Double perovskite crystals of nominal composition PbMn1/2Nb1/2O3 were grown by spontaneous crystallization. Their structure has been studied by X-ray diffraction on a powder prepared by grinding crystals. According to the results of the Rietveld structure analysis, the powder includes phases of perovskite Ph1 ( 75.5 wt Pm3̅m (221)) with compositions Pb1.000(4)Mn0.54(2)Nb0.46(2)O3 and Pb1.00(1)Mn0.32(5)Nb0.68(5)O3, respectively, and the impurity phase of PbO ( 3.2 wt
An X-ray structural analysis of tsepinite-K from the pegmatite no. 62, Karnasurt Mountain, Lovozero alkaline massif (Kola Peninsula), was performed. The mineral belongs to the labuntsovite group and differs from other its representatives in a “combined” structural type (partial occupancy of the key D position). Its structure was previously studied within the space group Cm. A repeated study of the structural features of tsepinite-K was carried out with the transition to space group Pm. The unit-cell parameters are a = 14.3265(3) Å, b = 13.802(2) Å, c = 7.783(1) Å, β = 116.949(3)°, V = 1371.8 Å3; R = 0.042 for 1677 reflections with F > 2σ(F). A more detailed ordering of cations over structure positions was established.
A kinematical theory of a restricted X-ray beam diffraction in a bent crystal has been developed, enabling calculations of reciprocal space maps. A solution is obtained in the form of a Fourier integral equation, where the integrand is written in terms of Fresnel integrals. Numerical simulation of the angular distribution of diffracted intensity in reciprocal space from a plane-parallel plate, as well as from slightly and strongly curved crystals, is performed based on the obtained solution and two-dimensional recurrence relations. The results of calculations performed by the two methods are compared.
This study utilized corn stover as the biomass feedstock to produce stover charcoal (CC) through modification with oxalic acid. The charcoal was then composited with polyvinylidene fluoride (PVDF) using a coating-crosslinking method, resulting in CC/PVDF composite membranes. By optimizing key parameters including crosslinking agent concentration, crosslinking time, and charcoal loading, a composite membrane with outstanding ammonia adsorption performance was obtained. The membrane’s morphology, chemical composition, and mechanical properties were characterized by scanning electron microscopy, infrared spectroscopy, and a universal testing machine. The results indicate that the optimal ammonia adsorption capacity of 40.4 mg/cm2 was achieved under the following conditions: a binder ratio of 1 : 2, a crosslinking agent concentration of 3
Applying the technique of gradient melt freezing in a fluorinating atmosphere, large single-phase blocks of cerium fluoride crystals with a characteristic brown color, associated with quenching of point defects, were fabricated, and the influence of the formed color centers on the optical transmission and ionic conductivity of CeF3 during thermal heating in a vacuum was studied. The material under study crystallizes in the sp. gr. P3̅c1 and is characterized by additional optical absorption in the wavelength range of 350−650 nm, caused by the formation of color centers. Annealing of CeF3 at temperatures up to 827 K leads to thermal destruction of defect centers and release of fluorine vacancies, which causes an increase in the ionic conductivity of the crystals by ∼2 times as compared to the initial level, reaching a value of σdc = 3.5 × 10−4 S/cm (500 K), while maintaining the ion-transfer activation enthalpy ΔHa = 0.37 eV. The obtained results can be used to develop modes of post-growth thermal annealing of Се3+-based crystalline materials.
The PbW _1-x MoxO4 solid solutions were simulated by the method of empirical interatomic potentials. The mixing functions of the volume and the enthalpy show deviations from the additivity, indicating that the solid solution is non-ideal. The investigation of the local environment of the ions demonstrated that the size of PbО8 polyhedra increases with increasing molybdenum content in the solid solution, and these polyhedra are significantly distorted in intermediate compositions. The МоО4 and WO4 tetrahedra are more stable; however, their sizes also slightly decrease with increasing concentration of molybdenum, and these tetrahedra are also distorted in intermediate compositions. The changes in the local structure of the solid solutions lead to changes in their properties compared to those of the initial components, and the distortions of the polyhedra may improve the spectroscopic properties of activators involved in these solid solutions.
