Study of the internal structure of lithium fluoride single crystal by laboratory X-ray topo-tomography

Defects in a synthetic LiF crystal have been studied by X-ray topo-tomography on laboratory X-ray sources with a spatial resolution of ∼10 μm. An algebraic reconstruction method was applied to reconstruct the defect 3D structure of the crystal based on the diffraction data. The results presented in this study are in good agreement with the topographic data obtained by the Lang method.     

Study of the diffraction contrast of dislocations in X-ray topo-tomography: A computer simulation and image analysis

Oblique X-ray diffraction images of individual dislocations in the symmetric Laue geometry from a plane-parallel silicon plate have been calculated based on the Takagi-Taupin equations and analyzed. Computer simulation is used to develop a general mathematical model of the formation of oblique images which correspond to sample rotation around the diffraction vector h in X-ray topo-tomography. The results of numerical calculations and analysis of different oblique images of straight-line dislocations, where the dislocation line vector τ lies in a plane parallel to input surface of {111}Si plate with a diffraction vector h 〈220〉, are presented.     

Tracing of dislocations in ammonium hydrogen tartrate single crystals

Sparingly soluble ammonium hydrogen tartrate (AHT) crystals are grown by the gel method, derived from the diffusion of ammonium chloride into the set gel containing tartaric acid. Crystals up to 23 × 5 × 3 mm³ in size are grown at room temperature. AHT crystals are cleaved along (010) planes and the cleavage surfaces are studied by using multiple beam interferometry. The interferograms have revealed that the cleavages are quite flat. An attempt is made to trace the trajectory of dislocations of isolated as well as matched pairs of (010) cleavages of AHT when etched in a mixture of formic acid and methyl alcohol (2:1) and 1.0 M SrCl₂ solutions. Optical and transmission electron micrographs of dislocations show oblique, parallel and continuous line characteristics. Rows of equally spaced dislocation pits are observed and the implications of this are discussed.     

Kinetics of etching of dislocations in magnesium orthosilicate single crystals

Cleavages of undoped, doped and natural magnesium orthosilicate crystals have been simultaneously etched in concentrated hydrochloric acid solution. The lateral and normal velocities of the growth of pits were then measured at different temperatures for the varieties of crystals. The time dependence of the growth of pit dimensions is found to be linear, while the temperature dependence of the growth of pits is found to be exponential, viz. V = A exp (− E/kT). The dissolution parameters, e. g. the activation energy (E) and the pre-exponential factor (A) for dissolution along the surface and along the dislocation lines have been computed. It is observed that: (1) the activation energy (E₁) of dissolution along [001] direction is found to be greater than the activation energy (E_b) along [100] direction, irrespective of whether the crystals are doped, undoped or natural, and (2) the doping of the crystal with manganese has lowered down the activation energy (E_d) to such an extent that it is less than E₁ and E_b. The implications are discussed.     

The role of dislocations in the formation of mechanical stresses during annealing of gallium arsenide single crystals

The effect of dislocations on the change of mechanical stresses in undoped semi-insulating gallium arsenide single crystals has been studied during their annealing in vacuum and in the arsenic atmosphere. The phenomena observed are explained by the effect of dislocations playing the role of channels for arsenic diffusion on the concentration of intrinsic point defects in the crystal regions surrounding dislocations. The mechanism of arsenic diffusion over dislocations allows one to consider dislocations as sources and sinks of arsenic without the translational and twinning processes and, thus, makes the well-known data on the reactions of dislocation interaction with point defects and the experimental structural data for single crystals more consistent.     

Characterization of dislocations in a LiNbO3 single crystal grown by micro pulling down method

LiNbO₃ single crystals grown by the micro pulling down (μ-PD) method have been revealed to be as free of dislocations and subgrain boundaries up to 500 μm in diameter. On the other hand, μ-PD LiNbO₃ single crystals grown along the x-axis in diameter of 800 μm were observed to be dislocated due to the size effect of crystal. The Burgers vectors of dislocations were determined to be [22 01], [101 1], and [01 11] by X-ray topography.     

Study of the possibility of growing germanium single crystals under low temperature gradients

The possibility of growing germanium single crystals under low temperature gradients in order to produce a dislocation-free material has been studied. Germanium crystals with a dislocation density of about 100–200 cm^?2 have been grown in a system with a weight control of crystal growth at maximum axial gradients of about 1.5 K/cm.     

On splitting of dislocations at a reverse motion in germanium single crystals

The dislocation dissociation after the reverse motion along a slip plane in Ge single crystals have been investigated. It has been established that the dissociation width as well as the constriction density were independent on the motion direction. The results obtained have been used to clarify the role of the dislocation splitting in the formation of the phenomenon of the dislocation mobility asymmetry.     

Effects of inertia of dislocations on the probability of unpinning by ultrasound

The paper is concerned with theoretical models of dislocation segments fixed at their ends to local obstacles of either the same or two different types. The dislocation inertial properties are shown to influence substantially the probability of unpinning from the local obstacles as the crystal is subjected to ultrasonic oscillations. The unpinning probability in the presence of oscillations can be much higher than that with a constant load of the same magnitude. In the case of two types of obstacles, the weaker can play a “provocative” role facilitating unpinning of dislocations from the stronger obstacles. The considered effects constitute admittedly one of microscopic mechanisms of the plasticizing and “softening” action of ultrasound.     

