Vol 64, No 1 (2019)

A technique for studying the carbon-fiber structure using X-ray diffraction and electron microscopy has been developed. Average crystal-structure parameters are determined for samples of commercial polyacrylonitrile carbon fibers by X-ray diffractometry. Electron microscopy study has confirmed the diffraction data and provided data on the fiber macrostructure. A comparative analysis of the structures and quality of the samples under study has been performed based on the results obtained.

The influence of the precipitant cation type in a series of chlorides (NaCl, KCl, LiCl, NiCl₂, CuCl₂, CoCl₂) on the structure of lysozyme solutions has been investigated by small-angle X-ray scattering (SAXS) under the conditions favorable for tetragonal crystal growth. The influence of the precipitant concentration on the volume fraction of the oligomers (dimers, octamers) formed before the protein crystallization has been studied. Along with monomers, significant amount of dimers (3.5–13.1%) and octamers (1.7–3.6%) is found in the lysozyme solutions when adding a precipitant. The volume fractions of all oligomers increase with an increase in the precipitant concentration and with a decrease in temperature. In the series of monovalent and divalent ions, the octamer volume fraction increases as K⁺ – Na⁺ – Li⁺ and Cu²⁺ – Ni²⁺ – Co²⁺, respectively.

A new scheme of two-beam X-ray diffractometry on the “X-Ray Crystallography and Physical Materials Science” (XCPM) beamline at the Kurchatov Synchrotron Radiation Source (KSRS) has been experimentally investigated. The scheme includes a standard double-crystal monochromator and a narrow slit installed in front of the sample. Measurements have been performed for the Si 111 and 311 reflections in the monochromator and the Si 111 and 220 reflections in the sample crystal. It is shown that this scheme allows one to obtain a near-proper diffraction reflection curve even in the case of symmetric diffraction if the Bragg angle for the monochromator exceeds the Bragg angle for the crystal sample by a factor of 2 or more. The experimental results coincide well with the theory.

A crystallographic structural analysis of low- and high-temperature phases of BaSi₂O₅ and Na₃SO₄F compounds has been carried out in order to confirm the thesis that the stability of natural-abundant structural types is mainly determined by the configuration of the heaviest components. The geometry of cation (skeletal) sublattices is presented; it is shown that they play a key role in the formation of structures and are relatively stable at temperature phase transitions.

The nuclear magnetic resonance (NMR) of ⁷Li and ⁶Li nuclei in a LiTaO₃ single crystal sample of congruent composition has been experimentally investigated. It is found that there are weak side lines in the NMR spectra of ⁷Li nuclei, which have not been observed before; these lines are due to cation sublattice defects. A computer simulation of NMR spectra has revealed the most likely configuration of \(({\text{Ta}}_{{{\text{Li}}}}^{{{\text{5}} + }} + {\text{4}}{{V}_{{{\text{Li}}}}})\) defect complexes that are present in nonstoichiometric LiTaO₃ crystals.

The defect structure of as-grown SrF₂ and nonstoichiometric phases Sr_1 – xLa_xF_2 + x (x = 0.11, 0.20, 0.32, 0.37, 0.47) single crystals, grown from a melt under identical conditions, has been studied by X-ray diffraction analysis. All crystals belong to the CaF₂-type structure, sp. gr. \(Fm\bar {3}m\). Deficit of fluorine anions is found in the 8c site in SrF₂. Interstitial anions are not visualized in SrF₂ in difference electron-density maps. The Sr_1 – xLa_xF_2 + x phases exhibit the presence of vacancies in the main anion motif and interstitial fluorine ions of three types: in two sites 32f (w, w, w) with different coordinates w and in one site 4b. A model of the defect structure of Sr_1 – xLa_xF_2 + x phase is proposed, according to which interstitial fluorine ions and impurity cations La³⁺ are grouped into clusters of the [Sr_1 – nLa_nF₂₆] tetrahedral configuration. Calculations based on structural data revealed that the average number of La³⁺ ions per cluster linearly increases from 2.6 to 3.13 with an increase in the LaF₃ concentration. The average crystal volume corresponding to one cluster decreases from 1170.6(3) to 336.1(5) ų. The volume of the anion cluster core decreases from 2.52(7) to 2.42(7) ų, passing through a minimum in the composition with x = 0.32, which is similar to that of congruently melting phase, and then increases to 2.44(9) ų at х = 0.47. Dynamic thermal displacement of matrix anions in Sr_1 – xLa_xF_2 + x is observed in the [111] direction towards the cubic void center in the anion sublattice. Therefore, according to the mechanism of electrical conductivity, anion jumps are most likely in this direction.

