_0.068F_2.068 phases with a structure of fluorite (CaF₂) type, containing europium ions in different oxidation states (Eu²⁺, Eu³⁺), have been grown for the first time from a melt by the Bridgeman method. The variation in their thermal conductivity k(T) in a temperature range of 50–300 K has been experimentally studied. The equivalent cation positions with a large difference in cation masses (m_Sr = 87.62 amu and m_Eu = 151.96 amu) lead to low k(T) values of ternary (Sr_1-yEu_y-x²⁺Eu_x3+F_2+x) and binary (Eu_0.864²⁺Eu_0.136³⁺F_2.136) crystals. One can obtain crystals with thermal characteristics specified in a wide range by adjusting the ratio of strontium and europium (regardless of the valence). Since the thermal conductivity of Sr_0.5Eu_0.432Eu_0.068F_2.068 and Eu_0.864Eu_0.136F_2.136 is independent of temperature in the range of 200–300 K, they can be recommended as reference materials with k(T) = const.

The polarization azimuths and ellipticities of the reflected and transmitted light have been calculated as functions of the angle of incidence for transparent crystals of classes \(\overline 4 \)2m and \(\overline 4 \). Analytical expressions for these parameters are obtained. It is shown that in the general case the polarization azimuths and ellipticities for a plate cut parallel to the optical axis differ for positive and negative angles of incidence of light.

The switching processes in calcium barium niobate crystals CaxBa_1–xNb₂O₆ with x = 0.32 have been investigated. The dielectric hysteresis loops observed in the samples exposed to alternating fields of different frequencies and constant amplitude have been analyzed. It is shown that the loop formation is caused by the sample heating. The sample temperature increases due to the dielectric loss energy release only up to a certain frequency, above which complete switching becomes impossible.

Transmission spectra of chiral nematic liquid crystals (CNLCs) and chiral smectic С* liquid crystals (CSLCs) are studied in a wide range of angles of light propagation with respect to the direction of the helical structure axis. To this end, both a planar LC cell and a cell consisting of two substrates in the form of rectangular prisms (30° × 60° × 90°) were used, which allowed measurements of the transmission spectra at the light propagation at angles up to 65° with respect to the helical axis direction. The transmission spectra are numerically simulated; the results are in good agreement with the experimental spectra. A technique for determining the tilt angle of molecules in smectic layers is proposed based on the comparative analysis of experimental and calculated transmission spectra of CSLC.

The formation of a developed electrical relief on the sapphire substrate surface is investigated. A technique is proposed for introducing Ti⁴⁺ impurity atoms into the sapphire crystal lattice by depositing titanium layers with a thickness of about 5 nm and their annealing in air (oxidizing atmosphere) to a temperature of 1400°C. It is shown that this preliminary treatment of the sapphire substrate surface results in epitaxial growth of (111) СdTe films parallel to the sapphire (0001) plane at a temperature of 350°C.

The formation of ferroelectric domains in triglycine sulfate crystal in the para–ferroelectric phase transition has been investigated in situ by piezoresponse force microscopy. The domain structure has been analyzed using the images of Fourier transforms of the scanned (010) surface of natural cleavage. The formation of a quasi-periodic one-dimensional structure of 180° domains with dominant components of one sign below T_C near the transition point is revealed and crystallographic reference to the laboratory coordinate system is performed.

Polycrystalline samples of PbLn₄Mo₃O₁₆ (Ln = La, Pr, Nd, or Sm), CdLn₄Mo₃O₁₆ (Ln = La, Nd, Sm, Eu, or Tm), and PbLa_4–xNd_xMo₃O₁₆ (0 ≤ x ≤ 3) with the cubic fluorite-like structure and sp. gr. Pn n have been obtained by solid-phase synthesis in air. The specific conductivity of lead compounds is established to reach 10^–2 S/cm (which is an order of magnitude higher than that for cadmium compounds). The conductivity value is shown to reach a maximum at a concentration ratio of rare-earth elements of Nd: La = 1: 1.

The dependence of the thermodynamic parameters of a free-standing nanocrystal of simple matter on its size, density, temperature, and surface shape has been studied. The following parameters have been analyzed: Debye temperature Θ, Gruneisen parameter γ, melting temperature T_m, surface energy σ, surface pressure P_sf, elastic modulus B_T, Poisson ratio μ, thermal expansion coefficient α_p, and specific heats c_v and c_p. Calculations performed for silicon have shown that the functions Θ, T_m, σ, and B_T decrease, whereas the functions γ, |P_sf|, μ, α_p, c_v, and c_p increase with an isomorphic decrease in the number of atoms N. The stronger the nanocrystal shape deviates from the most energetically stable shape, the more pronounced the change in the aforementioned functions with an isothermal decrease in N is.

The kinetic regularities of crystallization (growth order and constants) in a model solution of blood plasma have been investigated. The particular order and crystallization constant are determined to be n < 1 and k = 12.30–26.91 molⁿ L^–n s^–n, respectively. The impurities are found to form the following descending sequence with respect to their inf luence on the particular kinetic characteristics of crystallization: milky acid > magnesium ions > alanine, glycine, glucose, and citric acid. It is shown that the impurities affect to a greater extent the nucleation rather than the growth stage. It is also established that a change in рН within 7.0–8.0 does not affect the crystallization parameters of the system modeling the composition of human blood plasma.