Volume 64, Nº 3 (2019)

The noncrystallographic symmetries of chains of regular tetrahedra are determined by mapping the system of algebraic geometry and topology designs to the structural level. It has been shown that the basic structural unit of such a chain is a tetrablock: a seven-vertex linear aggregation over faces of four regular tetrahedra, which is implemented in linear (right- and left-handed) and planar versions. The symmetry groups of linear and planar tetrablocks are isomorphic, respectively, to the projective special linear group PSL(2, 7) of order 168 and the projective general linear group PGL(2, 7) of order 336. A class of structures formed by an assembly of tetrablocks having no common tetrahedra is introduced. Examples of tetrablock assembly over common face, leading to a Boerdijk–Coxeter helix, an α helix, and a helix used as one of collagen models are presented.

A new description of Penrose tiling, based on parameterization of tiling rhombuses, has been obtained. A method making it possible to describe clusters and coordination environments in Penrose tiling in terms of parameters is developed. The parameters of all possible types of the first coordination environments in Penrose tiling are found. A new proof is obtained that the form of layer-by-layer growth of Penrose tiling is a regular decagon, and the vertices of this decagon are calculated.

The real structures of epitaxial CVD films of germanium and diamond have been comparatively analyzed. The influence of the specific features of elastic-stress distribution in two-layer structures on the relaxation processes and dislocation distribution are considered. The influence of inhomogeneous (over thickness) plastic deformation, caused by the motion of dislocations under alternating elastic-stress field, on the formation of residual bending of substrates and films is shown by the example of epitaxial germanium structures. A significant difference in the relaxation processes in CVD films of diamond and germanium (crystallographic analog of diamond) is revealed.

New lead silicate {Pb₄(О(ОН)₂)}[SiO₄] was synthesized under hydrothermal conditions and structurally characterized, sp. gr. P2₁3, а = 8.9756(13) Å. According to the commonly used classification, the new compound belongs to orthosilicates. In the crystal structure, isolated SiO₄ tetrahedra are located between numerous Pb atoms. The structure can be considered as an anion-centered positively charged zeolite-like framework composed of {O₃Pb₇}⁸⁺ clusters, which are linked to each other and include three edge-sharing (OPb₄)⁶⁺ tetrahedra with one common vertex. The formula of this positively charged antiframework is {O(ОН)₂Pb₄}⁴⁺, and its cavities are occupied by negatively charged [SiO₄]^4– tetrahedra. This framework is similar to that of sodalite when relating the sites to the centers of gravity of the {O₃Pb₇}⁸⁺ clusters.

The title compound, C₁₇H₁₅NOS, crystallizes in the orthorhombic sp. gr. Pca2₁. Two molecules in the asymmetric unit have similar structure. Crystal structure contains weak C–H···O intermolecular and C–H···N intramolecular H- bonds. Molecular parameters obtained from theoretical calculations are close to those from X-ray analysis. Molecular electrostatic potential surface shows that the most negative regions are mainly located near N and O atoms while the most positive region is mainly located on S bonded C atom.

A two-layer Zr_0.8Sc_0.2O_1.9/Ce_0.9Gd_0.1O_1.95 heterostructure has been modeled by the molecular dynamics method in a box containing about 27 thousand atoms. It is shown that this system retains on the whole the crystallographic characteristics of layers doped with zirconia and ceria, having a fluorite structure. Crystal structure distortions are observed in a narrow boundary layer with a thickness of few angstrom. An analysis of pair correlation functions indicates that the oxygen sublattice in the heterostructure is disordered. The calculated values of the layer-by-layer diffusion coefficient of oxygen and the diffusion activation energy are compared with the data of both direct physical and computer experiments.

Microstructures have been formed on single-crystal silicon substrates of different crystallographic orientations, (111) and (100), under “soft” hydrothermal conditions. Well-faceted ZnO microcrystallites were obtained on the Si(111) surface, whereas the Si(100) substrate surface was covered with willemite (Zn₂SiO₄) crystallites, and zinc oxide microcrystallites were observed in only a narrow region ∼1 mm wide over the substrate edge. These results indicate clear influence of the crystallographic orientation of silicon substrates on the processes of crystal formation and microstructure growth under conditions of low-temperature hydrothermal synthesis. Low-threshold effective lasing has been detected on well-faceted zinc oxide crystallites under optical excitation.

