The kinetics of production and loss of O(<Superscript>3</Superscript><Emphasis Type="Italic">P</Emphasis>) oxygen atoms in air plasma

The method of electron paramagnetic resonance is used to measure the production rate and concentration of O(³ P) oxygen atoms at pressures from 50 to 320 Pa and discharge currents from 50 to 100 mA in the positive column of a dc glow discharge in air. The probability of heterogeneous recombination of oxygen atoms proceeding by the first kinetic order with respect to their concentration is determined in the region of flux afterglow and in the zone of air plasma.Translated from Teplofizika Vysokikh Temperatur, Vol. 42, No. 6, 2004, pp. 869–872.Original Russian Text Copyright © 2004 by I. N. Brovikova.     

Carrier transport in a diode base with locally nonuniform recombination properties

Nonuniformity of the recombination properties in the form of a layer of radiation defects is created in the base of a p ⁺−n diode. The change in the effective hole lifetime, measured by an injection-extraction method, is investigated and attributed to recombination in this layer. Pis’ma Zh. Tekh. Fiz. 23, 79–86 (May 12, 1997)     

Synthesis and structural studies of a new potassium uranyl selenate K(H5O2)[(UO2)2(SeO4)3(H2O)] with strongly deformed layers

Single crystals of a new uranyl selenate, K(H₅O₂)[(UO₂)₂(SeO₄)₃(H₂O)] (I), were prepared by isothermal evaporation at room temperature. The crystal structure of I was solved by the direct method (space group P2₁/c; a = 11.456(2), b = 10.231(1), c = 14.809(2) ?; β = 101.901(4)°, V = 1698.4(4) ?³; Z = 4) and refined to R ₁ = 0.0547 (wR ₂ = 0.0825) for 3375 reflections with |F _o| ≥ 4σ _F . The structure of I is based on layers of the composition [(UO₂)₂(SeO₄)₃(H₂O)]²⁻. The charge of the inorganic layer is compensated by potassium and oxonium ions located in the interlayer space. Each potassium ion is coordinated by seven oxygen atoms belonging to uranyl selenate layers, including uranyl oxygen atoms, which leads to bending of uranyl selenate structural elements.     

Oxygen isotope shifts of the Raman modes of YBa2Cu3O7−x

Raman scattering experiments have been carried out on sintered pellets of YBa₂Cu₃ ¹⁶O_7−x and YBa₂Cu₃ ¹⁸O_7−x obtained both by gas exchange and by growth with substituted oxides. The frequencies of the modes at 340, 435 and 502 cm⁻¹, which involve motion of the oxygen atoms and which shift significantly upon oxygen isotope substitution, have been measured for several sets of samples. The measured frequency shifts indicate that the isotope exchange on the O(2) and O(3) sites is more complete than the exchange on the O(4) site. The 502 cm⁻¹ line of the¹⁸O samples is observed to be broadened and this is attributed to¹⁸O-¹⁶O disorder on the O(4) sites. The results are discussed with reference to previous measurements of site activation energies and models for the exchange mechanism.     

Mechanisms for thermal stability of the electrophysical properties of overcompensated n-Si〈B, S〉

Thermal annealing was used to study silicon samples having different sulfur concentrations. It was established that the temperature at which the sulfur centers decay depends on the concentration of sulfur atoms N _s in the overcompensated silicon. As the distance between the impurities (N _s ^−1/3 ) decreases, the decay temperature increases. The effect can be attributed to characteristic features of the distribution of the compensating sulfur impurity atoms inside a region of fluctuation, formed in silicon during doping. Pis’ma Zh. Tekh. Fiz. 25, 1–4 (December 26, 1999)     

Heterogeneous recombination of oxygen atoms on an aluminum foil surface under low-temperature plasma conditions

According to direct kinetic measurements under conditions of a positive column of a low-pressure glow discharge, the probabilities of recombination of oxygen atoms (γ) on an aluminum foil surface have been determined and the effective activation energies of the recombination process have been calculated. It has been found that the γ value depends on the gas pressure in the system at a discharge current above 30 mA. The fluxes of the main active components of oxygen plasma onto the plasma-bounding surface have been calculated. It has been shown that an increase in the heterogeneous recombination probability is attributed to an increase in the flux of metastable oxygen molecules О₂(b¹∑_g⁺ onto the surface.     

