On the 70th anniversary of the Shubnikov Institute of Crystallography of the Russian Academy of Sciences

We are celebrating the 70th anniversary of the Shubnikov Institute of Crystallography. The institute has gone from being a small laboratory of piezoelectric crystals to a large internationally known scientific center. The 1990s was a decade of ruin, both literally and figuratively, for our institute; however, it is hard to believe that was the case when walking through the corridors of the institute today.     

Numerical analysis of the velocity of SiC growth by the top seeding method

Velocity of crystal growth of SiC in a process of solution growth was studied on the basis of a global model of heat and mass transfer in conjunction with a phase diagram of the Si–C system. The growth rate was estimated by flux of carbon to a seed crystal. The results of calculation showed that growth velocity was increased when temperature of a seed crystal was increased. The temperature dependence of growth velocity was mainly determined by the phase diagram of the Si–C system, although the flow pattern was slightly modified by changing temperature distribution in the furnace.     

Effect of the temperature on the TMOS sono-hydrolysis and a study of the structure of the resulting aged sonogels

The effect of temperature on the oxalic acid catalyzed sono-hydrolysis of tetramethoxysilane (TMOS) was studied by means of a heat flux calorimetric method. The activation energy of the process was measured as (24.5 ± 0.8) kJ/mol in the temperature range between 10 and 50 °C. The structural characteristics of the resulting sonogels, after long period of aging in saturated conditions, were studied by means of small angle X-ray scattering. The structure can be described as formed by ∼2.7 nm mean size mass fractal-like aggregates (clusters) of primary silica particles of ∼0.3 nm mean size, all imbibed in a liquid phase. The average mass fractal dimension of the clusters was found to be 2.58. The primary particle density was estimated as 2.23 g/cm³, in good agreement with the value frequently quoted for fused silica. The volume fraction of the clusters in the saturated sonogels was estimated as about 28%. The moment in which the meniscus of the liquid phase penetrates into the clusters under rapid evaporation process has been detected by an inflection in the first derivative of the curve of weight loss in a simple thermogravimetric test.     

On the Influence of the Crystallization Conditions on the Microstructure of the Directionally Solidified Eutectic of the LiF – LiYF4 System

This paper particularly deals with the transitions in degree and type of ordering of a given microstructure. As a model system the eutectic of the LiF – LiYF₄ system was used for studies of transitions from ‘anomal’ microtextures with non-cooperative production of seeds of both phases to the fibrous microtexture. It was shown that even if the matrix phase LiYF₄ contains a relative high volume ratio (36%) of LiF it is possible to create highly ordered fibrous microtextures with pronounced hexagonal ordering of particular LiF fibres. It was also shown that methods of image processing could be used for sensitive evaluation of small changes in degree of ordering of the microstructure. These procedures make possible a deeper understanding of the influence of the solidification conditions on the microstructure of the directionally solidified eutectic and creates possibilities for optimalisation of the technological process.     

Levitation of metallic melt by using the simultaneous imposition of the alternating and the static magnetic fields

This study developed a new levitation method, which used the simultaneous imposition of static and alternating magnetic fields. Dynamic behavior was measured for pure Cu and pure Ni melts levitated by the proposed method. The oscillation due to surface tension and convection in levitated Cu melts were hardly observed at static magnetic fields exceeding 1 T. Only the rotation of this axis parallel to the static magnetic field was observed under high static magnetic fields. The proposed method demonstrated that metallic melt could be statically levitated like a solid sphere. It was also found that stable levitation of paramagnetic Ni melt was rather difficult at static magnetic fields exceeding 5 T, because of the magnetization force.     

Investigations of the crystal growth of PbMoO4 by the Czochralski method

The thermal conditions at the liquid/solid interface are characterized by temperature measurements in the melt and on the growing crystal with 〈001〉-seed orientation. The thermal boundary layer was determined, from which the relation of effective thermal conductivity of the liquid and solid phase was found to be λ_l,eff/λ_s,eff = 0.25. The liquid/solid interface is extremly convex towards the melt and has a conical shape. When the crystal diameter reaches a certain value the cone was truncated with the formation of a facet in the centre. This typical interface shape is mainly the result of a difference between the effective thermal conductivity of melt and crystal.     

