Synthesis of chalcogenide and pnictide crystals in salt melts using a steady-state temperature gradient

Some examples of growing crystals of metals, alloys, chalcogenides, and pnictides in melts of halides of alkali metals and aluminum at a steady-state temperature gradient are described. Transport media are chosen to be salt melts of eutectic composition with the participation of LiCl, NaCl, KCl, RbCl, CsCl, AlCl₃, AlBr₃, KBr, and KI in a temperature range of 850–150°C. Some crystals have been synthesized only using a conducting contour. This technique of crystal growth is similar to the electrochemical method. In some cases, to exclude mutual influence, some elements have been isolated and forced to migrate to the crystal growth region through independent channels. As a result, crystals of desired quality have been obtained using no special equipment and with sizes sufficient for study under laboratory conditions.     

The migration of gas-filled brine inclusions in rock salt under a temperature gradient

In natural halite crystals two sorts of inclusions will move if there exists a temperature gradient. The direction of moving brine inclusions is orientated towards the heat source. Brine inclusions which additionally contain more than 10% gas move towards the cold end of a specimen. This model is well known but detailed information about the migration mechanism of these so-called SORBY inclusions have been lacking in literature so far. In this paper it will be demonstrated that SORBY inclusions having a diameter of up to 20 micrometers can move undisturbed through the crystal. SORBY inclusions with a diameter of more than 20 micrometers are fixed and form droplets with their tips directed towards the cold side of the specimen. Later when the tips have grown the inclusions will emit a new daughter inclusion which can move through the crystal. The velocity of migration depends on the generation of daughters. Extended kinds of SORBY inclusions are stable for a long time. However, these can decay into many individuals which will move through the crystal like the others.     

Lysozyme crystallization in a precision-controlled temperature gradient

The possibility to control the crystallization process of biomaterials by the temperature was examined by the mathematical simulation of lysozyme crystallization. The precision-temperature control with creation of a local temperature gradient in solution was demonstrated to allow the growth of lysozyme crystals with high structural perfection.     

Water and the glass transition temperature of silicate melts

Literature data on the effect of water on the glass transition in silicate melts are gathered for a broad range of total water content c_w from 3 × 10⁻⁴ to 27 wt%. In terms of a reduced glass transition temperature T_g^*=T_g/T_g^GN, where T_g^GN is T_g of the melt containing c_w≈0.02 wt% total water, a uniform dependence of T_g^* on total water content (c_w) is evident for silicate melts. T_g^* decreases steadily with increasing water content, most strongly at the lowest water content where H₂O is dominantly dissolved as OH. For water-rich melts, the variation of T_g^* is less pronounced, but it does not vanish even at the largest water contents reported (≈27 wt%). T_g^* vs. c_w is fitted by a three-component model. This approach accounts for different transition temperatures of the dry glass, hydroxyl and molecular water predicting T_g^* as a weighted linear combination of these temperatures. The required but mostly unknown water speciation in the glasses was estimated using IR-spectroscopy data for hydrous sodium trisilicate and rhyolite.     

Controlling the axial temperature gradient in inductively heated Czochralski systems

A heating system is described that allows to increase the temperature gradients in rf‐heated Czochralski setups during the growth. This system applies an active afterheater, operated by a separate induction coil parallel to that heating the crucible. By changing the inductivity of an additional coil located outside the growth chamber, the ratio of the rf currents flowing through the crucible and afterheater and, consequently, the geometry of the heat input to the setup can be altered. The efficiency of the heating systems has been demonstrated in various experiments.     

Effect of temperature gradient on the lamellar structure formation

The detailed definition of interplay between interface curvature and concentration field is presented. It explains an effect of horizontal component of temperature gradient on the lamellar structure formation during the steady-state eutectic growth. This definition is composed of two competitive terms connected with the heat and matter transfers, respectively. The above competition is interpreted by using the thermodynamics of irreversible processes. Finally, it is shown that the lamellar spacing depends on both the growth rate and temperature gradient for a given regular eutectic solidifying under stationary conditions. The proposed analysis is the fourth part of model developing in “Crystal Research and Technology” which is mentioned in the References.     

