Vol 55, No 8 (2019)

The photoconductivity and electrical transport parameters of indium monoselenide single crystals have been measured as functions of temperature in the range 77–300 K. The results demonstrate that, below 240–250 K, not only the dark electrical conductivity and free carrier mobility but also the key photoconductivity parameters of the semiconductor vary from sample to sample. With decreasing dark electrical conductivity, the temperature-dependent photoconductivity of the crystals begins to exhibit behavior atypical of ordered crystalline semiconductors. Such behavior is shown to be due to the fact that the n-InSe single crystals contain random macroscopic (large-scale) defects identical in phase composition and crystal structure to the major material.

We have studied phase equilibria in the silicon-rich part of the Yb–Pt–Si system. To this end, 22 alloys were prepared by a standard arc-melting process in an argon atmosphere and annealed at a temperature of 850°C. Physicochemical characterization of the samples included electron microscopy, X-ray microanalysis, X-ray diffraction, and differential thermal analysis. We have confirmed the existence of four previously known intermetallic compounds: YbPtSi, YbPt₂Si₂, Yb₃Pt₄Si₆, and Yb₂Pt₃Si₅. In addition, three new phases have been identified: YbPtSi₂, Yb₃₃Pt₁₇Si₅₀, and Yb₁₁Pt₃₃Si₅₆. The structure type of the YbPtSi₂ compound has been determined. We present statistical crystallographic data for the ternary phases and describe phase equilibria in the region studied in the 850°C isothermal section of the Yb–Pt–Si phase diagram.

Five new intermetallic phases of known structure types have been identified in a systematic study of the Ce–Ru–Ga system: CeRu_0.90Ga_1.10 (MgZn₂ structure, sp. gr. Р6₃/mmc, а = 5.44503(5) Å, с = 8.67188(13) Å, very narrow homogeneity range), Ce₇Ru₆Ga₇ (Pr₇Co₆Al₇ structure, constant composition, sp. gr. Р4/mbm, а = 13.61910(17) Å, с = 4.34957(5) Å), Ce₆Ru_0.18Ga_2.82 (K₂UF₆-derived structure, sp. gr. P\(\bar {6}\)2m, a = 8.09628(19) Å, c = 4.38470(13) Å), Ce₂₃Ru₁₀Ga (Pr₂₃Ir₇Mg₄ structure, sp. gr. P6₃mc, а = 9.867(6) Å, с = 22.153(14) Å) and Ce₂₆(Ru_0.59Ga_0.41)₁₇ (Sm₂₆(Co_0.65Ga_0.35)₁₇ structure, sp. gr. P4/mbm, a = 11.9529(4) Å, c = 15.0135(10) Å). In the structures of Ce₇Ru₆Ga₇, Ce₂₃Ru₁₀Ga, and Ce₂₆(Ru_0.59Ga_0.41)₁₇, some of the Ce atoms form very short Ce–Ru bonds (2.525(3), 2.549(5), and 2.506(2) Å, respectively), which is attributable to their intermediate valence state.

We have synthesized catalytically active membranes based on α- and γ-Al₂O₃ powders for the dehydration and dehydrogenation of butyl alcohol to butadiene and hydrogen. The open porosity of the samples obtained in this study is 41% in the case of α-Al₂O₃ and 38% in the case of γ-Al₂O₃. The open pore size is 4.6–5.1 μm in the α-Al₂O₃ material and 0.5–0.8 μm in the γ-Al₂O₃ material. We have implemented a hybrid, membrane–catalytic process for the dehydrogenation of butanol by combining reaction and hydrogen separation steps in a single device. It has been demonstrated that the dehydration of n-butanol on a γ-Al₂O₃ converter leads to the formation of a butylene fraction with a selectivity of 99.88–100% at a temperature of 300°C, which is 50°C lower than in the case of commercially available gamma-alumina granules. The dehydrogenation of butylene to butadiene on an α-Al₂O₃ membrane with selective hydrogen removal from the reaction zone has made it possible to raise the 1,3-butadiene output from 16.5 to 22.6 L/(h g_{act. comp.}), with the degree of ultrapure hydrogen extraction reaching ~16%. After the experiment was run for 20 h, no decrease in the catalytic activity of the system was detected, as distinct from commercial solutions, in which a regeneration step is necessary every 8–15 min.

Solid solutions with the whitlockite structure Ca₉Gd_{1 –х}Eu_x(PO₄)₇ and Ca₈MgGd_{1 –х}Eu_x(PO₄)₇ were synthesized by standart tecnique. The compounds have been characterized using various methods, including X-ray diffraction, second harmonic generation, and luminescence spectroscopy. The optical characteristics, excited-state lifetimes, and quantum yield were studieid. Under excitation at 395 nm, the compounds exhibit bright luminescence in the orange-red spectral region. The concentration of Eu³⁺ cation were optimized for obtaining the most efficient luminescence in the red spectral region: x = 0.75 for Ca₈MgGd_{1 –х}Eu_x(PO₄)₇ and x = 1 for Ca₉Gd_{1 –х}Eu_x(PO₄)₇. Efficient Gd³⁺ → Eu³⁺ energy transfer has been demonstrated, with an increase in photoluminescence intensity by a factor of 2.5. The quantum yield in the solid solutions reaches 49.5%, which is twice that in a commercially available Y₂O₃:Eu³⁺-based phosphor.

The compounds of different composition with garnet structure, doped with Ce³⁺, have been obtained by co-precipitation method with use of partial substitution of Y³⁺ and Al³⁺ onto La³⁺ and Ga³⁺. The crystal structure of the substituted materials has been studied in detail by neutron diffraction. The nature and concentration of substituent cations have been shown to influence the structural parameters and luminescence spectra of (Y_{1 –х}La_х)₃(Al_{1 –у}Ga_у)₅O₁₂:Ce³⁺ compounds. Changes in the geometry of the oxygen dodecahedra have the strongest effect on Ce³⁺ luminescence in the substituted materials.

We have studied the physical and chemical properties of aqueous titanium(IV) chloride solutions. The results demonstrate that, during TiCl₄ dissolution in water, only part of the titanium(IV) chloride hydrolyzes. Aqueous solutions with up to 60 wt % TiCl₄ contain unhydrolyzed TiCl₄ species. Titanium(IV) chloride solutions have an increased chemical activity for aluminum hydroxide and aluminum oxide compared to hydrochloric acid. It has been shown that reactions of metallic aluminum with titanium(IV) chloride solutions can yield lower titanium(III) chlorides.

We consider the potential of using boron oxide as a sintering aid for the fabrication of transparent magnesium aluminate spinel (MAS) ceramics and analyze the general tendency in the effect of B₂O₃ concentration on the average density, open porosity, and optical transmission of MAS ceramics. Boron oxide is shown to have an advantageous effect on the sintering behavior of MAS ceramics: raising its concentration leads to a decrease in open porosity and an increase in the average density of the material.

We have developed a technique for the preparation of a dielectric ceramic CaCu₃Ti₄O₁₂ filler and submicron dispersions of the filler in epoxy. The crystal structure of CaCu₃Ti₄O₁₂ has been refined using high-resolution neutron diffraction data with anisotropic atomic displacement parameters. We have studied the particle size distribution and rheological properties of CaCu₃Ti₄O₁₂ dispersions and, using rheological data, identified the surfactants that ensure Newtonian rheology of dispersions, convenient for filling porous templates. Surfactants have been shown to have a significant effect on dielectric characteristics of filled epoxy compositions.