Hardening of crystals caused by the dynamic aging of dislocations

A model relating the pronounced effect of mobile impurities on the processes of plastic deformation in crystals to the dynamic aging of dislocations is proposed. The model is based on the concept of internal stresses depending individually on the age of dislocations due to the difference in the impurity environment. The concentration dependence of the impurity contribution to the hardening of materials is calculated. The theory is illustrated by the experimental data for a GeSi solid solution.     

Effect of impurities on dynamic dragging of dislocations

The rearrangement of the force of dynamic dragging of a dislocation due to its elastic interactio in with impurity atoms is analysed. The collisions of a moving dislocation with impurities excite additional vibrations of a dislocation line. Such vibrations lead to the appearance of extra parts to dissipative function and effective friction force of a dislocation which depend on location and concentration of impurities. This effect is one of the reasons of the influence of impurities observed in the experiments on the dynamic branch of the moving dislocation curve.     

On the possibility of strain determination by section topography in the parallel Geometry

A new approach to the problem of strain determination in a crystal by means of X-ray radiation is presented. In the proposed experiment the border of the windows etched in silicon oxide layer which covers the surface of the silicon crystal, is image using a section topography and geometry in which the edge of the windows is placed parallel to the vertical X-ray beam axis. That geometry allows to obtain the images giving the possibility of more Precise and easier determination of strains, because the interference fringes (Pendellösung) are considerably narrow. Their position can be defined with better precision.     

The influence of elastic anisotropy on interaction of dislocations with point defects in BCC metals

With the use of the anisotropic theory of elasticity the energies and the force-distance profiles for the interaction of various types of straight dislocations with point defects of intrusion (atoms of N, C or O) and of replacement in BCC metals with a different degree of elastic anisotropy (Nb, V, Ta, α-Fe) have been calculated. The calculated force-distance profiles in most cases are not centrally symmetric, that leads up to a gliding asymmetry at the direct and reverse directions of dislocation motion. The crystal elastic anisotropy can change the relative mobility of dislocations of various types and accounts for a number of experimentally observed peculiarities of BCC metals stress-strain properties.     

The twisting of LiNbO3 single crystals grown by the Czochralski method

As a result of our experimental work of mainly practical importance we established that for LiNbO₃ single crystals the Mg and Fe dopants promote spirality whereas decreasing of the pulling velocity strongly reduces the propensity of the system to produce spiralshaped crystals.     

On the possibility of devitrification of ancient glass

The question whether some crystallization will take place in old glass during long-time storage (up to 1000 years) at low temperatures well below T_g is considered theoretically. Evaluations for the systems Na₂O · SiO₂ and Li₂O · 2SiO₂ are carried out using the model of homogeneous and heterogeneous nucleation theory. For these systems a barely observable crystallized volume fraction will be reached at 300 K only after storage for astronomical periods (10⁶ ? 10¹³ years), depending on the number of heterogeneities and the contact angle of heterogeneity. If the observed strong influence of H₂O content in glass on crystallization is also taken into account, crystallization will not take place for at least 1000 years of storage at 300 K. This limiting value is close to archaeological periods and will therefore merit careful attention in further investigations.     

The factors affecting the polarization transformations of X-rays in Laue diffraction from crystals with dislocations

The transformations of X-ray polarization with coherent σ-and π-components have been studied for the Laue diffraction from LiF crystals with a high dislocation density (10⁴–10⁵ mm^?2). The dependence of the parameter characterizing the diffraction-induced birefringence and the degree of coherence of the transmitted beam on the squared reciprocal-lattice vector (H ² = 4sin²θ/λ²) is determined. It is shown that the parameter characterizing the diffraction-induced birefringence in crystals with dislocations is determined by the static Debye-Waller factor (exp(-L)) and the effective thickness Δ of the crystal. The exponent L increases and the parameter Δ decreases with the dislocation density.     

The features of identifying lines in a diffraction image formed by a widely divergent X-ray beam

A method for identifying lines in a diffraction image formed by a widely divergent X-ray beam and a technique for measuring the crystal structure parameters in the case of asymmetric crystal position have been developed. It is established that, once the distances between a crystal and a photographic plate and between the points of intersection of the hyperbola branches in a diffraction image are known, one can determine the angle between the crystal’s zone axis and the wave vector, which leads to multiwave diffraction. Relations linking this angle with the parameters of two atomic planes are obtained. It is found that, to measure the parameters of atomic planes belonging to a given zone, one can use different sets of crossed hyperbolas formed by radiations K _α and K _β. The measurements and calculations performed for the same sample (Si crystal), mounted symmetrically and asymmetrically, confirm the reliability of the proposed method.     

Influence of non-stoichiometry on the recombination activity of dislocations in undoped semi-insulating GaAs crystals

The effect of non-stoichiometry of gallium arsenide melt composition on the recombination activity of dislocations in undoped semi-insulating GaAs crystals is studied and analyzed. It is shown that the deviation from the stoichiometric melt composition is one of the major factors affecting the recombination activity of dislocations in undoped semi-insulating GaAs crystals. namely: the recombination activity of dislocations is increased when the arsenic-rich melt is used, and, on the contrary, is decreased when the gallium-rich melt is used. The regularities observed are explained by the complex processes of interaction of dislocations with the non-stoichiometry-induced intrinsic point defects.