Crystallography Reports最新文献

The spectral dependences of transmission and absorption in the wavelength range of 200–2500 nm were measured for Ca₃TaGa₃Si₂O₁₄ crystals cut perpendicular to the optical axis in the initial state (without annealing) and after isothermal annealing in vacuum and in air. It was found that annealings in vacuum and air lead, respectively, to decrease and increase in the absorption band intensity. A spectrophotometric method for measuring and calculating the specific rotation angle ρ of the plane of polarization of light in gyrotropic crystals from the transmission coefficient spectra at different angles between the polarizer and analyzer is considered. The transmission spectra were normalized to eliminate the spectral shifts associated with measurement features. Spectral dependences of ρ, approximated by the extended Drude formula, were obtained for all three samples; the influence of the annealing atmosphere on the coefficients of this formula is established.

The results of studying the growth of para-quaterphenyl (4P) and its derivative—4,4'''-bis(trimethylsilyl)-para-quaterphenyl (TMS-4P-TMS)—crystals from solutions are presented. It has been established that TMS-4P-TMS crystals exhibit better growth characteristics as compared to 4Р. Parameters of the phase transitions of 4P and TMS-4P-TMS in closed crucibles were refined using differential scanning calorimetry. The crystal structure of TMS-4P-TMS in the triclinic space group P(bar {1}) (Z = 2) has been decrypted for the first time using single-crystal X-ray diffraction and was studied in a wide temperature range. A crystallographic analysis of the studied compounds in crystals was performed using the Hirshfeld surface method, and modeling of intermolecular interactions was performed.

The α-Na_0.35Dy_0.65F_2.30 crystal has been investigated for the first time by X-ray diffraction at 293 and 85 K and by electron diffraction at 293 K. The unified cluster model of the defect structure of fluorite-type nanostructured crystals, based on the polymorphism of ordered KR₃F₁₀ phases (R = Er, Yb), is extended by the matrix part model based on the KYF₄ structure. The unified cluster model is applied to construct the defect structure of α-Na_0.35Dy_0.65F_2.30. The matrix part of the crystal is found to contain Na⁺ and Dy³⁺ cations in a ratio of 1 : 1. Some of matrix anions are displaced to 32f sites (sp. gr. Fm(bar {3})m). Excess Dy³⁺ cations and Na⁺ cations form octa-cubic clusters [({text{N}}{{{text{a}}}_{{14-n}}}{text{D}}{{{text{y}}}_{n}}{{{text{F}}}_{{64 + n}}})] with cores in the form of distorted and regular {F₁₂} cuboctahedra. They are composed by interstitial anions at two 32f sites and one 48i site. The cluster component of the α-Na_0.35Dy_0.65F_2.30 crystal contains octa-cubic clusters of the f, f–i, and i types. The electron diffraction study revealed that α-Na_0.35Dy_0.65F_2.30 is a nanostructured crystal. Its cluster component includes platelike precipitates ~5 nm thick with superstructural ordering and unit octa-cubic clusters. A model is proposed for their structure. A decrease in the temperature to 85 K increases the number of interstitial F_(32f)1 anions in the matrix component of the crystal.

The crystal structure of a new mineral from the eudialyte group, amableite-(Ce) Na₁₅[(Ce_1.5Na_1.5)Mn₃]Mn₂Zr₃(square )Si[Si₂₄O₆₉(OH)₃](OH)₂ ⋅ H₂O, was refined using X-ray diffraction analysis in the R3 space group. This mineral was discovered in a hyperagpaitic pegmatite in the Saint-Amable massif, Canada. Amableite-(Ce) is a representative of the eudialyte group with the lowest calcium content; it differs from other representatives of this group by the dominance of lanthanides in a part of edge-sharing octahedra of the six-membered ring. The structure of amableite-(Ce) has a unit cell with the following parameters: a = 14.1340(2) Å, c = 30.378(1) Å, and V = 5255.6(3) ų. In this paper, we propose a model for the distribution of cations within the amableite-(Ce) crystal structure based on a low-symmetry P3 space group. We refined 162 independent atomic positions in the isotropic–anisotropic approximation using 3968 reflections with F > 3σ(F) and obtained R = 4.6% as the final result. Despite the close agreement between the two space groups, the transition from R3 to P3 allows us to obtain a more detailed information about the local distribution of a number of elements over different framework sites. The models of the crystal structure of amableite-(Ce) in the framework of R3 and P3 symmetries, as well as other low-calcium minerals from the eudialyte group that have been studied previously within several space groups, have been compared.