Raman spectra of monodisperse crystalline diamond powders have been studied in dependence of the sample sizes. Raman scattering was excited by a near-IR cw laser (λ₀ = 785 nm), which ensured suppression of the photoluminescence signal. Powders consisting of close-packed particles in the form of nanodiamonds (0.2–0.3 µm in size) and microdiamonds of specified sizes (up to 180 µm) have been studied. The recorded Raman spectra of the microdiamond powders are characterized by anomalously high intensity, which is related to the excitation radiation trapping in microparticles, the size of which exceeds significantly the lasing wavelength. The Raman spectra contain a sharp line at a frequency of 1332 cm^–1 and additional bands in the vicinity of fundamental-mode overtones.

An increase in the electro-optical switching speed with a decrease in the electrode grating period is observed in a cell with a homeotropically oriented nematic liquid-crystal (LC) layer and interdigitated electrodes. There are two modes in the electro-optical response: fast and slow. A numerical simulation has shown that the fast mode occurs because of the LC director deformation in the surface layer. On the contrary, the occurrence of slow mode is due to the fact that the deformation propagates much more slowly in the LC bulk. The observed effect has been demonstrated experimentally in the bidirectional switching mode using two crossed interdigitated electrodes, located at both sides of the LC layer.

The dynamical properties of a system of linear domains induced by shear oscillations in a planar layer of a nematic liquid crystal under the joint action of shear and piston oscillations at frequencies in the audio range, have been studied experimentally. The phenomenon of spatial modulation of the domains of an initial system along their axis under the piston oscillations producing periodic layer compression is investigated for the case where the directions of the director in a mesophase layer and the shear oscillations coincide. A relationship between the period of secondary two-dimensional structures at the effect threshold and the layer thickness as well as oscillation frequency is established. It is shown that the formation mechanism of these structures can be interpreted on the basis of a known phenomenological model, according to which the instability of the initial system of these linear domains is caused by the “mismatch” between the real period of domains and its optimal value, which varies jointly with the variations in the thickness of a periodically compressed layer.

The temperature dependences of structural and phase transformations in quartz successively implanted by zinc and fluorine during annealing in nitrogen have been studied. Plates were doped with ⁶⁴Zn⁺ ions to a dose of 5 × 10¹⁶ cm^–2 with an energy of 50 keV and then with ¹⁹F⁺ ions to the same dose but with an energy of 17 keV. After the implantation, individual Zn-containing particles about 100 nm in size were found on the sample surface. These particles decrease in size during annealing (by an order of magnitude after annealing at 800°C). The implantation leads to the formation of radiation-induced point defects and their clusters in the quartz bulk. Radiation-induced defects are gradually annealed during the heat treatment, and the phase of metallic zinc is transformed first to its zinc oxide (ZnO) at 600°C and then to willemite (Zn₂SiO₄) at 800°C.

Indium nanoparticles were obtained by vacuum evaporation technique of indium onto a glass carbon substrate in high purity Ar atmosphere. A technique is proposed for determining the nanoparticle dimensions and shape using a specially developed program. It is found that at a molten indium evaporation time from 10 to 30 s, indium nanoparticles with dimensions of 10–100 nm and crystallites with dimensions up to 500 nm are formed on the substrate. The largest number (up to 37%) of indium nanoparticles with dimensions of 50–100 nm is obtained at an evaporation time of 10 s; up to 52% of these particles have a shape different from spherical. The average diameter of indium particles increased from 140 to 190 nm with an increase in the evaporation time.

Different methods of scanning electron microscopy in combination with different sample preparation techniques have been compared by an example of decellularized rat diaphragm matrices. It is shown that a combination of scanning electron microscopy under natural environmental conditions, allowing one to investigate tissue samples in a state similar to native, with the conventional high-vacuum microscopy of samples obtained by drying at the critical point, provides the most complete microstructural analysis of native and decellularized tissue samples.