The structures of Rb-boroleucite Rb_1.0(B_0.333Si_0.667)₃O₆ and boropollucite Cs_0.87(B_0.290Si_0.710)₃O₆ were studied using single crystals, which were prepared under hydrothermal conditions. In the zeolite-like framework structures, Rb and Cs atoms are located in large window cavities formed by four, six, and eight tetrahedra. The B and Si atom randomly occupy general positions. Cubic space groups of both phases (acentric sp. gr. I\(\bar {4}\)3d for Rb and centrosymmetric holohedral sp. gr. Ia\(\bar {3}\)d for Cs) remain unchanged at room temperature and down to 120 K. Rubidium boroleucite has a stoichiometric formula. Boropollucite is characterized by a deviation from the stoichiometric composition, which is manifested in the predominance of Si atoms in tetrahedra and the defect occupation of the Cs site. The size of alkali metal cations plays a crucial role in the existence of a particular modification, which is consistent with earlier results.

5-Bromo-3-methoxysalicylidene-2-furfurylamine (HL⁵) and 3-nitrosalicylidene-2-furfurylamine (HL⁶) were synthesized and studied by X-ray diffraction. Both structures are stabilized by strong intramolecular N1–H1···O2 hydrogen bonds. Both molecules exist as zwitterions. The structure of the HL⁶ molecule has a contribution of the tautomeric quinoid form.

The crystal structure of 4,4'-substituted phenyl benzoate CH₂=C(CH₃)–COO–C₆H₄–COO–C₆H₄–O–С₉Н₁₉ has been investigated by X-ray diffraction. The crystal packing of this compound consists of alternating loose aliphatic and closely packed aromatic regions, which are typical of mesogenic crystals. However, weak directional interactions, responsible for the formation of structured melt, are absent in aromatic regions. Therefore, this compound does not exhibit mesomorphic properties both during melting and upon cooling the isotropic melt.

In the first part of the work, it was shown how the structure and functioning of enzymes can be described within the formalism of generalized forces and currents. The driving forces in II-type enzymes and DNA translocation in helicases are considered in the second part.

The positions of the substrates (ATP and ribose 5 phosphate) of phosphoribosylpyrophosphate synthetase from Thermus thermophilus were determined by molecular dynamics simulations. The simulation brought the system to an equilibrium state, with the binding poses of the ligands in the active site being stable. Based on the results of simulation of the complex, the environment of the substrates was analyzed and the amino-acid residues of the enzyme that form polar interactions with the substrates were identified. Candidate sites for mutagenesis, which can be mutated in order to broaden the substrate specificity toward ribose 5-phosphate, are proposed.

X-ray and electrophysical investigations of ceramics with a charge composition 95 mol % CeF₃ × 5 mol % SrF₂ synthesized by hot pressing of CeF₃ and SrF₂ powders (300 MPa, 900°С, 20 min) in vacuum and the same ceramics annealed in a fluorinating atmosphere (CF₄, 1000°С, 180 min) have been carried out. The initial two-phase ceramics consists of the tysonite phase (a = 7.130(1) Å, c = 7.295(1) Å) and up to 2 wt % of fluorite impurity phase (a = 5.77(1) Å). The ionic conductivity of the initial ceramics is σ_dc = 2.8 × 10^–4 S/cm at room temperature. Heat treatment of the ceramics in a fluorinating atmosphere eliminates the fluorite phase. After the annealing, the ceramics becomes single-phase and turns to the Ce_1 − ySr_yF_3 − y tysonite solid solution. The σ_dc value of the annealed ceramics with the nominal composition Ce_0.95Sr_0.05F_2.95 increases by a factor of ∼1.5 and coincides with the σ_dc value for a single crystal of the same composition.

