Geometrical aspect of solving the problem of real structure growth on the model of alkali metal halides of the NaCl type

The formation of a nanoscale packing polyhedron of layer-by-layer model-structure growth using a specified system of bonds between structural units, which determines the space partition in crystalline or quasicrystalline state of matter, was described in [1]. In a continuation of these studies, the application of the mechanism of sectorial layer-by-layer growth of the structural unit neighborhood graph to the formation of the spectrum of model coordination packing growth polyhedra of NaCl-type alkali metal halides is considered. A computer experiment based on the cell content clusterization and application of the radial algorithm of neighborhood graph growth showed that the existence of small (NaCl)₂ clusters in the form of planar dimers is preferential for the equilibrium growth shape of sodium chloride crystal as a regular cube in the precrystallization state of vapor phase.     

Inverse problem in the layer-by-layer growth model

The problem of finding structural features of growing structures for which a specified growth shape is allowable is considered within the model of layer-by-layer growth of tilings or packings and the generalization of this model to connectivity graphs. In particular, the problem of revealing growth shapes that are impossible for periodic structures is solved.     

A new model for the structure of chalcogenide glasses: The closed cluster model

A new model is proposed for the structure of low atomic coordination chalcogenide glasses. The closed clusters model is based on the various types of clusters, packed by van der Waals forces in a molecular-type packing. Arguments are given to support the new model, taken as example the typical chalcogenide glass As₂S₃.     

Controllable growth of ZnO nanowires with different aspect ratios and microstructures and their photoluminescence and photosensitive properties

ZnO nanowires with variable aspect ratios and microstructures have been prepared by a hydrothermal reaction of Zn foil and Na₂C₂O₄ solution at 140 °C. The ZnO nanowires are single crystalline with the wurtzite structure and grow in the [0 0 0 1] direction, and their aspect ratios and microstructures can be changed by tuning the reaction time and the Na₂C₂O₄ concentration. UV and blue-green emissions that depended on the Na₂C₂O₄ concentration are observed from the ZnO nanowires with different aspect ratios. The photosensitivity of ZnO ultralong nanowires with honeycomb-like micropatterns is found to be about 10 at 5 V.     

On automation of the procedure for crystal structure model refinement

The methods of automation of the procedure for crystal structure model refinement from experimental diffraction data, implemented in the ASTRA program package, are described. Such tools as statistical tests, parameter scanning, and data scanning reduce the time necessary for structural investigation. At strong correlations between parameters, especially when the data set is limited, parameter scanning has an advantage over the full-matrix refinement.     

A modular model of the crystal structure of the pyrochlore-murataite polysomatic series

This paper reports on the results of a structural investigation of an important group of synthetic compounds with derivative structures of the mineral murataite, a potential matrix for immobilization of radioactive wastes. A model describing the structure of the entire group of compounds is proposed on the basis of analyzing the results obtained using chemical (electron microprobe) analysis methods, high-resolution transmission electron microscopy (high-resolution electron images and microdiffraction patterns), and X-ray diffractometry of a large array of artificial murataite ceramics. In essence, the proposed structural model is as follows: in the structure, layers of two (pyrochlore and murataite) packing types alternate along the [111] direction of the cubic supercell, so that the resulting structures can undergo substitutional and displacive modulations. Experiments on irradiation of different murataite modifications with heavy ions demonstrate that these modifications have close values of the radiation resistance, which is higher than that of the pyrochlore coexisting with them.     

Crystal growth and dislocation structure of gallium antimonide

Single crystals of GaSb have been grown by the Czochralski method under reducing conditions. Crystals were grown in the 〈100〉, 〈111〉, and 〈112〉 directions. The 〈111〉 growth direction was found to be the most suitable for the successful and reliable crystal growth. Dislocation densities in 〈111〉 oriented crystals were examined by chemical etching. The etch pits density in these crystals didn't exceed the value of 1 × 10² cm⁻².     

Theoretical model of crystal growth shaping process

A theoretical investigation of the crystal growth shaping process is carried out on the basis of the dynamic stability concept. The capillary dynamic stability of shaped crystal growth processes for various forms of the liquid menisci is analyzed using the mathematical model of the phenomena in the axisymmetric case. The catching boundary condition of the capillary boundary problem is considered and the limits of its application for shaped crystal growth modeling are discussed.

The static stability of a liquid free surface is taken into account by means of the Jacobi equation analysis. The result is that a large number of menisci having drop-like shapes are statically unstable. A few new non-traditional liquid meniscus shapes (e.g., bubbles and related shapes) are proposed for the case of a catching boundary condition.     

Crystal structure of the growth surface of silicon carbide obtained by sublimation

Structural study of polycrystalline silicon carbide obtained by sublimation performed via X-ray luminescence and X-ray diffraction analysis. It is shown that chemical vapor deposition of silicon carbide results in the formation of grains with the (00.1), (01.1), and (12.3) crystallographic planes parallel to the growth surface. The grains with the (00.1) growth planes are characterized by perfect structure and by red luminescence. Domains with yellow luminescence have a mosaic structure with the (01.1) and (12.3) growth planes.     

Solution of the inverse problem for reconstructing the real structure of materials from the data of different X-ray methods

A software program has been developed for joint solution of the inverse problem on the basis of the X-ray diffractometry and reflectometry data. A system of basic objects supporting some functions of automatic data flow processing in the program and oriented to mathematical calculations has been created. It is shown by the example of experimental data for an In _x Ga_{1 ? x} As-In _y Al_{1 ? y} As/InP(001) sample that joint fitting of X-ray diffractometry and reflectometry curves makes it possible to reconstruct the model parameters of a multilayer with smaller rms errors. The data were processed taking into account the angular dependences of the background intensity and the aperture factors stemming from the geometry of the experiment.     