Induced of zeeman coherence in an ensemble of cesium atoms interacting with a two-frequency (microwave+rf) magnetic field

Interference between Zeeman states corresponding to the forbidden magnetic-dipole transition ΔF=0, Δm _F=2 is reported in connection with the simultaneous interaction of 6² S _1/2 cesium atoms with a resonant microwave field and an rf field which varies at twice the Larmor frequency and is directed perpendicular to a static magnetic field H ₀=0.2 Oe. Pis’ma Zh. Tekh. Fiz. 24, 89–93 (July 26, 1998)     

Experimental determination of the spatial distribution of electron defects in La2 ? xSrxCuO4 and Nd2 ? xCexCuO4 crystal lattices

The data of M?ssbauer emission spectroscopy on ⁶⁷Cu(⁶⁷Zn) and ⁶⁷Ga(⁶⁷Zn) isotopes show that holes appearing as a result of the Sr²⁺ substitution for La³⁺ in the La_{2 − x} Sr _x CuO₄ crystal lattice are localized predominantly at oxygen atoms occurring in the same atomic plane as the copper atoms. In contrast, electrons appearing as a result of the Ce⁴⁺ substitution for Nd³⁺ in the Nd_{2 − x} Ce _x CuO₄ crystal lattice are localized in the copper sublattice. These results are consistent with the model assuming that a mechanism responsible for the high-temperature superconductivity in La_{2 − x} Sr _x CuO₄ and Nd_{2 − x} Ce _x CuO₄ crystal lattices is based on the interaction of electrons with two-site two-electron centers possessing negative correlation energies (negative-U centers).     

Possible ionization of mercury atoms by elastic reflection in hyperthermal surface ionization experiments

An elastic collision model baed on data given by Danon et al. [Phys. Rev. Lett. 65, 2038 (1990)] was used to calculate the ionization potential of mercury atoms (V′) at the critical charge transfer distance as a function of the kinetic energy of the atoms (E). The linear dependence V′(E) obtained supports the model of surface ionization of mercury atoms by elastic reflection from a surface. Pis’ma Zh. Tekh. Fiz. 25, 42–45 (July 26, 1999)     

Synthesis,crystal structure,and absorption spectra of a complex neptunium(v) chromate,Na<Subscript>3</Subscript>[NpO<Subscript>2</Subscript>(CrO<Subscript>4</Subscript>)<Subscript>2</Subscript>](H<Subscript>2</Subscript>O)<Subscript>5</Subscript>

A new Np(V) chromate complex with outer-sphere sodium cations, Na₃[NpO₂(CrO₄)₂](H₂O)₅ (I), was synthesized from aqueous solution. Its composition and structure were determined by single crystal X-ray diffraction. The structure of I is based on anionic chains of the composition [NpO₂(CrO4)2] _n ³ⁿ⁻, running along [010] and forming layers parallel to the (101) plane. The Na⁺ ions and water molecules of crystallization are arranged between the layers. The coordination polyhedra of the Np atoms (pentagonal bipyramids) are combined pairwise by sharing common equatorial edges formed by two bridging oxygen atoms of bidentate chelate-bridging CrO₄ groups. The absorption spectra of I in the IR and visible ranges are presented.     

Structural and noise characterization of VO2 films on SiO2/Si substrates

Multi-technique structural and electrophysical investigations of VO₂ films on SiO₂/Si substrates are carried out to study the microscopic nature of fluctuator defects — sources of lowfrequency flicker noise. It is established that the noise intensity is determined by the magnitude of the microstress fluctuations 〈ε 〉={〈(δc/c)²〉}, where c is the lattice parameter along the c-axis parallel to [011] direction in the blocks of which the film is formed. The dimensions of the blocks were determined in the direction of the c-axis (t _c∼1000 Å). The suggestion is put forward that the samples contain two types of fluctuator defects: 1) V atoms jumping between the two nearest interstitial sites and 2) V atoms jumping between these interstitial sites near lattice defects. Pis’ma Zh. Tekh. Fiz. 23, 58–65 (July 12, 1997)     

Excitation of the 63 P 1 atomic level of mercury by pumping a mixture of mercury vapor and argon with a pulse-periodic nanosecond discharge

An experimental investigation was made of a pulse-periodic nanosecond discharge in a mixture of mercury vapor and argon. It is shown that the concentration of mercury atoms in the 6³ P ₁ state is around 50% of the total concentration. Pis’ma Zh. Tekh. Fiz. 24, 89–92 (March 12, 1998)     

Behavior of Anhydrous Uranyl Acetate at Heating in CH3CN. Crystal Structures of New Uranyl Acetates

