Vol 55, No 5 (2019)

The spatial distribution of impurity defect centers in Fe-doped chemical vapor deposited (CVD) ZnSe has been studied by two-photon confocal microscopy and scanning Fourier transform spectroscopy using an IR microscope. It has been shown that, as a result of doping with Fe, CVD ZnSe contains regions hundreds of microns in size that are parallel to the doping plane and differ in luminescence characteristics. The characteristics of these regions have been shown to correlate with the concentration of optically active Fe. Our results can be interpreted in terms of a model that assumes codiffusion of Fe and two types of impurity defect centers. We have pointed out that the data obtained in this study should be taken into account in interpreting results on the photoluminescence of semiconductors doped via thermal diffusion.

We have studied the high-pressure, high-temperature behavior of adamantane (C₁₀H₁₆) and the associated formation of diamond nano- and microcrystals. Diamond microcrystals have been synthesized at a pressure of 8 GPa and temperatures above 1300–1400°C, whereas large-scale synthesis of nanodiamond has been carried out at higher pressures, near 9.4 GPa, in the narrow temperature range 1250–1330°C. Our experimental data suggest that diamond microcrystals are formed in a fluid growth medium as a result of recrystallization of graphite, an intermediate carbonization product, and that nanodiamond formation is a direct result of adamantane carbonization.

We have demonstrated the feasibility of preparing powder alloys based on intermetallic phases in the Ti–Ni–Hf and Ti–Ni–Hf–Zr systems by a calcium hydride reduction process and investigated the physicochemical and technological properties of Ti₄₀Ni₅₀Hf₁₀, Ti₂₈Ni₅₀Hf₂₂, and Ti₂₈Ni₅₀Hf₁₂Zr₁₀ powders thus prepared. Experimental data show that the synthesis of the powders typically involves the formation of intermediate phases inherent in the binary phase diagrams of the constituent components of the alloys. According to X-ray diffraction data, the synthesized powders consist predominantly of B19' martensite, whose content depends significantly on the synthesis temperature. At given process parameters, the alloy with the lowest Hf dopant concentration is the most homogeneous.

Materials based on amorphous iron(III) phosphate have been prepared via precipitation in an aqueous medium and microemulsion, as well as in a microemulsion in the presence of a mesoporous carbon material. Heat treatment of amorphous FePO₄ leads to the formation of the hexagonal phase of crystalline FePO₄. It has been shown that sodium intercalation under operating conditions of sodium ion batteries is more effective in the case of materials based on amorphous FePO₄. The discharge capacity of the material based on amorphous FePO₄ synthesized by the microemulsion method in the presence of the G_157M mesoporous carbon material is ~183 mAh/g and is the sum of the capacity due to sodium intercalation into FePO₄ and that due to the reduction of functional groups on the surface of the mesoporous carbon.

The Gd₂Ti₂O₇ and Lu₂Ti₂O₇ titanates (pyrochlore structure, sp. gr. Fd3m) have been prepared by solid-state reactions in air at temperatures from 1673 to 1773 K using the Gd₂O₃, Lu₂O₃, and TiO₂ oxides as starting materials. Their high-temperature heat capacity has been determined by differential scanning calorimetry in the range 350–1000 K. The experimental heat capacity data have been used to evaluate the thermodynamic functions of gadolinium and lutetium dititanates.

Using high-pressure, high-temperature processing of a low water content amorphous tantalum hydroxide, TaO_0.5–2.0(OH)_4–1 · (1.0–2.5)H₂O, at p = 5.0–6.0 GPa and t = 800–900°C, we have obtained a mixture of two phases, Ta₂O₅ · 2/3H₂O (sp. gr. P6₃/mсm, а = 7.4736(2) Å, с = 7.6798(2) Å, Z = 3, V = 371.48(1) ų) and H₂Ta₂O₆ · 2/3H₂O (sp. gr. P6₃/mсm, а = 7.4998(2) Å, с = 7.6171(2) Å, Z = 3, V = 371.04(1) ų), both crystallizing in the hexagonal tungsten bronze (HTB) structure. According to thermogravimetric analysis results, the material contains 4.9(3)% water. After water removal, heating in air to 550°C causes the HTB phase to convert into δ-Ta₂O₅. X-ray diffraction data have been analyzed by the Rietveld method and the following reliability factors have been obtained: R_F = 0.0374 for Ta₂O₅ · 2/3H₂O and R_F = 0.0416 for H₂Ta₂O₆ · 2/3H₂O. We assume that the MO₃ stoichiometry of the basic HTB cell in these compounds is ensured by different mechanisms.

We present results of an ¹¹B and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy study of the local anion structure of a series of borosilicate glasses in the Rb₂O–B₂O₃–SiO₂ system with different R = [Rb₂O]/[B₂O₃] and K = [SiO₂]/[B₂O₃] molar ratios. Relative integrated intensities of NMR lines are used to evaluate the fractions of boron atoms in three- and fourfold coordinations and the concentrations of various types of Qⁿ silicate units. Comparison of the present data on the anion structure of the Rb₂O–B₂O₃–SiO₂ glasses studied here and data for previously studied glasses of analogous compositions in the Cs₂O–B₂O₃–SiO₂ system demonstrates that the difference in the degree of depolymerization in the anion structure of the glasses in these two systems is most pronounced at R = 1 and K = 2. At other values of these composition parameters (R = 0.43 and K = 1.43 or R = 2.33 and K = 3.33), the glasses with the compositions studied in the Rb₂O–B₂O₃–SiO₂ and Cs₂O–B₂O₃–SiO₂ systems are rather similar in local anion structure. The observed structural distinctions between the glasses make it possible to find out why they differ in glass transition temperature, as shown in a previous study by differential scanning calorimetry.

The electrical properties and phase states of Li_0.17Na_0.83Nb_уТа_{1 –у}O₃ (y = 0.1–0.5) ferroelectric perovskite solid solutions prepared by high-pressure, high-temperature synthesis have been studied using impedance spectroscopy. The ε'(T) and σ_sv(T) curves of the solid solutions have been shown to have anomalies due to phase transitions of the materials. Their Curie temperature has been found to decrease with increasing Ta concentration. The static electrical conductivity of the solid solutions has been measured as a function of temperature and the enthalpies of activation of charge carriers in them have been evaluated. The Li_0.17Na_0.83Nb_0.1Та_0.9O₃ solid solution has been shown to be a high-temperature superionic conductor. Possible mechanisms of the observed phenomena are discussed.

We have found conditions for high-speed measurements of Raman spectra of lithium niobate and lithium tantalate single crystals and ceramics. To measure Raman spectra, we used 180° scattering geometry and a minispectrometer equipped with an array detector. The Raman spectra obtained at two distinct orientations of the single crystals relative to the excitation beam direction have been shown to have different frequencies of transverse and longitudinal optical modes. The Raman spectra of the ceramics are mainly contributed by transverse optical modes.