Vol 62, No 2 (2017)

The year 2016 marked 60 years since the foundation of the journal Kristallografiya (Crystallography Reports) by Aleksei Vasil’evich Shubnikov. During this period, the studies that were foundations stones for new lines of research in the fields of crystal growth, structure, and physical properties and are well-known throughout the world have been published in the journal. One of these studies was devoted to the discovery of ferroelectric domains and their relationship with ferroelectric switching. Our brief review is focused on these issues.

The supramolecular chemistry of alumophosphates, which form framework 3D MT structures from polyhedral AlО₄(Н₂О)₂ clusters with octahedral O coordination (of M polyhedra) and PO₄ and AlO₄ with tetrahedral O coordination (of T polyhedra), is considered. A combinatorial–topological modeling of the formation of possible types of linear (six types) and ring (two types) tetrapolyhedral cluster precursors М₂Т₂ from MT monomers is carried out. Different versions of chain formation from linked (MT)₂ rings (six types) are considered. The model, which has a universal character, has been used to simulate the cluster selfassembly of the crystal structure of AlPO₄(H₂O)₂ minerals (metavariscite, m-VAR, and variscite, VAR) and zeolite [Al₂(P_O4)₂(H₂O)₂] · H₂O (APC). A tetrapolyhedral linear precursor is established for m-VAR and a ring precursor (MT)₂ is established for VAR and APC. The symmetry and topology code of the processes of crystal structure self-assembly from cluster precursors is completely reconstructed. The functional role of the O–H···O hydrogen bonds is considered for the first time. The cluster self-assembly model explains the specific features of the morphogenesis of single crystals: m-VAR prisms, flattened VAR octahedra, and needleshaped APC square-base prisms.

The surface layers of Si(001) substrates subjected to plasma-immersion implantation of helium ions with an energy of 2–5 keV and a dose of 5 × 10¹⁷ cm^–2 have been investigated using high-resolution X-ray reflectivity, Rutherford backscattering, and transmission electron microscopy. The electron density depth profile in the surface layer formed by helium ions is obtained, and its elemental and phase compositions are determined. This layer is found to have a complex structure and consist of an upper amorphous sublayer and a layer with a porosity of 30–35% beneath. It is shown that the porous layer has the sharpest boundaries at a lower energy of implantable ions.

Accurate X-ray diffraction study of langasite (La₃Ga₅SiO₁₄) single crystal has been performed using the data obtained on a diffractometer equipped with a CCD area detector at 295 and 90.5 K. Within the known La₃Ga₅SiO₁₄ model, Ga and Si cations jointly occupy the 2d site. A new model of a “multicell” consisting of two different unit cells is proposed. Gallium atoms occupy the 2d site in one of these cells, and silicon atoms occupy this site in the other cell; all other atoms correspondingly coordinate these cations. This structure implements various physical properties exhibited by langasite family crystals. The conclusions are based on processing four data sets obtained with a high resolution (sin θ/λ ≤ 1.35 Å^–1), the results reproduced in repeated experiments, and the high relative precision of the study (sp. gr. P321, Z = 1; at 295 K, a = 8.1652(6) Å, c = 5.0958(5) Å, R/wR = 0.68/0.68%, 3927 independent reflections; at 90.5 K, a = 8.1559(4) Å, c = 5.0913(6) Å, R/wR = 0.92/0.93%, 3928 reflections).

The Сu₂Te compound has been synthesized by alloying Сu and Te taken in stoichiometric ratios. A Сu₂Te sample has been homogenized by annealing at 773 K for 100 h. A pure copper phase in the form of thin filaments is found to segregate from the synthesized product (4 g) by the end of annealing. It is established by X-ray diffraction analysis that the copper-deficient sample is crystallized into the trigonal system with lattice parameters а = 8.328(1) Å, с = 7.196(1) Å, V = 432.2(1) ų, sp. gr. \(P\overline 3 m1\), Z = 8. The crystal structure is determined and the sample composition Сu_2–mTe (m = 0.25) is refined by the Rietveld method. The three independent copper atoms in the lattice are found to have different coordination numbers: 3, 4, and 5.