—The preparation and comprehensive structural analysis of nanostructured Nd16Fe76B8, alloy, synthesized chemically using nanodispersed Nd2O3, α-Fe2O3, and Fe3BO6 precursors, are described. The morphology and dispersion of the initial nanoparticles and the final product were studied using transmission electron microscopy. The phase composition and crystal structure were determined by X-ray diffraction, followed by refinement using the Rietveld method. The main phase of the alloy was shown to be tetragonal Nd2Fe14B, which is consistent with the results of X-ray diffraction analysis. The local state of iron was further studied by Mössbauer spectroscopy at 293 K: the spectrum is satisfactorily described by six Zeeman sextets corresponding to the positions of Fe atoms 16k 1, 16k 2, 8j 1, 8j 2, 4e, and 4c in the Nd2Fe14B structure. The hyperfine interaction parameters confirm that Fe belongs to the Nd2Fe14B phase and do not require the introduction of components characteristic of Fe2B/FeB borides. The obtained results demonstrate applicability of the chemically assisted reduction–diffusion approach for the production of nanostructured Nd–Fe–B alloys with a controlled phase structure.
The effect of thermal exposure of textured YBCO ceramics on its structure and properties after long-term (25 years) storage at room temperature has been studied. Effective restoration of the orthorhombic structure and improvement of the superconducting properties of YBCO ceramics via its saturation with O2 at 500°C for 10 h using powders of the same composition with an optimum oxygen content have been demonstrated. The 00l-type reflections 001, 002, 003, 004, 005, 006, 007, 008, and 009 revealed by X-ray diffraction analysis are indicative of a high degree of texturing of the ceramics. Oxygen saturation of the ceramic samples leads to a change in the conductivity type from semiconductor to metallic. It has been established that, with an increase in the degree of oxygen saturation, the intensity of the O(4) mode at 500 cm–1 and the mode at 340 cm–1 corresponding to vibrations of O(2)/O(3) atoms in the CuO2 planes significantly increase.
The influence of precursors and the nominal CoO:Fe2O3 ratio on the stoichiometry and morphology of cobalt ferrites was investigated. The alloys were produced by directed solar melting at the focus of the Big Solar Furnace followed by water quenching. The microstructure was studied using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), while the local composition was analyzed by energy-dispersive X-ray spectroscopy (EDS). It was shown that variation of the Co/Fe ratio determines the particle size, degree of agglomeration, and structural ordering. For compositions close to CoFe2O4, more homogeneous structures are formed with a higher degree of crystallinity. Control of the melting/quenching regime stabilizes the particle size distribution. The established “precursors–composition–morphology” relationships confirm the reproducibility of solar synthesis and enable targeted selection of the initial CoO and Fe2O3 ratios and molar fractions to align the local stoichiometry with the desired nominal composition.
The results of a comprehensive study of the electronic and crystal structure of cobalt ferrite alloys (CoFe2O4) synthesized by solar melting are presented. Using X-ray phase analysis and the Rietveld refinement method, the crystal lattice parameters were determined and secondary phases of CoO and Fe2O3 were identified. Electron density mapping was performed, which allowed us to establish the nature of the Co–O and Fe–O bonds, as well as to identify the covalent component in the crystal lattice. The data obtained indicate the influence of CoFe2O4 composition on the symmetry, phase purity, and electron density distribution.
—The crystal structure of a high-manganese variety of the eudialyte-group mineral ikranite has been studied. It is distinguished from original ikranite (Na,H3O)15(Ca,Mn,REE)6Fe _2^3 + Zr3( □ ,Zr)( □ ,Si)Si24O66(O,OH)6Cl⋅ nH2O (n = 2–3) by Mn3+ admixture and some structural features. The trigonal unit-cell parameters of ikranite-Mn3+ are a = 14.1872(1) Å, c = 30.11654(2) Å, V = 5249.60(6) Å3. The crystal structure was refined in space group Р3 (166 independent positions, R = 0.046, 4829 independent reflections with F > 3σ(F)).
A simple sol–gel method for the synthesis of ZnO films co-doped with Fe and Al to a total concentration of 6 at
Criteria are established for the existence of an acoustic (Stoneley) wave in an mmm-symmetry medium at the twist boundary coinciding with the plane of symmetry. The results obtained are applicable to media of other symmetries in which the tensor of elastic moduli has the same structure as in the mmm symmetry class.
The results of an analysis of the anomalous thermomagnetic Nernst–Ettingshausen effect in diluted magnetic semiconductors are reported. The fundamental relationships determining the effect’s amplitude and its dependence on the external magnetic field are considered. Analogies between the anomalous Nernst–Ettingshausen effect and the anomalous Hall effect are drawn. It is shown that a combined study and analysis of these phenomena allows for obtaining more detailed information about the nature of ferromagnetic ordering in thin films of a diluted magnetic semiconductor than when using either method separately. Studies of the anomalous phenomena near the Curie point of diluted magnetic semiconductors of the (A3, Fe)B5 and (Ga, Mn)As types have been performed.
