On the growth mechanism of sodalite single crystals grown by the hydrothermal method on single crystal seeds coated with informative informative interfacial layers

The growth mechanism of the early formation stages of sodalite single crystals grown by the method of hydrothermal synthesis on single crystal seeds coated with interfacial layers of polycrystalline silver has been studied at an electronmicroscopic scale. Coating with interfacial layers leads to a very weak adhesion between the overgrown single crystal and the surface of the interfacial layer on top of the seed, thus providing a unique possibility of detaching the overgrown single crystals from seeds and investigating the very early crystallization stages by the morphology of the growth surface. In local microregions of seed surfaces coated with interfacial layers discrete particles arise differing from one another in morphology, this being primarily associated with the electrical heterogeneity of seed surfaces. During crystallization, the space between the discrete particles was filled with the hydrothermal solution which represented a liquid interfacial layer exhibiting informative properties occurred under the influence of electrically active elements of the seed surface. At the boundary separating the liquid interfacial layers with particular informative properties from the rest of the solution volume, at early crystallization stages, together with the formation of discrete particles directly on the coated seed surface, growth of a continuous sodalite single crystal took place. The informative properties of seed surfaces, which are regularly modified due to coating with interfacial silver layers, determine the occurrence on local regions of seed surfaces (under appropriate crystallization conditions) of one or the other polymorphous modification: either hexagonal – cancrinite, or cubic – sodalite.     

The chemical action of oxygen on a heater during growth of refractory oxide crystals from melt

The behavior of the W-O₂ system has been investigated at 2400 K in the pressure range from 1 to 1 × 10^?5 bar. The chemical composition of the solid and vapor phases for the ratio W: O₂ = 1: 1 was calculated by minimizing the Gibbs free energy. It is shown that the only solid phase in the system is metallic tungsten (0.333–0.355 mol), whereas trioxide WO₃ dominates in the vapor phase; its concentration may reach 99%. It is concluded that providing an inert atmosphere in the growth chamber with a pressure of 1 bar decreases the concentration of atomic and molecular oxygen in the vapor phase and decreases its effect on the tungsten heater.     

Chemical transformations in the W-Al2O3 system under pressure P = 1 bar near the melting point of aluminum oxide

The W-Al₂O₃ system has been considered at a temperature of 2400 K and a pressure of 1 bar. The main chemical processes providing the interaction between the components of the system have been determined. It is shown that evaporation of Al₂O₃ into the gas phase gives rise to numerous reactions, which involve not only tungsten but also Al₂O₃ melt. It is concluded that such interactions can be reduced by decreasing the Al₂O₃ evaporation, which can be done by increasing the inert gas pressure. This approach makes it possible both to optimize the parameters of sapphire crystal growth and increase the lifetime of a tungsten heater and other units of crystallization systems.     

Possibility of Mo<Subscript>2</Subscript>O<Subscript>6</Subscript> formation at high temperatures in the range of pressures from 1 to 1 × 10<Superscript>−5</Superscript> bar

Gibbs free energy minimization was used to consider the formation of complex molybdenum oxide (Mo₂O₆) at 2400 K in the range of pressures from 1 to 1 to 1 × 10⁻⁵ bar for the basic component ratio Mo: O₂ = 1: 1. Several ways are shown to lead to Mo₂O₆ formation: when P = 1 bar, a synthesis reaction involving simple molybdenum oxides (MoO, MoO₂, MoO₃) is the main way; when P = 1 × 10⁻³ bar or lower, reactions of (MoO₃) _n (n = 3−5) complex oxides with metallic molybdenum and molybdenum monoxide (MoO) are.     

Chemical processes and composition of the gas and solid phases in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Mo system in the temperature range 2327–2500 K

The possibility of chemical reactions has been calculated and the composition of the gas and solid phases that are in equilibrium with an aluminum oxide melt is determined. It is shown that, for the Al₂O₃-Mo system in the pressure range 1?1 × 10^?5 bar, evaporation of Al₂O₃ is incongruent, and the fraction of this component in the gas phase decreases from 51.5 mol % (P = 1 bar) to 0.01 mol % (P = 1 × 10^?5 bar). The presence of molybdenum-containing compounds in the gas phase changes the balance of oxygen and aluminum in favor of the latter (the aluminum partial pressure increases by a factor of 1.5–3), as a result of which there may be an aluminum deficit in the solid phase of Al₂O₃ during crystal growth from a melt. The thermodynamic characteristics (K _p , ΔG, P _tot) of dominant chemical reactions have been calculated for the temperature range 2327–2500 K. Understanding of the chemical processes makes it possible to optimize the growth parameters of leucosapphire single crystals.     

Thermodynamic analysis of the W-Al2O3 system near the melting temperature of Al2O3. I. Evolution of the system in the pressure range of 1 × 10−1−1 × 10−4 bar

The changes in the main chemical reactions occurring upon the interaction between tungsten and the evaporation products of Al₂O₃ melt are considered at a fixed temperature (2400 K). The concentrations of the components coexisting in equilibrium in a closed system under isobaric-isothermal conditions are determined by stochastic simulation for low (× 10⁻¹−1 × 10⁻³ bar) and high (1 × 10⁻⁴ bar) vacuum. It is shown that the gas-liquid-solid system is in heterogeneous equilibrium for the basic component ratio W: Al₂O₃ = 1: 1 in the entire pressure range under consideration. A detailed study of the chemistry of this system should facilitate the choice of the optimal conditions for growing leucosapphire crystals from melt.     

Molybdenum-oxygen interaction during growth of refractory oxide crystals from the melt in vacuum

The behavior of the Mo: O₂ system in a temperature range of 2350?2500 K under a pressure of 1 × 10^?5 bar has been investigated. The compositions of gas and solid phases and the main chemical reactions describing molybdenum oxidation are determined for different ratios of the basic components. The thermodynamic calculations made it possible to recommend a applying a specific atmosphere during the growth of leucosapphire and aluminum yttrium garnet crystals from the melt by the Bagdasarov method.