Seven new, previously unknown, bicyclic and tricyclic heterocycles based on derivatives of 3-cyanopyrid-2-ones are obtained: 2-oxo-2,5,6,7,8,9-hexahydro-1H-cyclohepta[b]pyridine-3-carbonitrile, C₁₁H₁₂N₂O (1a); 2-[2-(4-chlorophenyl)-2-oxoethoxy]-6,7,8,9-tetrahydro-5Н-cyclohepta[b]pyridine-3-carbonitrile, C₁₉H₁₇ClN₂O₂ (2a); (3-amino-6,7,8,9-tetrahydro-5H-cyclohepta[b]furo[3,2-e]pyridin-2-yl)(4- chlorophenyl)methanone, C₁₉H₁₇ClN₂O₂ (3); 2-oxo-1,2,5,6,7,8,9,10-octahydrocycloocta[b]pyridine-3-carboxamide, C₁₂H₁₆N₂O₂ (4); 2-[2-(4-chorophenyl)-2-oxoethoxy]-5,6,7,8,9,10-hexahydrocycloocta[b]pyridine-3-carboxamide, C₂₀H₂₁ClN₂O₃ (5a); 1-[2-(4-chlorophenyl)-2-oxoethyl]-2-oxo-1,2,5,6,7,8,9,10-octahydrocycloocta[b]pyridine-3-carboxamide, C₂₀H₂₁ClN₂O₃ (5b); and 2-[2-(4-chlorophenyl)-2-oxoethoxy]-5,6,7,8,9,10-hexahydrocycloocta[b]pyridine-3-carbonitrile, C₂₀H₁₉ClN₂O₂, (6). All compounds are characterized by ¹H NMR spectroscopy, and their crystal structures are determined by X-ray diffraction.

The title coordination polymer {[Cd₁₂(tda)₈(H₂O)₁₁] · (H₂O)_6.25}_n (H₃tda = 1,2,3-triazole-4,5-dicarboxylic acid), has been hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction. Complex crystallizes in orthorhombic sp. gr. Pmn2₁ with Z = 4. The Cd₂ unit doublebridged by one carboxylate oxygen atom and two neighboring nitrogen atoms from the tda^3– ligands are linked by the tda^3–ligands to lead to the 2D (4,4) network in the ac plane. The almost coplanar Cd₂(μ₅-tda)₂ unit comprised of two Cd ions double-bridged by two tda^3– ligands through the neighboring nitrogen atoms is connected with the other four Cd₂(μ₅-tda)₂ units form the undulating 2D network in the ac plane. The (4,4) networks and undulating 2D networks are alternatively connected along the b axis by the tda^3– ligands coordinating to the Cd ions to form the 3D framework.

A proton transfer compound 2,6-dimethyl piperazine-1,4-diium perchlorate monohydrate was synthesized by slow evaporation at room temperature using 2,6-dimethyl piperazine as template. The asymmetric unit contains one organic dication, two crystal graphically independent perchlorate anions and one water molecule. Each organic entities is engaged in a large number of bifurcated and non-bifurcated N–H···O (O) and C–H···O hydrogen bonds with different species and enhanced the three dimensional supramolecular network. In addition, the diprotonated piperazine ring adopts a chair conformation with the methyl groups occupying equatorial positions.

Crystals of porcine pancreatic carboxypeptidase B (CPB) were grown by the capillary counter-diffusion method in the presence of polyethylene glycol and zinc acetate. The three-dimensional structure of CPB was determined at 1.40 Å resolution using the X-ray diffraction data set collected from the crystals of the enzyme at the SPring 8 synchrotron facility and was refined to R_fact = 17.19%, R_free = 19.78%. The structure contains five zinc atoms, two of which are present in the active site of the enzyme, and an acetate ion. The arrangement of an additional zinc atom in the active site and the acetate ion is different from that reported by Yoshimoto et al.

Fe₂O₃–CaO–SiO₂ glass ceramics containing nucleation agent P₂O₅/TiO₂ were prepared by sol-gel method. The samples were characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The activation energy and kinetic parameters for crystallization of the samples were calculated by the Johnson-Mehi-Avrami (JMA) model and Augis-Bennett method according to the results of DSC. The results showed that the crystallization mechanism of Fe₂O₃–CaO–SiO₂ glass, whose non-isothermal kinetic parameter n = 2.3, was consistent with surface crystallization of the JMA model. The kinetics model function of Fe₂O₃–CaO–SiO₂ glass, f(α) = 2.3(1–α)[–ln(1–α)]^0.57, was also obtained. The addition of nucleation agent P₂O₅/TiO₂ could reduce the activation energy, which made the crystal growth modes change from onedimensional to three-dimensional.

Stacking faults and dislocations in InGaAs/InAlAs/InAs heterostructures have been studied by electron microscopy. The use of different techniques of transmission electron microscopy (primarily, highresolution dark-field scanning transmission electron microscopy) has made it possible to determine the defect structure at the atomic level.