The oligomeric composition of nicking endonuclease Nt.BspD6I (nickase) was studied by small-angle X-ray scattering (SAXS). The obtained results were confirmed by native electrophoresis. An analysis of the SAXS data demonstrated the presence of monomers, dimers, and trimers in solution. The volume fractions of dimers and trimers increase with an increase in the concentration of the enzyme. The structures of the nickase homodimers and the nickase complex with DNA were refined by the molecular docking and molecular dynamics simulations. The homodimer interface includes amino acid residues of the N-domain of one nickase molecule and the C-domain of another molecule. Studies of the step of the nickase binding to the recognition site demonstrated that the position of the cleaved phosphodiester bond is distant from the active site of nickase. These results open up new possibilities for suppressing the nickase oligomerization and for the subsequent crystallization of the nickase–DNA complex.
Scanning and transmission electron microscopy were used to study the properties of hydroxycarbonate precursors of Gd, Y, Lu, Ce, Yb, Al, and Ga, as well as the resulting garnet-type oxide powders and ceramic materials. The precursors and oxide powders were synthesized by coprecipitation from solutions followed by heat treatment. The ceramics were produced by uniaxial pressing and sintering in an oxygen atmosphere at 1720°C without applying pressure. It was found that the chosen synthesis method yields powders with uniform microstructure, chemical, and phase composition, suitable for the production of ceramic materials with a linear optical transmittance of approximately 60
According to the second law of thermodynamics, the face of a growing crystal is always subject to total adsorption of impurities. Certain free energy fluctuations, capable of causing impurity desorption from the crystal face, allow building particles to temporarily attach to the kinks of dislocation growth steps. In a concentrated, supersaturated solution, fluctuations in the Gibbs free energy can create relatively long-lived transition-state clusters, which also function as impurities in the adsorbed state. It is possible to select a time interval during which the growth of a crystal face satisfies the conditions of the limit theorem of probability theory on the sum of random variables in a random number, according to which the probability density of the growth of a crystalline layer of a substance on a crystal face is described by an exponential function. This is how the transition from an equilibrium system, in which entropy fluctuations can be considered a reversible phenomenon, to a nonequilibrium system with an arrow of time occurs.
In the CuCl2–NH2(CH2)2NH2–H2O system, a family of crystals Cu(NH2(CH2)NH2)Cl2, Cu(NH2(CH2)2NH2)2Cl2⋅H2O, and Cu(NH2(CH2)2NH2)3Cl2⋅2H2O was obtained. X-ray diffraction analysis of all three compounds was carried out. It has been established that, depending on the ratio of copper chloride and ethylenediamine in the solution during synthesis, different coordination environments of copper are formed: square-planar and octahedral, in which different ligands are involved. The optical transmission spectra and thermal properties of the grown crystals were studied.
The results of a study of the photoluminescence characteristics of β-Ga2O3 ceramic samples synthesized by plasma thermal spraying on a sapphire substrate are presented. The photoluminescence spectra of the initial powder and the synthesized ceramics, upon excitation in the region of interband transitions, exhibit luminescence peaking at 430 nm and lasting for milliseconds. We attribute this luminescence to a triplet-singlet transition at the F-center. After high-temperature treatment of the synthesized ceramics, the intensity of this luminescence decreases, and the luminescence of chromium R lines in the 693 nm region increases, against a background of broadband emission at 680–740 nm with a tail extended to 850 nm. High-temperature annealing leads to possible recharging of trace amounts of variable-valence impurities; changes in the coupling of impurity ions with the environment, the degree of which depends on the fractional size of the synthesized ceramic; and changes in the ratio of impurity and structural F-type defects in the ceramic. This leads to the formation of a new configuration of metastable levels in the band gap and manifests itself in luminescence. Laser irradiation was used as a model to monitor the kinetics of the high-temperature treatment process. The research is focused on the potential use of ceramics as luminescent materials.
Molecular dynamics simulation was used to study ion transport processes in neodymium molybdate Nd5Mo3O _16 ±δ crystals with various types of defects. In oxygen-deficient samples, the carriers are anions in the O1-type positions. When additional oxygen ions are introduced into O4-type positions, the transport occurs due to O2-type anions, while when additional oxygen ions are introduced into O5-type positions, O2- and O5-type anions participate in transport processes. Anion jumps can occur over short distances of approximately 1 Å, as well as over distances of 2.3–2.7 Å. In some cases, ion movement takes the form of successive jumps with a total displacement of 4–6 Å.
