Crystallography Reports最新文献

The effect of the temperature on the single-crystal structure 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 decreasing temperature from 293 to 85 K was studied by X-ray diffraction analysis. A threefold decrease in the unit cell volume of the crystal was observed, while sp. gr. С2/с was retained.

The results of modeling the surface energy of the (100), (010), (001), (110), and (1(bar {1})0) faces of linear acene crystals (naphthalene, anthracene, tetracene, pentacene) using the OPLS force field method and density functional theory (DFT) of the B3LYP/6-31G(d,p) level are presented. 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 are obtained and analyzed using the classical thermodynamic approach, taking into account the surface energy anisotropy.

A study of the crystal structure of the high-calcium eudialyte from the Tamazeght massif (Morocco) revealed some structural features reducing the symmetry of this mineral. The trigonal unit-cell parameters are a = 14.1863(1) Å, c = 30.0703(2) Å, V = 5240.95(4) ų, sp. gr. P3. The structure, whose unit cell contains 161 independent atomic sites, was refined to the final residual factor R = 4.4% in the isotropic approximation of atomic displacements using 8202 reflections with F > 3σ(F). Ordering of Ca and Mn atoms in octahedra of six-membered rings; Fe and Mn atoms in М2 sites between translation-linked rings; as well as Na, Н₃O, La, and K in extraframework N sites was established. The chemical composition of the mineral studied is close to those of golyshevite and feklichevite, but differs from them by a lower calcium content and a higher manganese content. The main features of the mineral in comparison with the high-calcium eudialyte-group minerals investigated previously within the sp. gr. R3 are discussed. It is shown that the symmetry lowering in the structural model of high-calcium eudialyte-group representatives may be caused by the ordering of the cations occupying N sites of the extraframework part of the structure.

The results of a complex study of the dynamics of phase transitions in thin SmS films—the semiconductor–metal transition, induced by mechanical polishing, and the inverse thermally induced metal–semiconductor transition—are presented. It is found that the inverse phase transition occurs during sample cooling in the temperature range of 408–373 K. A change in the phase and elemental compositions of thin films is observed within the surface layer during the aforementioned phase transitions. The data obtained suggest that thin SmS films can be considered as structures with predictable and required dynamics of reversible phase transitions, making them promising candidates for engineering functional materials and elements of wide-range pressure sensors.

A method for fabricating 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 a two-stage heat treatment at temperatures of 600 and 1000°C, using boric acid, urea, nitrogen, and hydrogen. The h-BN hexagonal structure with an interplanar spacing of 3.3 Å, narrow size distribution, and uniform distribution of elements in the material have been confirmed by X-ray phase analysis, transmission electron microscopy, selected-area electron diffraction, and analysis of electron density maps. The proposed approach excludes the use of toxic ammonia and is energy efficient and suitable for industrial scaling. The obtained nanoparticles can be used in tribological coatings and lubricants.

A phase transition in a ~10-nm-thick Ge₂Sb₂Te₅ (GST) film upon heating it from room temperature to ~400°C has been studied using a pulsed electron diffractometer. During crystallization of a freely suspended amorphous sample, the formation of a hexagonal GST phase has been detected, in which mixing of Sb and Ge leads to a formal violation of the translational and unit-cell symmetries. However, upon heating an identical amorphous GST film on a carbon membrane, the crystalline state is represented only by a cubic phase. A qualitative explanation within the Phillips theory for such a nanoscale effect in GST has been proposed, which opens up new possibilities for controlling structural ordering in phase-change memory materials.

The results of studying the structural and phase composition and the optical and thermoluminescent properties of β-Ga₂O₃ ceramic samples, obtained by plasma gas-thermal spraying, from the perspective of the application of this ceramics as a luminescent material, are reported. Both the synthesized samples and the samples subjected to post-growth high-temperature and plasma treatment were studied. It is shown that the synthesized ceramic samples without additional processing do not provide the necessary sensitivity to radiation because of 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 subjected to post-growth high-temperature and plasma treatment, having a high thermoluminescence yield and thermoluminescence peak located at 120°C, are competitive with commercial thermoluminescent detectors when irradiated to doses in the range of 0.2–2.5 Gy.

The effect of impurity cations on the optical and electrophysical properties of La₃Ga₅SiO₁₄ crystals (sp. gr. P321), inactive and activated by Cr and Mn ions (1000 ppm), is shown. The optical properties of these crystals indicate different mechanisms of point defect formation, which leads to difference in their electrophysical properties. For the La₃Ga₅SiO₁₄ : Cr crystal, the dc electrical conductivity in the directions parallel (σ_||c) and perpendicular (σ_⊥c) to the optical axis in the temperature range of 673–825 K and the static permittivity (ε_||c and ε_⊥c) at 297 K were measured by impedance spectroscopy. The data obtained are compared with the results of previously published measurements for La₃Ga₅SiO₁₄ : Mn.

The first results obtained by a new experimental method for phase-contrast microscopy of microobjects using synchrotron radiation and a nanofocusing lens in a cone-beam geometry are presented. In the experiment, a secondary radiation source is formed in the lens focus located at a small distance from the microobject, enabling the acquisition of its magnified image. Under the near-field regime, the microobject structure can relatively easily be determined from the experimental image using the transport of intensity equation. The experiment was performed at the KISI-Kurchatov synchrotron radiation source. A model weakly absorbing microobject—commercially available carbon fiber of grade VMN-4—was used. The obtained fiber dimensions and structural features, resolved with submicron spatial resolution, are consistent with the electron microscopy data.