Vol 63, No 4 (2018)

By spark plasma sintering, HfB₂–45 vol % SiC ultra-high-temperature ceramic was prepared, from which wedge-shaped samples were cut. The behavior of the samples was examined in a flow of dissociated air produced by an induction plasmatron, where the surface temperature of the leading edges of the samples reached ~2700°C. The dependence of the temperature distribution gradient on the distance from the leading edges of the samples was experimentally investigated. For the samples after the experiments, the elemental and phase compositions were determined, and features of the surface microstructure in various regions of the sample and on its polished surface were studied.

A powderlike material of composition MgFe_1.6Ga_0.4O₄ was synthesized by gel combustion using a glycine–hexamethylenetetramine mixture. The gel produced by the synthesis was studied by thermal analysis (TGA/DSC) and IR spectroscopy. This mixture was shown to be efficient for obtaining homogeneous nanosized MgFe_1.6Ga_0.4O₄. The morphology of the powders was characterized by scanning electron microscopy and X-ray powder diffraction analysis.

Technological approaches have been developed to the preparation of single-phase lithium niobate batches doped with magnesium, zinc, iron, and rarer-earth elements (TR = La, Pr, Nd, Sm, Gd, Dy, or Er) from high-purity niobium-containing solutions to avoid niobium pentoxide separation, its mixing with lithium carbonate, and LiNbO₃ preparation from this mixture by a solid-phase reaction. Two process flowsheets have been proposed, being applicable over the entire ranges of the studied dopant concentrations. Optimal parameters for preparing single-phase LiNbO₃ batches of tailored compositions have been determined. The batches prepared by the developed technology are intended for growing doped optical-quality lithium niobate single crystals.

Structural features of eight binuclear complexes with the general formula [{MoO₂(L_biⁿ)}₂(μ-O)] (I–VIII) (L_bi is a bidentate chelate ligand, n = 1–8), in which the coordination number of Mo atoms is five, are considered. The parameter τ = (A–B)/60, where A and B are the greatest bond angles among the ten bond angles at the Mo atoms in coordination pentahedra, can be used as a criterion characterizing the coordination polyhedron of the molybdenum atom in complexes I–VIII. The parameter τ is zero for an ideal square pyramid and unity for an ideal trigonal bipyramid.

Our computational studies into the separation of two- and three-component mixtures of rareearth salts by recycling liquid–liquid chromatography (RLC) with multiple sample injection show that this method considerably enhances metal separation efficiency and makes it possible to concentrate one of the components of mixture.

MALDI-TOF was used to study molybdenum dioxide (MoO₂) containing a nanosized fraction. The composition of cationic clusters of nonstoichiometric lower molybdenum oxides in the gas phase was determined, and the thermodynamic stabilities and configurations of isomers were calculated for selected symmetric molecular structures and for cations MoSO₈⁺and Mo₅O₉⁺. Molecular orbital analysis was performed for two trigonal-bipyramidal clusters Mo₅O₈ and Mo₅O₉. Changes in molybdenum–molybdenum interatomic distances in going from MoO₈⁺ and Mo₅O₉⁺ cations to neutral clusters are discussed.

Nanocomposites based on nanocrystalline ZnO and CdSe and InP nanocrystals (quantum dots) have been synthesized by chemical precipitation and high-temperature colloidal synthesis. The microstructure parameters of the oxide matrix and the size of the CdSe and InP nanocrystals have been determined. A correlation was established between the spectral dependence of the photoconductivity of nanocomposites and the optical absorption spectra of quantum dots. The influence of CdSe and InP quantum dots on the interaction of ZnO with NO₂ under visible light irradiation has been studied. It has been shown that the synthesized nanocomposites can be used to detect NO₂ under illumination with green light without additional thermal heating.

Investigations of the evolution of the phase composition, texture, and physical properties of SCAS monoliths with a set of spectroscopic, diffractometric, and thermal methods have shown that during the leach test an amorphous phase is accumulated and its composition changes. This is beneficial as regards stability of SCAS monoliths to the action of hydrosphere and makes them promising materials for the manufacture of matrices for immobilization of hazardous (toxic) wastes.

Phase equilibria in the TlInSe₂–TlInP₂Se₆ system have been studied for the first time by classic physicochemical analysis methods (differential thermal analysis (DTA), X-ray diffraction, and microstructural analysis (MSA)), and its phase diagram has been plotted. The system belongs to the eutectic type (Rooseboom V type) and forms boundary solid solutions based on the initial complex selenides.

For HF solutions in DMF, concentration-dependent fractions of DMF molecules (α(DMF)) that remain unassociated and that enter heteroassociates (HAs) of 1 : 1, 4 : 1, and 12 : 1 molecular stoichiometries were obtained by two independent methods, namely, from an analysis of IR spectra and by calculating the material balance. The experimental way was shown to be enough exact in determining the ratio between the solvent molecules in four different states up to ~83 mol % HF. The equilibrium compositions of HF–DMF solutions were estimated over the entire range of concentrations. Starting with [HF] of ~25 mol %, more than one-half HF molecules are associated, and at [HF] of ~50–92 mol %, at least 90% of the HF molecules are associated. The equilibrium composition of HF–organic solvent (Solv) solutions in which HAs of 1 : 1, 1 : 4, and 1 : 12 molecular stoichiometries are formed, can be described by a single set of α(HF–Solv) versus concentration plots.

The processes of formation of iron(II) complexes in aqueous glycine solutions in the pH range of 1.0–8.0 at 298 K and ionic strength of 1 mol/L (NaClO₄) are studied using Clark and Nikolskii’s oxidation potential method. The type and number of coordinated ligands, the nuclearity, and the total composition of the resulting complexes are determined. The following complex species are formed in the investigated system: [Fe(OH)(H₂O)₅]⁺, [FeHL(H₂O)₅]²⁺, [Fe(HL)(OH)(H₂O)₄]⁺, [Fe(OH)₂(H₂O)₄]⁰, [Fe₂(HL)₂(OH)₂(H₂O)₈]²⁺, and [Fe(HL)₂(H₂O)₄]²⁺. Their formation constants are calculated by the successive iterations method using Yusupov’s theoretical and experimental oxidation function. The model parameters of the resulting coordination compounds are determined.