A novel sol-gel method for the synthesis of γ-aluminium oxide: development of the sol-gel transformation and characterization of the xerogel

A novel method for the synthesis of aluminium oxide gel has been developed, whereby the sol-gel transformation was investigated. Aluminium tri-sec-butoxide was used as precursor while acetone was chosen as solvent. The synthesis was carried out in a special reactor, which allowed the dosing of steam. ²⁷Al NMR spectroscopy showed that during the sol-gel process the signal at δ∼3 ppm increases strongly corresponding to the formation of hexacoordinated aluminium species. Beside hydrolysis and condensation reactions, the coordination of acetone to a strong Lewis acid aluminium site occurs, which was shown by FTIR and ²⁷Al NMR spectroscopy. Viscosimetric analysis showed that at the beginning of the sol-gel process short polymers are observed while before the gelation a three-dimensional polymer network is formed. After pyrolyzing the gel a high surface area γ-aluminium oxide xerogel was formed. The effect of heating on the morphology and structure was examined by nitrogen physisorption (BET and pore size distribution), XRD and ²⁷Al MAS NMR spectroscopy.     

Study of the metastable region in the growth of GaN using the Na flux method

It was revealed that the metastable region, in which liquid phase epitaxy (LPE) of GaN single crystals proceeds without the generation of polycrystals, expands with growth temperature in the Na flux method. The metastable region appears when LPE growth proceeds at temperatures above 1073 K, although generation of polycrystals inevitably occurs on a crucible at temperatures less than 1073 K. The highest growth rate of 14 μm/h in a small experimental setup was achieved at a temperature of 1163 K with a nitrogen pressure of 5.5 MPa due to complete suppression of the growth of polycrystals on a crucible, even though the supersaturation at this condition reached a fairly high level.Also, an LPE crystal with a flat surface could easily be obtained under high-temperature conditions.     

Characteristic of the photoconductivity of thallium monoselenide

Systematic investigations on thallium monoselenide single crystal photoconductivity were reported at 300 K as a function of excitation intensity, side illumination, and wavelength. The samples were prepared using Bridgman technique. The photoconductivity as a function of the modulation frequency of the incident light was plotted at different values of light intensity. The effect of light intensity on the photocarrier lifetime was studied. The relation between the photocurrent and applied voltage at different values of light intensity was represented graphically. Side illumination effect in all these relations was checked. The spectral distribution of the photoconductivity of TlSe in the spectral region 1.24 ev to 1.77 ev was measured, and the energy gap was evaluated. All results were discussed and analyzed. Studies for such samples are very important and necessary in order to understand their use as photoconductor.     

Measurement of Flexo-coefficients for Nematic LCs without the Influence of the Impurity Ions by Using the Chromatographic Isolation Method

The sum of the flexo-coefficients (e₁₁+e₃₃) was measured by the capacitance characteristic depending on the applied dc voltage in the HAN cell. The voltage for the minimum value of the capacitance was shifted by the influence of flexoelectric effect. One of the important problems for the evaluation for the flexo effect was the influence of impurity ions. Then, the chromatographic isolation phenomenon was used to separate impurity ions when the LC material was injected into the empty cell by the capillary action. The coefficient (e₁₁+e₃₃) of ZLI-4792 was measured, and a value of 26.0 pC/m was obtained.     

The effect of the translational symmetry of crystalline silicon on the structure of amorphous germanium in the interfacial region

The structure of an amorphous Ge layer near an interface with a Si(111) crystal was studied by quantitative high-resolution electron microscopy. It was found that the translational symmetry of a Si crystal leads to the crystal-like order in the positions of Ge atoms in the interfacial region, the width of which is about 1.4 nm. In this region, the average orientation of interatomic bonds tilted with respect to the interface compensates for the difference in the bond lengths in crystalline Si and amorphous Ge and is responsible for the tetragonal distortion of the most likely atomic positions.     

Measurement of the refractive index of glass fibers by the Christiansen-Shelyubskii method

The Christiansen principle was employed to measure the refractive index of borosilicate glass fibers (13–41 μm diameter) over the visible range. The refractive index for glass fibers at 589.3 nm was measured by temperature and wavelength scan and values obtained were in close agreement. The refractive index for glass fibers as a function of wavelength was measured to an accuracy of < 10⁻⁴. The uniformity of the refractive index for a bundle of fibers of slightly different diameter was calculated using the modified Shelyubskii method and compared to experimental values. Theoretical calculations of the transmission by the present work suggest that, for high optical clarity and transmission of Christiansen cell (or transparent composite consisting of glass fiber and polymer), the refractive index must be controlled to the fifth decimal place. For example, the maximum transmission of a fiber/liquid mixture cell at 25°C can increase from 89 to 97% when the standard deviation is reduced from 13 × 10⁻⁵ to 9 × 10⁻⁵.     

