Kristallografiâ

Digital processing of the 2D noisy X-ray diffraction images (2D-XDI) of a single point defect in Si(111) crystal, registered at the level of dispersion of statistical Gaussian noise of the detector using filtering methods such as singular value decomposition and 1D-line-by-line smoothing of test 2D-XDIs, is carried out. The efficiency of digital filtering of 2D-XDI is evaluated and analyzed by means of control parameter FOM (figure-of-merit) value of reconstruction of the displacement field function of a point defect of Coulomb type f_h(r–r₀), (h – diffraction vector, r₀ – radius-vector of the defect position in the sample). It is shown that the filtering technique using the singular value decomposition of 2D-XDI works significantly better than the 1D linear-by-line smoothing method of 2D-XDI, which, apparently, in relation to our problem requires further research on its improvement.

A classification of special intercrystalline boundaries in centrosymmetric crystals of all syngonies 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 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.

Extended defects in large synthetic diamonds grown at high pressures and high temperatures (HPHT) using thermal gradient method were studied by X-ray topo-tomography with a laboratory setup. It is shown that temperature and growth rate are the principal factors influencing crystalline quality of the diamonds. Synthesis at high temperatures and low rate allows growth of single crystals with low density of dislocations, stacking faults and microtwin lamellae. Annihilation kinetics of the extended defects during annealing at high pressure implies important role of vacancies diffusion. The annihilation of such defects leads to formation of perfect dislocations.

The kinetics of kinks and domain walls in quasi-one-dimensional systems is described within the framework of a model intermediate between the sharp kink model and the continuum model of an elastic string. The effects resulting from the discrete structure of crystalline materials are considered, including the periodic inhomogeneity of the energy relief for kink migration. Within the framework of a transparent approximation using a minimum number of internal variables, the dependence of the Peierls barriers on the driving force is calculated and the transition between static and dynamic regimes is described. The theory is based on the universal Frenkel-Kontorova model and can be applied to extended systems of various natures.

The structure of lithiated fluorite-like molybdates of the Ln₅Mo₃O_16+δ family was studied using accurate X-ray diffraction analysis in the temperature range 20–777°C (293–1050 K). Thermally activated redistribution of oxygen ions over basic and interstitial positions and reversible change in the position occupancies upon heating–cooling were established. Thermal stability, ionic oxygen conductivity and dielectric properties of ceramic samples of undoped and Li-containing Nd₅Mo₃O_16+δ phases were studied in the temperature range 20–900°C. The results demonstrate the possibility of controlling the functional properties of materials for use in medium-temperature solid oxide fuel cells.

The paper presents the results of many years of work on the development of a new method of temperature control and the apparatus developed on its basis for the automatic growth of protein crystals, in which the possibility of separate control of both the process of nucleation and growth of the formed crystals is realized.

Memory effects in magnetoplasticity of NaCl crystals with different impurity content are studied. Dislocation paths and microhardness of crystals after their exposure to a constant magnetic field or to crossed ultralow magnetic fields are measured. In two crystals, noticeable relaxation displacements of dislocations, introduced after exposure, are found. In two other crystals with a lower impurity concentration, the paths remain at the background level, but in one of them, exposure causes an increase in the density of mobile dislocations. Similar magnetic exposure also leads to a decrease in the microhardness of crystals, but to different extent. Interpretation of observations is reduced to a spin-dependent transformation of impurity centers in a magnetic field, which plasticizes the crystal. Introduction of dislocations after magnetic exposure leads to their relaxation displacements from unstable positions, and at strongly weakened pinning centers, dislocations straight away take up positions close to equilibrium.

Critical phenomena in cubic helimagnets with nonequivalent magnetic atoms are investigated within the framework of the Weiss mean-field theory. The reason for the appearance of temperature dependences of elastic moduli and the pitch of the magnetic helicoid is found and the form of these dependences, determining the change in the conditions for the appearance of magnetic skyrmions in type II multiferroic Cu₂OSeO₃, is predicted.

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 modulus change. The circular photovoltaic current J^C does not change in phase depending on the amount of optical activity, but depends on 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 crystals Bi₁₂SiO₂₀, Bi₁₂GeO₂₀ and Bi₁₂TiO₂₀ (class 23). Similar J^L calculations were performed for the right crystals Pb₅Ge₃O₁₁ (class 3), La₃Ga₅SiO₁₄ with impurities of Pr, Fe, Cr and Mn, Ca₃TaGa₃Si₂O₁₄ (class 32), Er(HCOO)₃·2H₂O (class 222) when light propagates in the direction of the optical axis. Examples of the J^C value for Pb₅Ge₃O₁₁, La₃Ga₅SiO₁₄ crystals with impurities of Co, Cr, and Fe, as well as α-HgS (class 32), are given. It is shown that consideration of optical activity is necessary when studying the photorefractive effect in crystals.

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

A theoretical analysis of the motion of an ensemble of edge dislocations in an aged binary alloy under conditions of high-energy external influences 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 a minimum on the resulting dependence.

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

The article presents the results of 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 with nanosecond laser pulses using electron microscopy and energy-dispersive X-ray microanalysis. It is shown that in a two-phase alloy, a layer of nanocrystalline MgO with a thickness from 5 nm to hundreds of nm is formed on the surface in the α-Mg matrix region. A recrystallized layer of columnar crystallites with a thickness of about 1 μm and a lateral size of 0.2–1 μm with inclusions of cubic MgO is formed under it. In the area of the intermetallic compound (phase Mg₁₂YZn–REE type 18R), a three-layer amorphous-crystalline structure is formed: an amorphous layer 15–20 nm thick on the surface, under it a crystalline layer of columnar crystallites 0.1–0.3 μm thick and with lateral dimensions of 0.1–0.5 μm, then an amorphous layer of intermetallic compound about 1 μm thick.