A low temperature synthesis of zirconium oxide coating using chelating agents

Coordination chemistry was applied to avoid undesirable precipitation in the hydrolysis reaction of tetrabutoxy zirconium (TBZR). Among the chosen β-dicarbonyl ligands, acetylacetone (AcAc) and ethyl acetoacetate (EAcAc) were found to be effective, while diethyl malonate (DEMA) was not. We also studied the hydrolysis, complex formation and curing process. Coating films obtained from these solutions were not only transparent and alkaline resisting, but also curable at low temperature.     

The transportation of brine inclusions in rock salt in a temperature field of a heat source

There are three kinds of brine inclusions in a natural crystal of halite, as follows: a) inclusion totally filled with brine; b) inclusions partially filled with brine and gas (SORBY inclusions) and c) inclusions totally filled with gas. Type a) and b) migrate in the crystal under the influence of a temperature gradient. Type c) inclusions do not move. This paper deals with the question whether the migration causes tracks in the crystal or not. In general, no tracks are caused directly by the motion of inclusions. However, there are some phenomena which can be explained as tracks. The reasons of these phenomena are examined and described.     

Study of deformation fields as a function of the temperature gradient in the mirror block of an interferometer

The formation of moiré fringes in an X-ray interferometer is studied. It is shown experimentally that the well-known expression for calculating the period of moiré pattern cannot be always used to calculate the period of moire patterns obtained in an X-ray interferometer. The change in the moiré period caused by the temperature gradient in the crystal block of an interferometer is calculated.     

The effect of mass transfer on the temperature gradient of a crystal growing from melt

The change in the temperature gradient on the crystal side while the rate of crystal growth from melt is varied has long been debated. Abe and Takahashi have recently reported an unambiguous experimental demonstration that the temperature gradient is a decreasing function of the growth rate, which is different from previous theories, experimental results, and widely held notion of other researchers. The present paper provides a theoretical basis for this seemingly peculiar effect of the growth rate on the temperature gradient. The essential matter is the effect of mass transfer, the role of which had been commonly disregarded in old studies. Although the rate of mass transfer is not large compared to that of heat conduction, it is proven that the temperature gradient is subjected to the mass transfer in a definite manner. Our analysis shows that the effect becomes significant when the crystal diameter is large, which is consistent with the experimental observation. Another effect of the mass transfer is the change in the shape of melt/crystal interface. In old studies, the temperature gradient was determined by Stefan's equation; however, this treatment confuses the cause and effect. The temperature gradient should be determined by the fundamental equation of heat conduction. When the gradient is determined in this way, the shape of the melt/crystal interface spontaneously adjusts to satisfy Stefan's equation.     

Growth of p-type InP single crystals by the temperature gradient method

P-type InP single crystals doped with zinc or cadmium have been prepared from a solution using indium as a solvent in which a temperature gradient was maintained for the transport of the InP solute. The average growth rate was between 1.1 and 3.0 mm/week. The room temperature hole concentration of the crystals obtained was in a range of 10¹⁷ to 10¹⁸ cm^-3. The distribution coefficients of zinc and cadmium were 0.8 and 0.004, respectively. The temperature gradient method makes it possible to dope the crystals with volatile impurities in a controlled manner.     

Photo-induced changes of a.c. conduction in amorphous chalcogenides and hydrogenated silicon

Photo-induced changes of a.c. conductance have been observed for both amorphous Se and hydrogenated Si (a-Si:H). After cessation of the illumination, no metastable change of a.c. conductance for Se and a decrease in the a.c. conductance for a-Si:H which could be related to the Staebler-Wronski effect have been found. These are discussed in terms of the negative-U defects.     

Structure and dynamics of oxide melts and glasses: A view from multinuclear and high temperature NMR

From a presentation of the various nuclear magnetic resonance (NMR) experiments that allows to characterize the local structure and dynamics of oxide glasses and melts, we show that it becomes possible to evidence not only the details of the coordination state of the constituting atoms but also the nature of polyatomic molecular motifs extending over several chemical bonds.     

