Crystallography Reports最新文献

The relationship between dislocation and autowave models of plastic flow is established. It is shown that the activity of a deformed medium required for generating autowave processes of deformation is provided by the dislocation structure of the medium. The relationship between the dispersion of autowaves and the stages of plastic flow and the dislocation structures observed at each of them is analyzed and explained. A mechanism for excitation of low-frequency auto-oscillations in a deformed medium due to the elastic interaction of dislocation ensembles with moving dislocations is proposed. The mutual complementarity of the autowave and dislocation approaches to the description of plasticity is discussed.

A classification of special intercrystalline boundaries in centrosymmetric crystals of all crystallographic systems is constructed based on the symmetric properties of planar lattices. It is shown that the set of orientation parameters identifying special boundaries is determined by the orientation of the plane formed by the matching atoms of the contacting crystals. Unlike for the general type of boundaries, the number of these parameters is either two or three. It is shown that the lattice of matching bicrystal nodes appears only in crystals with high-order axes of symmetry. Possible misorientations of the contacting crystals are found, depending on the symmetry of the crystallographic plane for different crystallographic conditions.

The effect of optical activity on the linear and circular bulk photovoltaic effect in crystals without a center of symmetry is investigated. It is shown that there is a phase shift of the linear photovoltaic current J^L, which is opposite for the right- and left crystals, and a change in modulus. The circular photovoltaic current J^C does not change in phase in dependence of the optical activity value but depends on the absorption and circular dichroism. The dependences of the J^L current on the polarization of incident light are calculated taking into account the optical activity for the right and left Bi₁₂SiO₂₀, Bi₁₂GeO₂₀, and Bi₁₂TiO₂₀ crystals (class 23). Similar calculations of J^L were performed for the right crystals Pb₅Ge₃O₁₁ (class 3); La₃Ga₅SiO₁₄ with impurities of Pr, Fe, Cr, and Mn, Ca₃TaGa₃Si₂O₁₄ (class 32); and Er(HCOO)₃⋅2H₂O (class 222) in the case where light propagates in the direction of the optical axis. Examples of the J^C value for crystals Pb₅Ge₃O₁₁; La₃Ga₅SiO₁₄ with of Co, Cr, and Fe impurities; and α-HgS (class 32) are given. It is shown that consideration of optical activity is necessary when studying the photorefractive effect in crystals.

An X-ray diffraction technique utilizing a synchrotron radiation source has been proposed and implemented to study the processes of magnetic domain structure evolution in external fields. Highly perfect single crystals of iron borate FeBO₃ were used as model objects. A series of  X-ray and magneto-optical experiments was performed to investigate the evolution of the magnetic domain structure in weak external magnetic fields. It has been established that the movement of domain walls leads to a stepwise broadening of the diffraction reflection curves of FeBO₃ crystals. It is demonstrated that X-ray diffraction studies of the magnetic domain structure can be useful for characterizing magnetic materials where direct observation of domains by magneto-optical and electron-microscopic methods is hindered.

The problem of enhancing the efficiency of detecting radiation in the long-wavelength part of the optical spectrum is considered. The second-harmonic generation in two-dimensional photonic crystals based on iodic acid is investigated in order to convert long-wave radiation into the visible range for subsequent detection by conventional silicon detectors. The problems of synthesis of two-dimensional photonic crystal structures for practical problems of nonlinear optics and photonics are discussed.

The results of computer processing for plane-wave X-ray topography imaging of a Coulomb-type point defect in a Si(111) crystal, recorded by an X-ray detector against a background of the Gaussian noise, and their subsequent filtering using the 1D-, 2D-sized guided and a heuristic wavelet 4th-order Daubechies atomic function, are presented and analyzed. The topography image filtering efficiency is determined by the parameter of the relative square deviations (averaged over all pixels) of the pixel intensities (RMS) of the processed and reference (noise-free) 2D images. Practical methods for selecting filtration parameters are proposed, with the aid of which the considered methods work well enough to be used in practice for the noise processing of plane-wave X-ray topography images, meaning their subsequent use for the 3D digital recovering of nanosized crystal defects.

A theoretical analysis of the motion of an ensemble of edge dislocations in an aged binary alloy under conditions of high-energy external impacts has been performed. Within the framework of the theory of dynamic interaction of defects, an analytical expression is obtained for the dependence of the dynamic yield strength on the density of dislocations. It is shown that a high concentration of Guinier–Preston zones in an aged binary alloy leads to the occurrence of a minimum on the obtained dependence.

The results of electron microscopy and energy-dispersive X-ray microanalysis studies of the structure and composition of the surface and near-surface layers of the Mg–Y–Zn–Nd–Yb–Zr alloy system with a long-period phase after irradiation by nanosecond laser pulses are reported. It is shown that in two-phase alloy, a nanocrystalline MgO layer with a thickness from 5 nm to several hundreds of nanometers is formed on the surface in the α-Mg matrix region. A recrystallized layer of columnar crystallites with a thickness of ~1 μm and a lateral size of 0.2–1 μm, with inclusions of cubic MgO, is formed under this layer. In the area of the intermetallic compound (Mg₁₂YZn–REE phase of the 18R type), a three-layer amorphous–crystalline trilayer is formed; it consists of a (15–20)-nm-thick amorphous surface layer, a (0.1–0.3)-μm-thick crystalline layer of columnar crystallites with lateral sizes of 0.1–0.5 μm under it, and a ~1-μm-thick amorphous intermetallic layer.

The results of studying the recombination-enhanced dislocation motion in GaN and 4H-SiC have been analyzed. It is shown that in both crystals, when irradiated by a low-energy electron beam, dislocations can shift at liquid nitrogen temperature. The activation energies of the dislocation glide stimulated by electron beam irradiation are estimated. Results demonstrating practically activation-free migration of double kinks along a 30° partial Si-core dislocation in 4H-SiC are presented. It is shown that localized obstacles significantly affect the dislocation motion in GaN both under the action of shear stresses and under irradiation. The excess charge carriers introduced into GaN by irradiation not only help to overcome the Peierls barrier but also stimulate dislocation unpinning from obstacles.

The paper presents the results of the many-year development of a new method for the temperature control of the crystal nucleation and growth. This method has made the basis of a setup, which allows the separate control of protein crystal growth processes in an automatic mode.