Finely crystalline anhydrous uranyl acetate UO₂(OOCCH₃)₂ (I) was prepared by recrystallization from acetonitrile at 140-145°C. Its X-ray diffraction pattern was indexed in the monoclinic system: a = 7.4311(5), b = 12.6622(9), c = 4.1985(2) Å, = 92.01(1)°, V = 394.8(2) ų, Z = 2, _{c a l c} = 3.265 g cm^{- 3}; probable space group C2, Cm, or C2/m. Presumably, in structure I, the coordination polyhedra of U atoms (hexagonal bipyramids), sharing common equatorial edges, are linked to form infinite chains via bridging oxygen atoms of acetate ions. Under the same conditions, the presence of water in acetonitrile results in formation of crystalline [UO₂(OOCCH₃)₂·HOOCCH₃] (II) and (NH₄)₂[(UO₂)₅(₃-O)₂(OOCCH₃)₈] (III), whose composition and structure were determined by single crystal X-ray analysis. In the structure of II, one acetate ion is bidentate chelate and the other, bidentate bridging; the coordination number (CN) of the U atom is 7. In the structure of III, there are three crystallographically independent U atoms with CN 7 and 8. The coordination polyhedra of the U atoms, sharing common edges and vertices, are linked via bridging O^{2 -} ions and oxygen atoms of acetate ions.     

Use of high-purity AlxGa1−x as layers in epitaxial structures for high-power microwave field-effect transistors

The fabrication of high-purity layers of Al_xGa_1−x As solid solutions in the range 0⩽x⩽0.38 by molecular beam epitaxy is reported. The low-temperature photoluminescence spectra of these layers reveal predominantly the free exciton recombination line (X). The narrow width of the X line, the high intensity ratio of this line to that of the band-acceptor transition line, and the linear dependence of the X line intensity on the excitation power density in the range between 1×10⁻⁴ and 100V·cm⁻² indicate a low concentration of background impurities in these layers. Using this material in pseudomorphic AlGaAs/InGaAs/GaAs heterostructures for high-power microwave transistors produced devices with a specific saturated output power of 0.9 W/mm at 18 GHz. Pis’ma Zh. Tekh. Fiz. 25, 8–15 (August 12, 1999)     

The role of basal-plane oxygen atoms in determining the ferroelastic and microstructural properties of Y-Ba-Cu-O

The basal-plane oxygen atoms in YBa₂Cu₃O_7−x behave like a lattice gas, with very high diffusivity, especially for oxygen-deficient specimens. Implicit in this behaviour is the property that even a small amount of stress applied along theb-axis (b >a) can make these oxygen atoms hop from the (0,1/2,0) sites to the (1/2,0,0) sites. This is suggested as the primary mechanism responsible for the ferroelastic switching observed in this crystal. Since the material is an oxide of a mixed-valence element (Cu), the common occurrence of overall nonstoichiometry is only to be expected. Also, as discussed by Khachaturyan and others, except at very high temperature, the oxygen atoms and the vacancies will always have a tendency for ordering and/or precipitation into configurations which approach near-perfect stoichiometry locally. However, not all evidence for ordering is in conformity with the predictions of Khachaturyan’s concentration-wave model. The experimental data are examined critically. Further experiments to resolve the discrepancies are suggested.     

Single step synthesis of nanosized CeO2–MxOy mixed oxides (MxOy?=?SiO2, TiO2, ZrO2, and Al2O3) by microwave induced solution combustion synthesis: characterization and CO oxidation

Various CeO₂ –M _x O _y (M _x O _y  = SiO₂, TiO₂, ZrO₂, and Al₂O₃) mixed oxides were prepared by microwave induced solution combustion method and analyzed by different complimentary techniques, namely, X-ray diffraction (XRD), Raman spectroscopic (RS), UV–Vis diffuse reflectance spectroscopy (UV-DRS), X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG-DTA), and BET surface area. XRD analyses revealed that CeO₂ –SiO₂ and CeO₂ –TiO₂ mixed oxides are in slightly amorphous form and exhibit only broad diffraction lines due to cubic fluorite structure of ceria. XRD lines due to the formation of cubic Ce_0.5Zr_0.5O₂ were observed in the case of CeO₂ –ZrO₂ sample. RS results suggested defective structure of the mixed oxides resulting in the formation of oxygen vacancies. The UV-DRS measurements provided valid information about Ce⁴⁺ ← O²⁻ and Ce³⁺ ← O²⁻ charge transfer transitions. XPS studies revealed the presence of cerium in both Ce³⁺ and Ce⁴⁺ oxidation states. The ceria–zirconia combination exhibited better oxygen storage capacity (OSC) and CO oxidation activity when compared to other samples. The significance of present synthesis method lays mostly on its simplicity, flexibility, and the easy control of different experimental factors.     

<Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">n</Emphasis> junctions in ZnO implanted with group V ions

n-Type ZnO〈Ga〉 films were implanted with 150-keV N⁺ (As⁺) ions to a dose of 7 × 10¹⁵ cm⁻² and then annealed in atomic oxygen at different temperatures. p-Type conductivity was obtained at annealing temperatures in the range 770–870 K. The parameters of the p-type layers were determined by photoluminescence spectroscopy, secondary ion mass spectrometry, and Hall effect measurements. According to the Hall data, the p-type layers had a resistivity of ∼30 Ω cm, carrier mobility of ∼2 cm²/(V s), and carrier concentration of ∼10¹⁸ cm⁻³. The electroluminescence spectra of the p-n junctions produced by ion implantation showed a band at 440 nm, due to recombination via donor-acceptor pairs.     

Critical importance of the single-level approximation to account for the highly nonmonotonic dependences of carrier lifetimes on recombination impurity concentration

This paper is a continuation of an analysis regarding an increase in the lifetimes of nonequilibrium electrons π_n and holes πp by several orders of magnitude, observed with increasing concentration of recombination centers. It is shown that a substantial increase in πn and πp may also occur for three charge states of the recombination impurities N, and the curves πn=f(N) and π n=f(N) may each have two minima and maxima. Pis’ma Zh. Tekh. Fiz. 23, 39–45 (April 12, 1997)     

Synergy of Ultrathin CoOx Overlayer and Nickel Single Atoms on Hematite Nanorods for Efficient Photo-Electrochemical Water Splitting

To solve surface carrier recombination and sluggish water oxidation kinetics of hematite (α-Fe₂O₃) photoanodes, herein, an attractive surface modification strategy is developed to successively deposit ultrathin CoO_x overlayer and Ni single atoms on titanium (Ti)-doped α-Fe₂O₃ (Ti:Fe₂O₃) nanorods through a two-step atomic layer deposition (ALD) and photodeposition process. The collaborative decoration of ultrathin CoO_x overlayer and Ni single atoms can trigger a big boost in photo-electrochemical (PEC) performance for water splitting over the obtained Ti:Fe₂O₃/CoO_x/Ni photoanode, with the photocurrent density reaching 1.05 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode (RHE), more than three times that of Ti:Fe₂O₃ (0.326 mA cm⁻²). Electrochemical and electronic investigations reveal that the surface passivation effect of ultrathin CoO_x overlayer can reduce surface carrier recombination, while the catalysis effect of Ni single atoms can accelerate water oxidation kinetics. Moreover, theoretical calculations evidence that the synergy of ultrathin CoO_x overlayer and Ni single atoms can lower the adsorption free energy of OH* intermediates and relieve the potential-determining step (PDS) for oxygen evolution reaction (OER). This work provides an exemplary modification through rational engineering of surface electrochemical and electronic properties for the improved PEC performances, which can be applied in other metal oxide semiconductors as well.     

The Second Order Recombination and Spin-Exchange Relaxation of Atomic Deuterium at Low Temperatures in a Low Magnetic Field

We measured the 2nd order recombination rates and spin-exchange relaxation of atomic deuterium (D) in a ⁴ He coated sample cell, using the hyperfine resonance of (F = 1/2, m_F = –1/2) – (F = 3/2, m_F = –1/2) transition in a low magnetic field (3.9 mT) at temperatures between 0.6 K and 1.2 K. At lower temperatures below 0.9 K, the density decay of D atoms was dominated by D-D recombination on the liquid He surface. We found that the surface recombination cross length was 1_DD = (5.5 ± 1.3) × 10 ^–9 cm and the adsorption energy of D on ⁴ He surface was _a = 3.97 ± 0.07 K. Compared with prior measurements at high magnetic fields by other groups, 1_DD at low field was orders of magnitude smaller than what was expected when the scaling of 1/B² dependence of the direct recombination mechanism was used, and in addition, _a was significantly larger. This was attributed to the onset of the resonant recombination mechanism for the D-D surface recombination at high fields. Above 0.9 K, D-D volume recombination and recombination of D with hydrogen impurity became dominant processes of the density decay of D. The transverse relaxation times were measured and we determined the D-D spin-exchange relaxation rates, G_DD = (1.4 ± 0.6) × 10 ^–10 cm ³ sec ^–1 . It was smaller than theoretical calculations.