Vol 62, No 4 (2017)

Definitions are given, based on which algorithms have been developed for constructing computer models of two-dimensional quasilattices and the corresponding quasiperiodic tilings in plane, the point symmetry groups of which are dihedral groups D_m (m = 5, 7, 8, 9, 10, 12, 14, 18), and the translation subgroups are free Abelian groups of the fourth or sixth rank. The angles at the tile vertices in the constructed tilings are calculated.

The coordination encirclements in a graph of Penrose tiling vertices have been investigated based on the analysis of vertice parameters. A strong parameterization of these vertices is developed in the form of a tiling of a parameter set in the region corresponding to different first coordination encirclements of vertices. An algorithm for constructing tilings of a set of parameters determining different coordination encirclements in a graph of Penrose tiling vertices of order n is proposed.

The phase compositions of the basic material (alloy) of a pre-Mongolian Old Russian encolpion cross found near Suzdal and its black inlay have been investigated by methods of scanning and transmission electron microscopy and energy-dispersive X-ray microanalysis. The cross material is lead–zinc–tin α-bronze, and the black is copper sulfide with lead inclusions. As a result of the corrosion, tin and copper oxides and zinc sulfides, which made the cross dark gray, were formed on its surface. It is shown that the corrosion depth is as large as 50 μm; thus, laps must be prepared to accurately determine the morphology and phase composition of the corrosion layer. Decoration with black was apparently performed by pouring with copper sulfide melt or melting a powder preliminarily placed in grooves forming an image on the surface.

The tetragonal and monoclinic modifications of Nd₂MoO₆ single crystals have been investigated by X-ray diffraction. The analysis has been on laboratory sources (2D CCD diffractometers: Xcalibur S (Shared Research Center, Institute of Crystallography, Russian Academy of Sciences) and BRUKER Smart APEX 2 (Shared Research Center, Institute of General and Inorganic Chemistry, Russian Academy of Sciences)) and on a synchrotron radiation source (ESRF, Grenoble, PILATUS@SNBL diffractometer). Local displacements of all atoms in the oxymolybdate structure have been revealed for the first time using electron density difference maps. Possible violation of the translational periodicity in the distribution of Mo and Nd atoms over types is indirectly confirmed, and statistical disorder of oxygen atoms is revealed. The results of the study confirm the previous suggestion that polymorphic transformations occur in oxymolybdates at a very low rate and, apparently, continuously in a wide temperature range. With an increase in temperature, layers undergo ordering according to the cation type as a result of the phase transition from the monoclinic to the tetragonal phase.

The crystal structure of a representative of eudialyte group, which was found by A.P. Khomyakov at the Rasvumchorr mountain of the Khibiny alkaline massif (Kola Peninsula), has been studied by X-ray diffraction. The trigonal unit-cell parameters are found to be a = 14.2328(5) Å, c = 60.217(2) Å, V = 10564.08(3) Å3, sp. gr. R3m. The structure has been refined with the isotropic and anisotropic approximation displacement parameters to the total reliability factor R = 5.6%, based on 2989 reflections with |F| > 4σ(F). The idealized formula (Z = 3) is determined as [(H₃O)₁₁Na₁₀K5]Cа₁₂(Na₃Fe₂Mn)Si₄Zr₆(Si₄₈O₁₄₄)(OH,Cl)₅(H₂O)₅. The unit cell of the mineral is doubled due to the presence of two modules of different composition and structure in it (alluaivite-and kentbrooksite-like ones) and the formation of a polyhedral cluster in the kentbrooksite module. A comparison with two hydrated minerals having a modular structure shows that these minerals, being similar in their chemical composition, differ in the cation ordering over the sites of the two modules. The sample under study contains potassium in only one module, while oxonium groups are distributed in both modules but over different sites.

The crystal structure of (2Z)-(3,3-dimethyl-3,4-dihydroisoquinolin-1(2Н)-ylidene)nitrosoacetonitrile is determined (T = 150 K, R1 = 0.0435). The compound crystallizes as the E-cis and Z-cis isomers of the nitroso tautomer. Two independent molecules differ in the rotation of the nitroso group about the С–С bond between the dihydroisoquinoline group and the substituent. Analysis of bond lengths reveals their conjugation and prevailing contribution of the zwitter-ionic structures with the positive charge located on the isoquinoline fragment and the negative charge located on the substituent. Molecules having E and Z conformations are connected by the N–H···O hydrogen bond (N···O 2.922(3) Å) into dimers. The N–H group of the other molecule forms the intramolecular N–H···O hydrogen bond (N···O 2.544(3) Å) which closes a sixmembered cycle. Electronic absorption spectra are studied, and quantum-chemical modeling of the compound is performed.

Adenine phosphoribosyltransferase (APRT) belongs to the type I phosphoribosyltransferase family and catalyzes the formation of adenosine monophosphate via transfer of the 5-phosphoribosyl group from phosphoribosyl pyrophosphate to the nitrogen atom N9 of the adenine base. Proteins of this family are involved in a salvage pathway of nucleotide synthesis, thus providing purine base utilization and maintaining the optimal level of purine bases in the body. Adenine phosphoribosyltransferase from the extremely thermophilic Thermus thermophilus strain HB27 was produced using a highly efficient E. coli producer strain and was then purified by affinity and gel-filtration chromatography. This enzyme was successfully employed as a catalyst for the cascade biosynthesis of biologically important nucleotides. The screening of crystallization conditions for recombinant APRT from T. thermophilus HB27 was performed in order to determine the enzyme structure by X-ray diffraction. The crystallization conditions, which were found by the vapor-diffusion technique, were then optimized to apply the counter-diffusion technique. The crystals of the enzyme were grown by the capillary counter-diffusion method. The crystals belong to sp. gr. P12₁1 and have the following unitcell parameters: a = 69.86 Å, b = 82.16 Å, c = 91.39 Å, α = γ = 90°, β = 102.58°. The X-ray diffraction data set suitable for the determination of the APRT structure at 2.6 Å resolution was collected from the crystals at the SPring-8 synchrotron facility (Japan).

