Crystallography Reports最新文献

The compaction kinetics, phase composition and mechanical characteristics of pure SiC and pure TiC ceramics, as well as SiC–TiC composite ceramics, obtained by spark plasma sintering (SPS), were studied depending on the component content and SPS-modes. It was found that at a fixed pressing pressure (P_SPS = 45 MPa) in a given range of temperatures and sintering times (1800 ≤ T_SPS ≤ 2000°C and 0 min ≤ t_SPS ≤ 10 min), the relative density of sintered ceramics increases with increasing temperature and sintering time and reaches maximum values of 88, 99.9, and 96.2 of the calculated value for SiC, TiC, and SiC + TiC(10 wt %), respectively. It is shown that sintering of composite ceramics in the SiC–TiC system occurs through the formation and subsequent high-temperature decomposition of intermediate phases of binary carbides. It is demonstrated that the microhardness of SiC–TiC composite samples is determined as an additive value between the hardnesses of the included phases SiC (HV = 2948 ± 238 N/mm²) and TiC (HV = 2465 ± 256 N/mm²).

The possibility of synthesizing gallium oxide semiconductor layers by the classical thermal method of molecular layering with trimethyl gallium and ozone precursors and controlling the electrical properties of the synthesized layers is shown. Homogeneous layers of specified thickness have been deposited in a single cycle onto a silicon substrate, monocrystalline quartz, and high-aspect-ratio 3D substrates in the form of microchannel plates. The synthesized gallium oxide layers are amorphous, conforming to the Ga₂O₃ stoichiometry; have a band gap of 4.9 ± 0.2 eV; and do not exhibit the impurity conductivity. The possibility of obtaining gallium oxide films with the impurity conductivity by molecular layering according to a specified program, with alternation of the chemical composition of the precursors used, is demonstrated.

The re-study of the crystal structure of the second find of low-calcium and rare-earths bearing voronkovite in the Lovozero alkaline complex, Kola Peninsula has revealed new features of its structure, which lower the symmetry of the mineral. Ordering of Ca, Mn, Ce, and Na atoms in octahedra of six-membered rings, as well as Zr, Fe, and Na atoms in M2 positions, combining translationally identical six-membered rings as well as Nb, Ti, and Si atoms and vacancies in axial fragments of the structure, has been established within the P3 symmetry. The trigonal-cell parameters of the mineral are a = 14.1617(1) Å, c = 30.1815(1) Å, and V = 5242.09(4) ų; the number of independent positions is 184. The crystal structure has been refined to the R factor of 3.74% in the isotropic atomic displacement approximation using 8257 reflections with F > 3σ(F). The main features of voronkovite in comparison with the minerals investigated previously within the framework of the sp. gr. P3 are discussed.

Disordered crystalline systems, which exhibit a wide range of optical properties when formed under conditions similar to natural, have become an object of intensive recent research aimed at creating new-generation materials for photonics. A promising direction in this field is the development of compounds of cubic garnet structure type, which is formed basically from two- and four-charged cations in nature. The principles of creating multifunctional crystalline materials with a garnet structure and possibilities of targeted control of their properties through compositional disorder in the cationic subsystem of the crystal are considered. A comparative analysis of the material obtained at the National Research Centre “Kurchatov Institute” (NRC KI) and commercial scintillation materials for PET scanners is performed. New areas of possible application for energy production using isotope sources are demonstrated.

The diversity of free crystal shapes in nature is statistically described by the coefficient of variation in the central distances to symmetrically equivalent faces of simple crystallographic forms present on the crystal. The coefficient of variation for natural crystals lies in a limited range of values from 0.02 to 0.2. This regularity, implemented during growth, dissolution, and in the equilibrium state, is accompanied by a number of natural phenomena, which is explained by the fluctuation model of crystal growth. An increase in the coefficient of variation leads to temperature rise; nonstationarity of the process; and dissymmetrizing phenomena, which include gravitational field, anisotropy of facet supply, and geometry of free space. A decrease in the coefficient of variation causes prolonged growth or dissolution and symmetrization effects, which manifest themselves under the complex influence of several factors, including mechanical impacts on the crystal.

All 318 992 records of triclinic crystal structures, presented in the Cambridge Crystallographic Data Centre (CCDC-2023), were analyzed for presence of structural second-order phase transitions occurring without changing the crystal system. Four crystals (10 records) undergoing a classical second-order phase transition were found. The phase transition order was not established for 35 crystals (97 records). The low-symmetry phases of the structures undergoing a classical second-order transition revealed pseudosymmetry relative to the symmetry operators of the high-symmetry phase with large degrees of electron density invariance, η ≥ 0.896(1), with the group–subgroup relationship for symmetry groups retained. A technique for determining the most mobile structure fragment during a second-order phase transition is proposed.