The effect of thermal treatment in zinc vapor on the stoichiometry, defect and impurity structure, and activator cathodoluminescence of ZnSe doped with iron by the thermal diffusion method has been studied. Using a variety of measurement methods, it is found that, on the one hand, the separate regions in which the edge luminescence intensity drops become larger. On the other hand, the stoichiometry is improved and the integrated luminescence intensity, as well as the activator luminescence intensity, increase. The data obtained agree with the general approach to the mechanism of the influence of thermal treatment on the properties of doped ZnSe.

A method is proposed to estimate (using the autocorrelation function (ACF)) the thickness of layers determining the electron density profile of a thin film. The method does not require additional conditions and is applicable to an arbitrary reflectivity curve. It is based on the ACF synthesis by a superposition of Gaussians, whose parameters are related to the characteristics of the film layers and interfaces. The potential of this method is demonstrated by a number of model examples.

Magnetite nanoparticles have been obtained in hybrid micelles of polylactide-block-polyethylene oxide (PL-b-PEO) and sodium dodecyl sulfate (SDS) in a one-stage process. Dynamic light scattering, transmission electron microscopy, and small-angle X-ray scattering data show that hybrid micelles form micellar associates, and the size of magnetite nanoparticles in hybrid micelles of the block-copolymer varies from 0.5 to 10 nm. The specific magnetization of solid samples has been measured.

Mixed crystals (NH₄)₂Ni_xCo_{1 –x}(SO₄)₂ · 6H₂O have been grown from aqueous solutions of different compositions by reducing the solution temperature in two growth regimes: static and dynamic. The optical characteristics of crystal samples are analyzed, the dehydration temperatures are measured, and the defect structure of the crystals is studied.

The nucleation and growth of crystallites from a model saliva solution in the presence of amino acids (in concentrations corresponding to the physiological level and exceeding the norm by a factor of 5) have been studied. It is found that additives may either decrease or increase the crystallization rate, depending on the nature of amino acid, its concentration, and the initial supersaturation of the solution.

The known data on the morphology of paratellurite single crystals grown from melt by the Czochralski method are analyzed. The structure of faces is studied by scanning electron microscopy and interference profilometry, and their inclination angles to the pulling axis are measured. Based on calculations, the found faces are indexed as {113}.

Some archaeological heritage objects (miniature golden vial, encolpion crosses, boulatiry) have been studied by neutron tomography and radiography. It is shown that this method can reliably be used for articles made of nonferrous and precious metals. The study of a miniature golden incense vial dated I AD yielded important data on the presence of internal filling; as for the crosses, the absence of the so-called “holy relics” in these peculiar reliquary vessels was established. The study of the Old Russian iron boulatiry did not give any information on the presence or absence of traces of images under the corrosion layer on stamp working places. The data obtained are important for further historical interpretations and choosing methods of preservation of archaeological heritage objects, museum exposure, and presentation.

A new approach to the structural analysis of crystals based on diffraction data obtained in a series of experiments performed at different temperatures (i.e., to the multitemperature structural characterization) is proposed and implemented. Calculations are performed using symmetry-averaged diffraction data. The full-matrix least-squares method is applied to calculate simultaneously the composite structural model from several datasets. The main purpose of the proposed algorithm is to determine which of the refined parameters are general (temperature-stable) and which reflect the dataset individuality (i.e., change with temperature). The method is applied to determine the occupancies of mixed atomic sites in rubidium titanyl phosphate doped with zirconium, to search for the mobile atoms responsible for the ion transport in this crystal, and to study the structural evolution of the boron framework in crystals of rare-earth element dodecaborides (RB₁₂) of several compositions.