The structure, thermal expansion coefficient, and electrical conductivity of Y_0.9Ca_0.1Cr_1 – уCo_уO₃ (у = 0–0.9) compounds with the perovskite structure have been investigated in air in the temperature range of 100–1000°С. The linear thermal expansion coefficient of Y_1 – хCa_хCr_1 – уCo_уO₃ lies in the range of (8.77–16.1) × 10^–6 K^–1. The maximum electrical conductivity is attained for the Y_0.9Ca_0.1Cr_0.6Co_0.4O₃ composition. It is shown that the electrical conductivity at low (25–270°С) temperatures and weak substitution of cobalt for chromium is mainly due to the hopping of one electron hole from Cr⁴⁺ to Cr³⁺. At high (565–1000°С) temperatures and cobalt contents, this conductivity mechanism is supplemented with the conductivity caused by the hopping of an electron hole from Cо³⁺ to Cо²⁺.

The phase equilibria in the K₃H(SO₄)₂–Rb₃H(SO₄)₂–H₂O cross section have been investigated under isothermal conditions at 25°C. The concentration limits of crystallization of solid solutions with the general formulas (Rb_1 – xK_x)₃H(SO₄)₂ and (Rb_1 – xK_x)₂SO₄ are determined. The dependences of the equilibria of saturated solution on the initial production conditions are revealed. The growth conditions for large single crystals of complex acid rubidium–potassium sulfates are determined.

Sm_1 – ySr_yF_3 – y (0 < y ≤ 0.31) crystals have been grown from melt by directional solidification in a fluorinating atmosphere. The crystals have been studied by X-ray diffraction and optical spectroscopy, and their fluorine-ion conductivity σ_dc, density ρ, and refractive indices n_D have been measured. It is established that Sm³⁺ ions are not reduced to Sm²⁺ during crystal growth. The reversible polymorphic α ↔ β-SmF₃ transition does not make it possible to obtain bulk (>1–3 mm³) samples of the tysonite phase (of LaF₃ type) at y < 0.02. The dependences ρ(y) and n_D(y) for the crystals are descending. The dependence σ_dc(y) exhibits nonmonotonic behavior; the maximum σ_dc value (1.6 × 10^–4 S/cm) at 293 K is observed for the Sm_0.98Sr_0.02F_2.98 crystal. At y = 0.31, an eutectic composite 69SmF₃ × 31SrF₂ is formed, whose conductivity is σ_dc = 6 × 10^–8 S/cm, a value smaller than σ_dc for the crystal with y = 0.02 by a factor of ~3 × 10³. The carrier concentration n_mob and its mobility μ_mob have been calculated for Sm_1 – ySr_yF_3 – y (0.02 ≤ y ≤ 0.25) within the hopping conductivity model. For the crystal with the highest conductivity (Sm_0.98Sr_0.02F_2.98), n_mob = 4.0 × 10²⁰ cm^–3 and µ_mob = 2.5 × 10^–6 cm²/(V s) at T = 293 K.

The crystallization processes in solutions modeling the human blood plasma composition have been investigated. It is revealed that the solid phases consist of OH-deficient water-containing carbonate apatite. The influence of impurities (magnesium and glutamic acid ions) on the crystallization of calcium phosphates is analyzed. The presence of additives in model solution is found to affect the phase composition of samples. The solubility of synthetic samples in solutions of different nature and verapamil preparation is studied. The kinetic characteristics of this process are established, and the dependence of dissolution rate on the experimental laboratory conditions is shown.

Fragments of stamped amphorae (south Pontic (Sinope, III–I BC) and Mediterranean (Thasos, IV–III BC, and, presumably, Chios, III–II BC)), found in excavations on the Crimean Peninsula, have been investigated using X-ray diffraction, generalized semiquantitative X-ray fluorescence analysis, and inductively coupled plasma mass spectrometry. The data obtained have been subjected to comparative analysis. A comparison of the mineralogical and elemental compositions of samples has revealed the characteristic distinctions between the products fabricated at different ancient pottery centers.

Hydrophobic cores and hydrophobic clusters play a key role in globular protein folding, providing a framework for functionally important amino acid residues of proteins and enzymes. A program was developed for clustering amino acid residues in protein structures, which takes into account their hydrophobicity and utilizes the DBSCAN algorithm. The program is described and its scope and applicability are outlined.