Vol 54, No 4 (2018)

The magnetization of CuCr_2–xSb_xS₄ solid solutions has been measured as a function of temperature between 300 and 5 K in a weak (3980 A/m) and a strong (7960 A/m) magnetic field. We have identified the type and character of the magnetic transformations observed in the system and determined the temperature and composition limits of the stability regions of the magnetically active phases involved and the cation and valence distributions in them. A magnetic phase diagram of the synthesized materials has been mapped out, where the largest area (0 < x < 0.23) after the paramagnetic region is occupied by solid solutions based on the CuCr₂S₄ ferromagnet. In the composition range (0.23 < x < 0.40) adjacent to the infinite clusters—CuCr₂S₄ ferromagnet and CuCr_1.5Sb_0.5S₄ antiferromagnet—the phase diagram contains medium and small finite ferro- and antiferromagnetic clusters forming a short-range magnetic order or spin glass. The compositions based on the CuCr_1.5Sb_0.5S₄ antiferromagnet lie in the composition range 0.4 < x < 0.5.

Lead telluride films have been grown on Si (100) and BaF₂ (100) substrates by a modified hot-wall method using a graphite reaction chamber. According to X-ray diffraction, X-ray microanalysis, and scanning electron microscopy characterization results, the average growth rate of PbTe films having compositions within the homogeneity range of lead telluride increases with increasing lead vapor partial pressure and decreases with increasing tellurium vapor partial pressure, independent of the nature of the substrate. The rate of PbTe film growth has been shown to be maximal in the initial stage of the process and decrease monotonically over time, independent of the nature of the substrate. Independent of the growth time, the average growth rate of the PbTe films on the Si (100) substrates is considerably higher than that on the BaF₂ (100) substrates. Reflection high-energy electron diffraction data indicate that the texture of the PbTe films on Si (100) corresponds to the substrate orientation and that the misorientation angle of the mosaic blocks does not exceed 20°. On the BaF₂ (100) substrates, we observe epitaxial PbTe film growth with the orientation relationship (100), [011] PbTe ║ (100), [011] BaF₂.

Our results on the atomic order in La_0.6–xNd_xSr_0.3Mn_1.1O_3–δ manganites with partial Nd substitution for La (x = 0–0.4) demonstrate that all of the materials consist of differently sized groups: microcrystalline groups with long-range atomic order, D = 300–600 Å in size; clustered groups with mesoscopic atomic order and D = 100–200 Å; amorphous clustered groups with D = 20–30 Å; and a disordered material with short-range atomic order, on a length scale of two or three interatomic distances (D ~ 10 Å). It has been shown that, in the case of partial Nd substitution for La (x = 0.1–0.2) in the manganite, clusters 100–200 Å in size, as well as those of the rhombohedral phase 20–50 Å in size, initiate an FM → AFM magnetic phase transition in the range 200–250 K. Neodymium substitution for half of the lanthanum (in La_0.3Nd_0.3Sr_0.3Mn_1.1O_3–δ) has been found to result in the development of self-organization processes, with a reduction in cluster size fluctuations in the range ΔD ≈ ±50 Å in the orthorhombic ferromagnetic and rhombohedral antiferromagnetic phases.

We have grown single crystals of barium dihydrogen phosphate and studied its thermal transformations during heating to 500°C and its electrotransport properties. Ba(H₂PO₄)₂ (Pccn) has been shown to undergo no phase transitions up to its dehydration temperature. The thermal decomposition of Ba(H₂PO₄)₂, accompanied by dehydration, involves two steps, with maximum rates at ~265 and 370°C, and results in the formation of barium dihydrogen pyrophosphate and barium metaphosphate, respectively. The total enthalpy of the endothermic dehydration events is–244.6 J/g. Using impedance spectroscopy, we have studied in detail the proton conductivity of polycrystalline and single-crystal Ba(H₂PO₄)₂ samples in a controlled atmosphere. Adsorbed water has been shown to have a significant effect on the proton conductivity of Ba(H₂PO₄)₂ up to 130°C. The proton conductivity of the Ba(H₂PO₄)₂ single crystals has been shown to be anisotropic. The conductivity anisotropy correlates with specific structural features of the salt. Higher conductivity values, 3 × 10^–9 to 2 × 10^–7 S/cm in the range 60–160°C, have been observed in the [100] crystallographic direction, exceeding the conductivity along [010] by an order of magnitude. The activation energy for proton conduction is 0.80 eV.

We have calculated the total elemental composition and assessed its statistical characteristics for a class of high-purity volatile substances—organoelement compounds, exemplified by metalorganic alkyl compounds— using Exhibition–Collection of Extrapure Substances data. The average and total contents of elemental impurities in the purest samples and an array of such compounds have been estimated. Using the division of elemental impurities into classes, we have estimated the contributions of individual classes to the total impurity content in an array of samples of metalorganic alkyl compounds. This allowed us to more accurately characterize the impurity composition of individual samples and an array of samples, refining statistical characteristics of its description.

Highly dispersed η-Al₂O₃-based nanopowders have been prepared via glycine–nitrate combustion followed by heat treatment in air. The resultant materials have been characterized by X-ray diffraction, Fourier transform IR spectroscopy, scanning electron microscopy, simultaneous thermal analysis, and other techniques. We have optimized the glycine-to-nitrate ratio (G/N = 0.2) and found heat treatment conditions for combustion products (isothermal holding at a temperature of 700°C for 6 h) that allow one to obtain single- phase nanocrystalline η-Al₂O₃ powders with an average particle size of 5 ± 1 nm and specific surface area of 54 ± 5 m²/g. The acid–base surface properties of the η-Al₂O₃ nanopowder have been analyzed using pyridine sorption–desorption processes as an example. The specific concentrations of weak, intermediate, and strong Lewis acid centers on the surface of the η-Al₂O₃ nanocrystals have been shown to markedly exceed those on the surface of commercially available γ-Al₂O₃ (A-64). The synthesized nanopowders can thus be used as effective supports of acid catalysts.

We have prepared film-forming solutions for the growth of dense and porous thin organoalkylenesiloxane (OAS) films based on copolymers of methyltrimethoxysilane and 1,2-bis(trimethoxysilyl)ethane (BTMSE) by a sol–gel process. The chemical composition and microstructure of the OAS films have been studied by IR spectroscopy and spectral ellipsometry in relation to the mole fraction of BTMSE and the water: methoxy groups ratio in solution. The results demonstrate that partial substitution of ethylene bridges for silicon–oxygen bonds in OAS leads to distortion of the regular ladder-like structure characteristic of polymethylsilsesquioxane films and the presence of residual silanol groups, which causes an increase in the dielectric permittivity k of the matrix material. The relative porosity in porous OAS films produced via evaporationinduced self-assembly has been shown to be determined by not only the amount of surfactant added but also the presence of a sufficient amount of silanol groups, participating in the attachment of surfactant molecules, in the matrix copolymer solution. In this connection, an important factor determining the structure of the OAS matrix and its pore structure is control over the amount of water involved in the cohydrolysis process. It has been shown that the samples with a relative porosity of 38% prepared from a film-forming solution containing 47 mol % BTMSE (m = 0.7) and 30 wt % surfactant have k ≈ 2.3 and are potentially attractive materials for use as insulators in integrated circuits.