The α-Na_0.4R_0.6F_2.2 crystals (R = Ho–Lu, Y) have been studied by X-ray diffraction analysis at 293 and 85 K. A unified cluster model of nanostructured crystals with a fluorite-type structure, based on the polymorphism of KR₃F₁₀ (R = Er, Yb), was used to model their defect structure. The α-Na_0.4R_0.6F_2.2 matrix component contains Na⁺ and R³⁺ ions in a ratio of 1 : 1. Part of the matrix anions are shifted from the 8c to 32f site (sp. gr. (Fmbar {3}m)). Excess R³⁺ cations form, jointly with Na⁺, octa-cubic clusters with cores in the form of cuboctahedra {F₁₂}, consisting of interstitial anions at the 48i site. The α-Na_0.4R_0.6F_2.2 cluster component is formed by octa-cubic clusters of type i. The electron diffraction study showed that the clusters are shaped as plates about 5 nm thick with superstructural ordering. Their structural model based on the K_0.265Gd_0.735F_2.47 structure was proposed. Experimental confirmation of the affiliation of  (alpha {text{-N}}{{{text{a}}}_{{0.5-x}}}{{R}_{{0.5 + x}}}{{{text{F}}}_{{2 + 2x}}}) to nanostructured crystals was obtained for the first time by electron diffraction. When temperature decreases from 293 to 85 K, the type of the cluster component of the defect α-Na_0.4R_0.6F_2.2 structure with R = Ho–Lu, Y does not change. At 293 K, the boundary of the change in the defect structure type in the (alpha {text{-N}}{{{text{a}}}_{{0.5-x}}}{{R}_{{0.5 + x}}}{{{text{F}}}_{{2 + 2x}}}) series is located between R = Dy (Z = 66) and Ho (Z = 67). With a decrease in temperature from 293 to 85 K the position of the boundary does not change.

Comprehensive studies of three Attic plastic vessels dated to the 4th century BC from the State Historical Museum collection have been performed. The use of  X-ray tomography (XRT), large-scale X-ray fluorescence (XRF) mapping, energy-dispersive X-ray microanalysis (EDX) under scanning electron microscopy (SEM), and synchrotron X-ray diffraction (XRD-SR) analysis made it possible to study in detail their state of preservation (including restoration traces) and the manufacturing technology, as well as to identify the pigments used in painting the surface. Based on the pigment residues, identified visually and on XRF maps, a reconstruction of the polychrome painting of the vessels was proposed. Because of the poor state of preservation of the coating on the surface of two vessels, the reconstruction of their possible polychrome painting was performed based on a comparison with known analogues.

The X-ray Rietveld method has been used to refine the structure, determine the lattice periods, and study the phase composition of the samples of multicomponent polycrystalline solid solutions TbCo(_{{(2-x)}})In_x (х = 0, 0.05, 0.1, 0.15, 0.20, 0.25, 0.3, 0.35, 0.4), which are characterized by large values of magnetostriction saturation. With an increase in the indium concentration, the content of the TbCo₂ phase with a Laves phase structure decreases, the content of the TbCo₃ phase increases, and a Tb₁₁Co₄In₉ phase is formed. The lattice period in the TbCo₂ compound (sp. gr. Fd(bar {3})m) changes nonlinearly: increases in the range of x = 0–0.1 from a = 7.209(8) Å to a = 7.216(1) Å due to the replacement of cobalt atoms with indium atoms, having a larger radius. Then, in the concentration range of x = 0.15–0.4, it decreases to a = 7.205(1) Å at х = 0.4 due to the replacement of terbium atoms with indium and formation of structural defects.

This paper describes the phase composition, morphological, chemical, and crystallographic properties of silicide phases in Cr–Fe–Si alloy ingots obtained through directional solidification, both before and after annealing. The samples had a general formula Cr_xFe(_{{1-x}})Si₂ with x = 0.1, 0.2, 0.3, and 0.4. Characterization of samples using various techniques (powder X-ray diffraction, transmission and scanning electron microscopy with electron backscatter diffraction, energy dispersive X-ray spectroscopy) showed that the use of a higher Cr concentration (x = 0.3 and 0.4) leads to suppression of the formation of the ε-FeSi metallic cubic phase. The amount of Cr involved in the substitution of Fe in α-, β-, and ε-Fe silicides does not depend on the nominal Cr concentration introduced into initial melts. Unlike the samples that underwent the free crystallization process, neither pure Si nor some other silicides (for example, Cr₅Si₃) or pure Si were found in the ingots of directional crystallization. The orientation relationships between the phases and the directions of growth of precipitates before and after annealing were established. Electrical conductivity, Seebeck coefficient and power factor were determined for the as grown and annealed ingots.