Kristallografiâ

We present the first results of a new experimental method for phase-contrast microscopy of microobjects based on synchrotron radiation and a nanofocusing lens in a conical geometry. In the experiment, a secondary radiation source is formed at the lens focus, located at a short distance from the microobject, enabling the acquisition of its magnified image. Under near-field conditions, the structure of the microobject can be relatively easily retrieved from the experimental image using the transport-of-intensity equation. The experiment was conducted at the KISI-Kurchatov synchrotron radiation source. A model weakly absorbing microobject, namely a commercially available carbon fiber of grade VMN-4, was used. The fiber dimensions and structural features were obtained with submicron spatial resolution, in agreement with the electron microscopy results.

Serial section focused ion beam tomography was performed for a three-dimensional reconstruction of the microstructure of defects associated with the formation of fatigue cracks in samples destroyed during fatigue tests. The geometric parameters of defects containing Hf, Nb, Ti, Al, and Ni identified during 3D reconstruction were determined. The morphology of individual particles is represented by a set of shapes that form flat (carpet-like) conglomerates up to tens of microns in size, which cannot be detected by non-destructive testing methods. The revealed morphological features make it possible to propose a set of measures to increase the service life of parts made of granulated heat-resistant nickel alloy EP741NP, which is an important practical result of the study.

The effect of temperature on the structure of single crystals of tris(ethylenediamine)nickel(II)dichloride dihydrate [Ni(en)₃]Cl₂·2H₂O, or [Ni(H₂NCH₂CH₂NH₂)₃]Cl₂·2H₂O, was investigated. The rearrangement of the structure with a decrease in temperature from 293 to 85 K was studied using X-ray structural analysis. A threefold decrease in the volume of the unit cell of the crystal was found while maintaining the space group C2/c.

The effect of polyvinyl alcohol (PVA) cross-linked with sulfosuccinic acid on the structure, morphology, physical, mechanical and electrochemical properties of composite membranes made of domestic perfluorinated copolymer, an analogue of Nafion, and PVA was studied. An increase in the amount of cross-linked PVA in the membrane leads to an increase in proton conductivity. The crystallinity of the composites depends on the proportion of the cross-linking agent. The morphology of the membrane surfaces varies significantly: the lower surface has a uniform microstructure, and the upper surface forms three-dimensional folded structures during self-organization of polymer chains in the surface layer. According to energy-dispersive analysis, the two layers of the membrane differ significantly in chemical composition, which is illustrated by the distribution profiles of fluorine across the membrane thickness. The observed structural and morphological features of the membranes explain the differences in their proton conductivity.

The article presents the findings of an investigation of SmS thin films phase transition dynamics, focusing on the semiconductor–metal transition induced by mechanical polishing and the reverse thermally induced metal–semiconductor transition. The results demonstrate that the reverse phase transition occurs during the sample cooling within the temperature range of 408–373 K. The change of phase and element composition of thin films is observed during the aforementioned phase transitions within the surface layer. The findings suggest that SmS thin films can be considered as structures with predictable and required phase transition dynamics, making them promising candidates for engineering functional materials and elements of pressure sensors operating over a wide range.

The paper presents the results of modeling the surface energy of the (100), (010), (001), (110) and (110) faces of linear acene crystals (naphthalene, anthracene, tetracene and pentacene) using the OPLS force field method and the density functional theory of the B3LYP/6-31G(d,p) level. The modeling was performed using the single crystal X-ray diffraction refined crystal structures of linear acenes. For anthracene, tetracene and pentacene crystals, the surface energy of the (001) face was experimentally estimated using the contact angle method. Expressions for the critical sizes of crystal nuclei in homogeneous and heterogeneous processes under conditions of growth from vapor and solution were obtained and analyzed using the classical thermodynamic approach and taking into account the anisotropy of the surface energy.

The molecular dynamics method is used to study the features of ion transport in lithium tetraborate Li₂B₄O₇ crystals with vacancy disorder. It is shown that ion transport caused by lithium ions is anisotropic. The highest values of diffusion coefficients are observed along the c-axis and amount to D_Li ~ 1×10^–6 cm²/s at temperatures close to the melting point. It is shown that lithium ions jump over distances from 1.5 to 3.5 Å via the vacancy mechanism, and the length of correlated jumps can reach 6 Å.

For the first time, the extended fine structure of the absorption spectra for a protein monolayer on a liquid surface has been measured. We used as a test object human serum albumin treated with zinc chloride solution at a critically low concentration (3.6 × 10^–7 M), which is comparable to the zinc concentration in blood serum. A multi-pass technique for detecting fluorescence radiation under total external reflection conditions was applied. The temporal stability of the Zn K-edge absorption spectra was examined using weighted regression analysis. It is shown that the error of the oscillating part of the total spectrum did not exceed 1%, which allowed to determine the radius of the first coordination sphere with an accuracy of ±0.01 Å.