The structural and electrical characteristics of In_0.53Ga_0.47As epitaxial films, grown in the low-temperature mode on InP substrates with (100) and (411)A crystallographic orientations at flow ratios of As4 molecules and In and Ga atoms of γ = 29 and 90, have been comprehensively studied. The use of InP(411)A substrates is shown to increase the probability of forming two-dimensional defects (twins, stacking faults, dislocations, and grain boundaries), thus reducing the mobility of free electrons, and As_Ga point defects, which act as donors and increase the free-electron concentration. An increase in γ from 29 to 90 leads to transformation of single-crystal InGaAs films grown on (100) and (411)A substrates into polycrystalline ones.

Square lattices of magnetic nanoparticles undergoing dipole–dipole and antiferromagnetic exchange interactions have been investigated. The main types of equilibrium states with zero and nonzero total magnetic moments have been revealed. It is shown that transitions between different equilibrium states may occur by means of a uniform magnetic field due to the presence of configuration bistability. Orientational transitions between equilibrium states under the action of a magnetic-field pulse on the system and transient oscillation processes have been studied.

The influence of the concentration substitution of Ag ions for Cu ions in the structure of Ag_2–хCu_хS (х = 0.45, 0.8, and 1.07) solid solutions on the temperature of polymorphic phase transitions is demonstrated. The phase composition of Ag_2–хCu_хS (х = 0.45, 0.8, and 1.07) solid solutions has been investigated by X-ray diffraction at different temperatures. It is shown that the structure of the polymorphic modification of Ag_0.93Cu_1.07S composition and the phase transition temperature correspond to the structure and parameters of AgCuS single crystal. The structures of the modifications of compositions with a high silver concentration (Ag_1.55Cu_0.45S and Ag_1.2Cu_0.8S) and the phase transitions temperatures differ from the structure and corresponding parameters of AgCuS single crystal.

The Alexander–Haasen theory, which describes the deformation kinetics of silicon crystals, has been generalized for impurity crystals. The deformation kinetics of an impurity sample is calculated in a wide range of parameters, including the cases of partial and complete entrainment of impurities by moving dislocations. The developed model, despite its simplicity, adequately describes the qualitative transformation of the stress–strain curves of impurity silicon crystals in dependence of the impurity concentration and other material parameters. The manifestation of negative velocity dependence of the yield stress, observed in natural experiments, is analyzed.

A modification of the Langmuir–Schaefer method for the fabrication of high-quality protein films on a solid substrate was proposed and applied to lysozyme. The procedure relies on the use of a pre-prepared protein solution, the parameters of which correspond to crystallization conditions. A lysozyme Langmuir monolayer was shown to be formed with the involvement of complexes, namely, dimers and octamers of protein molecules that are present in such protein solutions. These complexes apparently retain the structure after spreading a protein solution onto an aqueous subphase in a Langmuir trough. The thickness of the film after the transfer of the monolayer, which was formed by the proposed procedure, onto a solid substrate corresponds to the diameter of the octamer and this film is dense, continuous, and uniform, as was demonstrated by several methods: X-ray reflectivity, total external reflection X-ray standing wave, and atomic force microscopy. A layer of chloride ions that formed under the Langmuir monolayer was found at the air–protein film interface. This fact confirms an important role of the precipitating agent (chloride ions) in all steps of the formation of lysozyme films.

A thermodynamic analysis of the processes occurring in the Мо–W–Al₂O₃ system at T = 2400 K and a total pressure of 1 bar, set by controlled reducing Ar + H₂ atmosphere, has been performed. It is found that the basic components of the system do not interact directly, although may be actively involved in chemical reactions with participation of other components to undergo numerous cyclic oxidation–reduction processes. Particular attention is paid to the processes involving such chemically active reagents as Н₂O₂, HO₂, H₂ (H), gaseous Al, and its hydrides (AlH, AlH₂, AlH₃).

A technique has been developed to refine the unit-cell parameters of single crystals with minimization of the influence of instrumental errors on the result. The corresponding computational procedure HuberUB is added to the software package of Huber-5042 diffractometer with a point detector and closedcycle helium cryostat Displex DE-202. The parameters of unit cell, its orientation, the goniometer zero angles, the sample eccentricity, the distances in the goniometer, and the radiation wavelength were refined by the nonlinear least-squares method, which allows imposition of constraints on the unit-cell parameters, depending on the crystal symmetry. The technique is approved on a LuB₁₂ single crystal. The unit-cell parameters are determined in a temperature range of 20–295 K, with an absolute error not larger than 0.0004 Å (the relative error is of ~5 × 10^–5). The estimates of the unit-cell parameters obtained by the proposed method are evidenced to be unbiased. Some specific features of the behavior of parameters in the ranges of 120–140 and 20–50 K are revealed, which correlate with the anomalies of the physical properties of the crystal.