The influence of the deviation of seed orientation from the [100] direction on the formation of a dislocation structure of gallium arsenide single crystals grown by the Czochralski method has been revealed. The intensive multiplication of dislocations and formation of a block structure occur at deviation by an angle of more than 3° in the region that is radially shifted to one of crystal sides. The linear density of dislocations in the walls changes from 1 × 10⁴ cm^–1 in low-angle boundaries to 6 × 10⁴ cm^–1 in subboundaries.

It is shown that anomalous piezoelectric properties of epitaxial nanostructures arise on the morphotropic phase boundary (MPB) due to the strong flexoelectric effect on dislocation walls. The MPB (typical of many materials) exhibits a coexistence of various phases and partition of these phases to minimum sizes. This minimum size l_с (nanoscale) is found using the dislocation theory; it coincides with the distance between individual dislocations in dislocation walls, which is much larger than the Burgers vector b, regardless of the type of crystalline material. The flexoelectric coefficients f are estimated taking into account dimensional relations and experimental data on the rotations of ferroelectric nanodomains in multiferroics. These estimates coincide with classical values. The critical value l_с ~ 10b specifies the measured dependence on the dielectric susceptibility χ_e, f ~ χ_e^1/2. The quantity χ_e depends on the frequency of the ac electric field applied to a sample and on the dislocation density. The Ba_0.6Sr_0.4TiO₃/Ni_0.8Zn_0.2Fe₂O₄ ceramic composite shows typical frequency dispersion of χ_e in a wide frequency range. The frequency dependence of flexoelecric coefficients is shown to reproduce the frequency dependence of permittivity at high frequencies.

The electrical conductivity of sodium–strontium germanate Na₄SrGe₆O₁₅ (sp. gr. P6₃/m) has been studied by impedance spectroscopy in the frequency range of 10²–4 × 10⁴ Hz and a temperature range of 450–600 K. Na4SrGe6O15 crystals were obtained by hydrothermal technique in the Na₂O–SrO–GeO₂–H₂O system (temperature t = 300–600°C and pressure p = 1.4 × 10⁸ Pа in the dissolution zone). The ionic conductivity of ceramic Na₄SrGe₆O₁₅ samples is σ = 2.2 × 10^–6 S/cm (at 573 K), the activation energy of Na⁺ ion transfer is E_a = 0.70 ± 0.03 eV.

Sapphire is widely used in production of optical windows for various devices due to its mechanical and optical properties. However, during operation the surface can be affected by fats, oils, and other organic contaminations. Therefore, it is important to improve the oleophobic properties of sapphire windows. In this study, we investigate the interaction of a supersmooth sapphire surface with oleic acid droplets, which imitate human finger printing. It is established that chemical–mechanical polishing with additional annealing in air, which leads to the formation of an atomically smooth sapphire surface, makes it possible to significantly improve the oleophobic properties of the surface. The results are analyzed using the Ventsel–Deryagin homogeneous wetting model.

The photosensitivity of a mixed composition of donor CuPc and acceptor C₆₀, its degradation in time (for 16 months), and partial recovery (issues that were out of consideration previously) have been investigated. The sample is a simple nanostructure without encapsulation (quartz–SnO₂–CuPc:C₆₀–Al), the degradation of which is easier to analyze. It is shown that the process under study involves two mechanisms: slow (because of the decrease in the carrier lifetime) and relatively fast (because of the decrease in the internal field) degradations of the sample photosensitivity.

The application of time–frequency wavelet analysis for solving the reflectometry inverse problem is considered. It is shown that a simultaneous transform of specular intensity curve, depending on the grazing angle and spatial frequency, allows one to determine not only the thickness but also the alteration order of individual regions (layers) with characteristic behavior of electron density. This information makes it possible to reconstruct the electron density profile in the film cross section as a whole (i.e., to solve the inverse reflectometry problem). The application of the time–frequency transform is illustrated by examples of reconstructing (based on X-ray reflectivity data) the layer alternation order in models of two-layer films with inverted arrangement of layers and a four-layer film on a solid substrate.

The factors responsible for the formation of different configurations of boundaries between adjacent crystallites during their growth from melt by Bridgman and Czochralski methods have been considered by an of example Fe–20 wt % Ga alloy and Ni bicrystals. It is found that the configuration of intercrystallite boundary is related to the features of crystallite growth, caused by the strained state of intercrystallite and interphase (crystal–melt) boundaries, the difference in the linear thermal expansion coefficients of the crystallite boundaries and bulk, and the shape (geometry) of the bicrystal cross section. It is suggested that the strained state of boundaries and the formation of substructure in crystallites during directional crystallization from metal melt are significantly affected by their deformation under the melt weight.