Optimization of the cascade crystallizers

For the cascade crystallization process a set of technological optimization conditions was established. The possibility of succesful application of proposed optimization conditions was pointed out by comparison with a mathematical model of Na₂SO₄ · 10 H₂O crystallization in multichamber crystallizer presented in the literature. The method has a general applicability and can be used for the optimization of any cascade-linked system, provided that the mathematical model was detailed enough to permit the operation of numerical derivation.     

Investigation of the deformed region on the (111) plane of MgO crystals by cathodoluminescence method

The (111) plane of MgO single crystals was deformed by a concentrated load. It was observed that the number of rows of a cathodoluminescence rosette is a factor of two lower than that for the etch pit one. The explanation of this phenomenon is given. The polarity of plastic deformation was revealed using both an etch pit technique and cathodoluminescence method. It is shown that deformation causes a change of the cathodoluminescence spectra. The spectra, obtained from crystal deformed by indentation and from compressed crystal, are similar.     

On the distribution coefficient of sulphur during the crystallization of GaP from nonstoichiometric gallium melts

GaP grown by the SSD technique was doped by sulphur in a wide concentration range. Using chemical, electrical and radiochemical methods the concentration and temperature dependence of the distribution coefficient k during this crystallization from the nonstoichiometric melt was determined. With the crystallization temperature at 1000°C for k a value 6.9 ± 1.4 was found independent on the sulphur quantity added. The distribution coefficient decreases clearly with increasing temperature.     

Molecular dynamics study of the influence of mobile cations on the reconstruction of an irradiated silicate glass

The effect of ballistic collisions in a simplified nuclear glass was investigated by molecular dynamics. Systematic results were obtained in the 0–16 keV energy range. Following a damage peak, reconstruction of the glass structure was observed in terms of the overall degree of polymerization. The reconstruction was facilitated by the presence of mobile cations. The dynamics of restoration of the SiO₄ tetrahedrons during displacement cascades and during the formation of a structure from a random configuration can be fitted to curves corresponding to the same analytic formula. This similarity allowed us to examine the influence of mobile cations (alkalis or alkaline earths) on the formation of SiO₄ entities in smaller systems formed from random configurations. The formation rate of SiO₄ tetrahedrons accelerates with the Na₂O or CaO percentage to reach a maximum rate above about 10 mol% Na₂O or CaO. This threshold corresponds to the disappearance in the glass structure of zones comprising a central bridging oxygen surrounded by two first-neighbor bridging oxygen rings. No mixed alkali effect was observed in systems containing both Na and Cs because the formation of SiO₄ does not require long-range diffusion of mobile cations.     

Evaluation of the highly coherent surface structure of the GaAs (411)A plane using scanning tunneling microscopy

We have observed the surface structure of (411)A GaAs by atomically resolved scanning tunneling microscopy for the first time. A highly ordered structure of straight atomic rows stretching along the [01 ] direction was observed. The structure was found to be composed of two kinds of alternately arranged rows with different contrasts. From this image, a surface structure model was proposed. It was also indicated that the surface roughness of a (411)A surface must be smaller than that of a (100) surface due to the corrugated surface structure in the former case.     

Photographic observation of the growth of GaP-Needles from the wet hydrogen process

GaP-needles were grown by the transport of GaP material with hydrogen in an open tube system. The needle growth was observed with a camera. Some needles were discovered whic grew without a visible Ga-droplet on their tips. It is assumed that these needles grew by the surface diffussion of GaP-complexes along the side planes of the needles.     

Vapor phase growth of GaN using GaN powder sources and thermogravimetric investigations of the evaporating behaviour of the source material

The growth of GaN from the vapor phase is a promising technique for producing both bulk GaN crystals and GaN layers. For establishing a growth method from the vapor phase the source material and reactor setup are of great importance. Highly pure and self synthesized GaN powder was chosen as source material. The evaporation behaviour of the GaN powder was studied by means of thermogravimetry (TG). A vertical growth reactor was set up according to the results of numerical simulations of the temperature distributions and flow patterns. Freely nucleated GaN platelets of some millimetres in length were grown. Furthermore, thin GaN layers were deposited directly on a sapphire substrate. This nucleation layer was successfully overgrown by low pressure solution growth. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)