Using the method of co-precipitation from aqueous solutions of nitrates, nano-sized polysubstituted solid solutions containing from 3 to 8 di- and trivalent cations in different quantitative ratios, with the general formula Mⁿ_1−xR^m_xF_2+x, where M = Ca, Sr, Ba, Pb, R = La, Gd, Dy, Yb, n = 3 and 4, m = 0, 1, 2 and 4 and x varies from ~0.07 to ~0.3 for different cation compositions, were obtained. All of them retain the fluorite structure (structural type CaF₂, space group Fm3m). The possibilities of precipitation of mixed solid solutions in different combinations of cations and using different fluorinating agents are considered. It is shown that the obtained solid solutions are medium- and high-entropy phases.

A method for obtaining hexagonal boron nitride (h-BN) nanoparticles 2–10 nm in size with a crystallinity of up to 99% has been developed. The method is based on two-stage heat treatment at temperatures of 600 and 1000°C using boric acid, urea, nitrogen and hydrogen. X-ray phase analysis, high-resolution transmission electron microscopy, electron diffraction in the selected area, and analysis of electron density maps confirmed the hexagonal structure of h-BN with an interplane distance of 3.3 Å, a narrow size distribution, and a uniform distribution of elements in the material. The proposed approach eliminates the use of toxic ammonia, is energy efficient and suitable for industrial scale. The resulting nanoparticles can be used in tribological coatings and lubricants.

The article presents the results of the study of aqueous solutions of guanylurea hydrophosphite (GUHP), in particular the isothermal cut of the phase diagram in the system of initial reagents for the synthesis of GUHP salt at 25 and 45°C. The solubility of GUHP in water was studied depending on pH. The growth conditions of bulk crystals were determined. The indexing of GUHP crystal faces was carried out. For the first time, the features of the real structure of GUHP single crystals were studied depending on the growth conditions.

The paper presents the results of studying the structural and phase composition, optical and thermoluminescent properties of β-Ga₂O₃ ceramic samples obtained by plasma gas-thermal spraying from the perspective of its application as a luminescent material. Both synthesized samples and those that had undergone post-growth high-temperature and plasma treatment were studied. It is shown that synthesized ceramic samples without additional processing do not provide the necessary sensitivity to radiation due to the high concentration of their own F-type defects, and the thermoluminescence peak is located at a temperature of 350°C, which is inconvenient for registration. Samples of β-Ga₂O₃ ceramics with post-growth high-temperature and plasma treatment, high thermoluminescence yield and thermoluminescence peak located at 120°C are competitive in comparison with thermoluminescent detectors on the market when irradiated with doses in the range of 0.2–2.5 Gy.

The article presents the approaches of a complex study of the 3D structure of a polymeric spongy matrix based on polylactide using X-ray microtomography without staining in combination with a non-invasive method of scanning pulsed acoustic microscopy and energy-dispersive X-ray microanalysis. Comparative studies of pure matrix samples, in vitro samples kept in a culture medium and after their implantation in vivo were conducted. The use of a complex of non-invasive methods revealed changes in the microstructure, including matrix degradation processes in a culture medium, after incubation with cells and implantation in a laboratory animal.

The disadvantages of the “strategy” scan-lists for diffraction experiments created by the diffractometer software are shown. The reason for these disadvantages is that the traditionally used target function has a limited, local meaning, for example, to obtain the best coverage of the reciprocal space. An approach is proposed that implements the principle of statistical randomization of the experiment and makes it possible to achieve the strategic goal of structural analysis – obtaining a model capable of reflecting the subtle details of the atomic structure. A scan-list balanced by most factors has been found, which in less time leads to obtaining experimental data of significantly higher quality than traditional lists. The use of a reference crystal, previously measured dozens of times on diffractometers around the world, has shown the advantage of experimental data obtained by a new method. Increasing the balance and accuracy of the data resulted in a maximum improvement in the values of the refinement criteria to R1/wR2 = 0.53/0.59% and Δρ = –0.47/+0.30 e/ų. The achievement of the strategic goal of the research of the reference crystal (confirmation of the anharmonic model of atomic displacement parameters) could be confirmed not only by the “purification” of difference Fourier syntheses of electron density, which is sometimes visual and subjective, but also by a statistically flawless decrease in the R-factors of refinement by 30–40 relative %. Data of such high quality are needed to study the dynamics of structural models under external conditions, for the detection and modeling of phase transitions, critical points, bio- and chemical activity of compounds, verification of computational methods of structures.