Simulation of mass transfer in a kinetic experiment based on migration of flat layers of a solution in a melt in a temperature gradient field

Mass transfer upon migration of flat liquid layers of a solution in a melt under conditions of slow change in the average temperature at a constant rate has been analyzed for the dislocation and nucleation mechanisms of interface processes. It is shown that the experimental data on the dependence of the migration rate of flat layers on their thickness can be adequately described within the theory developed for strictly steady thermal conditions. A technique is proposed for determining the overall limitations on the mass transfer at interfaces and the migration rate in the diffusion mode from the experimental results obtained in a mixed migration mode.

     

Determination of refractive index and electronic polarisability of oxygen for lithium-silicate melts using ellipsometry

The refractive index of Li₂O–SiO₂ melts has been measured using the ellipsometer constructed for high-temperature use. Measurements were carried out on four samples having different compositions over a wide temperature range (1300–1800 K). The standard deviation for the values measured was within ±0.0005. Additions of Li₂O were found to cause increases of refractive index in the temperature range investigated. At Li₂O concentrations <25 mol% the refractive indices of the melts increased with increasing temperature, similar to SiO₂. However, this temperature dependence became smaller with additions of Li₂O and at Li₂O concentrations more than 30 mol% the refractive indices of the melts decreased with increasing temperature. The temperature dependence of the refractive indices has been discussed from those of the density and the molar electronic polarisability on the basis of the Lorentz–Lorenz equation. In addition, the electronic polarisability of oxygen derived from the molar electronic polarisability increased with increasing temperature in each melt, suggesting that the ionicity of oxygen increases as temperature increases. Furthermore, additions of Li₂O were found to cause increases in the electronic polarisability of oxygen, due to the formation of non-bridging oxygen ions.     

General concept of increasing crystal symmetry with an increase in temperature

A general concept of the symmetry transformation of a material during its thermal expansion and final polymorphic transition is proposed based on numerous experimental data. Within this concept, the atomic nature of the transformation of the crystal structure with a change in temperature is considered. Particular attention is paid to the thermal expansion of materials, along with the well-known tendency of a thermally induced increase in the crystal symmetry, which is generally considered to be the directionality of material transformation during polymorphic transitions.     

Redox reactions during temperature change in soda-lime–silicate melts doped with copper and iron or copper and manganese

Glasses with the base composition 16Na₂O · 10CaO · 74SiO₂ doped with copper and iron or copper and manganese were studied by high temperature UV–vis–NIR spectroscopy. The spectra exhibited distinct absorption bands attributed to the respective transition metal ions present (Cu²⁺, Fe²⁺, Fe³⁺, Mn³⁺). In glasses doped with only one polyvalent element, the absorption decreases linearly with increasing temperature, the absorption bands are shifted to smaller wave numbers and get broader. In glasses doped with two types of transition metals, the situation is the same up to a temperature of around 550 °C. At larger temperature, the Cu²⁺-absorption in glasses also co-doped with iron increases again, while in glasses doped with both copper and manganese the absorption is approximately the same as in glasses solely doped with copper. It is shown that this is due to redox reactions between polyvalent species. These reactions are frozen in at temperatures <550 °C.     

Modeling habit forms of sapphire crystals using the principles of periodic-bond-chain method

The potential of the periodic-bond-chain method for calculating the sequence of manifestation of faces of corundum single crystals is considered. The leading role of the faces of the pinacoid, high rhombohedron, and hexagonal prism is demonstrated. The calculation results are compared with the experimental data on faceting the lateral surface of cylindrical sapphire single crystals grown by the Stepanov method and with the faceting data for crystals grown by the flux method and natural crystals.

     

Lattice dynamics and ion transport in structurally disordered chalcogenides of copper and silver

The results of studying the copper and silver selenides and their solid solutions are reported. The methods of inelastic neutron scattering and neutron diffraction analysis are used to study the dynamics and structure of the crystal lattice. The relation of the properties of dynamics and the crystal-lattice structure to the ion transport are analyzed.