The effects arising at the collision of a stream of metal particles 10–100 µm in size, moving at a speed of 1–3 km/s, with a solid target are explained. It is suggested that, at the instant of impact on the target, a particle loses some electrons and retains a positive charge for some time due to the presence of an oxide shell. The electron flux passing through the target at the impact instant generates an electromagnetic field. A particle with a charge of ~10^–9 C, having penetrated a solid target, presses on the channel wall with a force of ~500 MPa and moves in it under the action the forces caused by the target material polarization. The combination of high pressure and displacement leads to a significant decrease in the particle–wall friction force. The proposed hypothesis, if confirmed, may help to find ways to protect spacecraft electronic devices from impacts caused by streams of fast dust particles.

Nanowires made of Fe_xCo_yCu(_{{(100-x-y)}}) and Fe_xNi_yCu(_{{(100-x-y)}}) alloys have been investigated. Specific features of the formation of these structures by template synthesis are studied. Elemental analysis of the nanowires grown at successively increasing voltages revealed a significant decrease in the amount of copper, as well as a change in the ratio of the main magnetic elements. X-ray diffraction analysis showed that FeCoCu is a three-component solid solution, whereas FeNiCu contains three phases of solid solutions: FeCu with a Cu content up to 80%, FeNi with a high iron content, and NiCu in the amorphous or fine-grained state with a Ni content up to 80%. Mössbauer spectroscopy revealed that addition of copper may cause a change in the misorientation angle of magnetic moments in nanowires; this finding correlates with the magnetometry data.

The photopolymerization of Langmuir–Schaefer films of symmetrical diacetylene N-arylcarbamate molecules containing different numbers of methylene groups has been studied. UV irradiation of the diyne films led to their effective transition to the stable state of blue phase polydiacetylene. The number of CH₂ groups in the molecules affected the efficiency of the transition of the monomer film to the polymer and the polymer chain conjugation length. The scanning electron microscopy of the film morphology revealed a horizontal arrangement of the diyne molecules on substrates in the state of domains, the sizes of which depend on the number of CH₂ groups in the molecules.

The experience in developing and applying methods for measuring the refractive coefficients of high and middle category crystals based on multi-angle spectrophotometric reflectance methods (using the reflection spectrum from one face at a close-to-normal angle of incidence of light and the reflectance method where light falls at the Brewster angle) is peresented. Advantages and limitations of the methods and requirement to samples are described. It is shown that the reflectance method at a close-to-normal angle of incidence is applicable to optically isotropic media. The Brewster angle method is applicable to high and middle category crystals. The measurement accuracy is determined for both methods. Their applicability is verified for high- (Gd₃Al₂Ga₃O₁₂ : Ce) and middle category (La₃Ga_5.5Ta_0.5O₁₄) crystals.

Digital processing of 2D X-ray projection images of a Coulomb-type point defect in a Si(111) crystal detected against the statistical Gaussian noise background has been carried out using a guided filter and a wavelet filter with the 4th-order Daubechies function. The efficiency of 2D image filtering has been determined by calculating the relative square deviations of the intensities of the filtered and reference (noise-free) 2D images averaged over all points. A comparison of the calculated relative root-mean-square deviations of the intensities has shown that the investigated methods work quite well and can be effectively used in noise processing of X-ray diffraction images for 3D reconstruction of nanoscale defects in crystal structures.

In order to investigate the influence of the structure and morphology of patterned sapphire substrates on the luminescent performance of GaN-based light-emitting diodes, light-emitting diodes with the same structure were prepared on flat and patterned sapphire substrates, respectively. The reasons for the formation of dislocations and V-pits in the devices were analyzed, and the current and luminescent characteristics of two devices with different substrates were tested and compared. The relationship between the luminescent characteristics and internal defects in the devices was also analyzed. Research has shown that patterned substrates significantly reduce the dislocation density inside light-emitting diodes, and the light output power and external quantum efficiency of light-emitting diodes grown on patterned substrates are significantly improved. However, dislocation may not be the main reason for efficiency drop.