Geometrical properties of dislocations in potassium bichromate crystals

The geometry of growth dislocations present in potassium bichromate crystals grown from aqueous solution has been studied by etching. Etch topographs composed of dislocation etch grooves and grooves representing sector boundaries and growth bands have been analysed. It was found that most of the dislocation lines are straight and have well-defined directions in each sector. Refractions of dislocations at growth sector boundaries and at other defects have been observed.     

Crystal growth,structure and characterization of p-Toluidinium picrate

p-Toluidinium picrate (PTP), is a proton transfer complex of 2,4,6-trinitrophenol as an electron acceptor with p-toluidine as electron donor, crystallizing in the monoclinic system with four molecules in the unit cell (space group P2₁/c). The vibrational patterns of the organic crystal PTP in comparison with that of the parent compound clearly evidences the complex formation. Loss of hydroxyl proton at O1 leading to specific electron delocalization around C1 is observed. Crystallographic data are reported as a=12.9304(6)  Å, b=15.7176(7)  Å, c=7.5403(4)  Å, β=101.837(5)°. The crystalline cohesion is achieved by N–H…O and C–H…O hydrogen bonds and the ions are linked into three dimensional network. Intermolecular hydrogen bonding between nitrogen of p-toluidine and phenolate ion of picric acid results in charge transfer. A sharp endotherm in the DSC curve, no decomposition up to the melting point and poor absorbance in the visible region indicate the suitability of the material for potential applications.     

Geometrical Aspects of the Patterns of Conduction Paths in Fast Ion Conductors

A method for a straightforward calculation of geometrically privileged conduction paths in fast ion conductors is presented. It makes use of a special property of Dirichlet domains, namely that their vertices and edges define all polyhedral voids and all least narrow connecting passages within a rigid framework. The results of the procedure are exhaustive in the geometrical sense and may be used as a basis for physical investigations. The method is illustrated by two simple examples, α-AgI and sodium β-alumina. Restrictions are discussed.     

An operational scheme to determine the locally preferred structure of model liquids

We present an operational method to determine the ‘locally preferred structure’ of model liquids, a notion often put forward to explain supercooling of a liquid and glass formation. The method relies on finding the global minimum in the (free) energy landscape of clusters of atoms or molecules embedded in a liquid-like environment. We propose a more systematic approach of the external potential mimicking the influence of the surrounding bulk liquid on the cluster than in our previous work. The procedure is tested on the one-component Lennard-Jones liquid and we recover, without a priori input, that the locally preferred structure is an icosahedral arrangement of 13 atoms.     

A dynamic model of alumina inclusion collision growth in the continuous caster

The number and morphology of non-metallic inclusions are the key factors to estimate the quality of the steel product, so it is of particular importance to have a profound knowledge about the collision and aggregation among the inclusions in the molten steel. In this paper, a three-dimensional mathematic model was developed to predict the dynamic growth of alumina inclusions in the continuous caster. This model includes Eulerian approach for turbulent flow field and inclusion’s concentration and number density fields, and Lagrangian approach for inclusion’s collision growth. The numerical results indicate that the convective transfer leads to the ‘W’ distributions of inclusion in the continuous caster. This phenomenon is consistent with Javurek’s experiment. For a single inclusion, the larger inclusion has more chances to trap other inclusions, and the trapped inclusions tend to be the smaller inclusions with diameter less than 10 μm. The aggregated inclusions are dendritic with some tips and arms, which have been observed in Yin’s experiment. Cluster formation of alumina inclusion depends on the turbulent flow, which contributes to the enhancement of the collision rate and determines the inclusion growth mechanism.     

Formation of the block structure during growth of basal-plane-faceted sapphire ribbons with different widths

The defect structure of basal-plane-faceted sapphire ribbons 10, 30, and 50 mm wide is investigated. It is shown that the high density of blocks in these crystals is associated with the specific features of the plastic deformation of the crystals with this orientation during crystal growth. In the basal-plane-faceted ribbons, the plastic deformation begins with the slip in the prismatic system. Dislocations of this system form characteristic low-angle boundaries, which then transform into block boundaries. It is revealed that the slip in the easiest basal system, which is characteristic of ribbons with all other orientations, is impossible because of the geometric constraints. Block-free basal-plane-faceted ribbons 50 mm wide are produced by decreasing thermal stresses in growing crystals.     

The role of surface and interface structure in crystal growth

Crystal growth occurs at the interface of a crystal and its growth medium. Due to the abrupt termination at the surface, at the interface the properties of the crystal will typically deviate from the bulk and this can affect the growth behaviour. Also the properties of the growth medium at the interface will typically differ from the bulk. In growth from solution, for example, the liquid will show ordering induced by the crystal surface or have a different composition. Here techniques to study such growth interfaces will be discussed together with examples of the effect that the properties of the interface can have on the growth.     

Mathematical model and numerical simulation of faceted crystal growth

A new mathematical macroscopic model is proposed to describe the nonstationary process of faceted crystal growth by the methods of directional crystallization with a slow change in external thermal conditions and low pulling rate of a cell through the growth system. The facet-growth rate is determined by the Stefan condition, integral over the face. Two boundary conditions are set for temperature: the continuity condition and the relation between the heat-flux jump and the supercooling at the facet points. The supercooling is determined by solving the entire heat problem. A facet is selected as a planar part of the phase boundary. The kinetic coefficient at the facet may depend on the supercooling. The energy conservation law is valid within the model developed. Examples of calculations of some model problems are presented.