Electronic structure of α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>: Ab initio simulations and comparison with experiment

Al₂O₃ films 150 Å thick are deposited on silicon by the ALD technique, and their x-ray (XPS) and ultraviolet (UPS) photoelectron spectra of the valence band are investigated. The electronic band structure of corundum (α-Al₂O₃) is calculated by the ab initio density functional method and compared with experimental results. The α-Al₂O₃ valence band consists of two subbands separated with an ionic gap. The lower band is mainly formed by oxygen 2s states. The upper band is formed by oxygen 2p states with a contribution of aluminum 3s and 3p states. A strong anisotropy of the effective mass is observed for holes: m _h⊥ ^* ≈ 6.3m ₀ and m _h‖ ^* ≈ 0.36m ₀. The effective electron mass is independent of the direction m _e‖ ^* ≈ m _e⊥ ^* ≈ 0.4m ₀.     

Quantum yield and rate constant of the singlet <Superscript>1</Superscript>Δ<Subscript><Emphasis Type="Italic">g</Emphasis></Subscript> oxygen luminescence in an aqueous medium in the presence of nanoscale inhomogeneities

The quantum yields and lifetimes of photosensitized luminescence of the ¹Δ _g state of singlet oxygen in an aquatic media with a controlled concentration of dielectric anisotropy centers (polyethylene glycol) have been measured using the methods of laser fluorometry. It is established that the quantum yield and the rate constant (k _r ) of the a ¹Δ _g → X ³Σ _g ^- luminescence of ¹O₂ increase as the polymer concentration increases. The effect is analyzed within a general approach involving a relationship between kr and dielectric properties of the medium and is explained by the increased density of photon states and the local field factor in the space around O₂(а ¹Δ _g ).     

The oxygen content and its ordering in the chain planes in fine-grained HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

A comparative analysis of the results of the X-ray and Mösbauer studies of the high-temperature superconductor (HTSC) YBa₂Cu₃O _y and YBa₂Cu_{3 ? x} ⁵⁷Fe _x O _y (x = 0.015, T _c ≈ 91.5 K) samples with different average grain sizes <D> in the micron and submicron ranges has been performed. The regularities in the change in the lattice parameter c and in the degree of occupation of different oxygen sites in the CuO_δ chain planes taking place at the decrease in <D> have been studied. The quantitative interrelation between the parameter c and the oxygen content δ in the CuO_δ planes exceeding the amount of the mobile oxygen due to the interplane oxygen redistribution is established.     

Exchange Bias in Layered GdBaCo<Subscript>2</Subscript>O<Subscript>5.5</Subscript> Cobaltite

It is established that excess oxygen content δ influences the exchange bias (EB) in layered GdBa-Co₂O_{5 + δ} cobaltite. The EB effect arises in p-type (δ > 0.5) cobaltite and disappears in n-type (δ < 0.5) cobaltite. The main parameters of EB in GdBaCo₂O_5.52(2) polycrystals are determined, including the field and temperature dependences of EB field H _EB , blocking temperature T _B , exchange coupling energy J _i of antiferromagnet–ferromagnet (AFM–FM) interface, and dimensions of FM clusters. The training effect inherent in systems with EB has been studied. The results are explained in terms of exchange interaction between the FM and AFM phases. It is assumed that the EB originates from the coexistence of Co³⁺ and Co⁴⁺ ions that leads to the formation of monodomain FM clusters in the AFM matrix of cobaltite.     

Initiation of combustion of a hydrogen-air mixture with ozone impurity by UV laser radiation

The ignition kinetics of hydrogen-air mixtures with a small amount (0.5%) of ozone that are exposed to laser radiation with wavelength λ _I = 248.4 nm is analyzed. The formation of excited O(¹ D) atoms and O₂(a ¹Δ _g ) molecules due to O₃ molecule photodissociation is shown to greatly intensify the chain reactions and noticeably decrease the induction period and ignition temperature compared with the case when the radiation is absent even if the radiation energy applied to the gas is low, E _s = 0.5?1.0 eV per O₃ molecule. The efficiency of such a way of combustion initiation is much higher than at local heating of the medium by laser radiation but, at the same time, is considerably lower than the efficiency of the method based on excitation of O₃ molecule asymmetric oscillations.     

Measurement of attenuation cross-sections of some fatty acids in the energy range 122–1330 keV

The mass attenuation coefficients (μ_m) have been measured for undecylic acid (C₁₁H₂₂O₂), lauric acid (C₁₂H₂₄O₂), tridecylic acid (C₁₃H₂₆O₂), myristic acid (C₁₄H₂₈O₂), pentadecylic acid (C₁₅H₃₀O₂) and palmitic acid (C₁₆H₃₂O₂) using ⁵⁷Co, ¹³³Ba, ¹³⁷Cs, ⁶⁰Co and ²²Na emitted γ radiation with energies 122, 356, 511, 662, 1170, 1275 and 1330 keV, respectively. The accurate values of the effective atomic number (Z_eff), atomic cross-section (σ_t,), electronic cross-section (σ_e) and the effective electron density (N_eff) have great significance in radiation protection and dosimetry. These quantities were obtained by utilizing experimentally measured values of mass attenuation coefficients (μ_m). A NaI(Tl) scintillation detector with 8.2% (at 662 keV) resolution was used for detecting of attenuated γ-photons. The variation in Z_eff and N_eff of fatty acids with energy is discussed. The experimental and theoretical results are in good agreement within 2% deviation.     

Quantum-Chemical Simulation of the Solvent Effect on Spontaneous Emission of Singlet Oxygen

A molecular simulation of the solvent effect on radiative rate constant k _r of singlet oxygen is carried out. This study included a search for the most probable conformations of the complexes of molecules of singlet oxygen and ten solvents and calculation of dipole moments M of transitions a¹Δ _g –b¹Σ _g ⁺ (M _a–b ) and a¹Δ _g –X³Σ _g ^- (M _a–X ) of the oxygen molecule for them. Averaging of M _a–b by conformations, taking into account the probability of their formation for complexes without atoms with a large atomic number (Cl, S), yields values that, as a rule, correlate well with the behavior of k _r in the experiment. Taking into account the possibility of decreasing the distance (compared to equilibrium) between molecules in a collision complex at room temperature made it possible to achieve satisfactory agreement of the calculated and experimental data also for complexes with CCl₄, C₂Cl₄, and CS₂. The obtained data indicate that a number of factors affect k _r . The correlation of k _r with molecular polarizability in a number of cases is due, on the one hand, to its effect on the strength of dispersion interactions in the complex and, on the other hand, to the fact that it to some extent reflects the position of the upper filled orbitals of the solvent molecule. Both factors affect the degree of mixing of the π orbitals of the singlet oxygen molecule with the orbitals of the solvent molecule, which, as was found earlier, facilitates the activation of the a¹Δ _g –b¹Σ _g ⁺ transition and the borrowing of its intensity by the a¹Δ _g –X³Σ _g ^- transition.     

Thermodynamic study of compounds of the Nd-Ba-Cu-O system

Standard enthalpies of formation for solid solutions of composition Nd_{1 + x} Ba_{2 ? x} Cu₃O _y (x = 0–0.8, y = 6.65–7.24) from oxides were determined by solution calorimetry. The heat capacity of NdBa₂Cu₃O_6.87 phase was measured in the range 5–320 K by low-temperature adiabatic calorimetry. The absolute entropy S ^o(T), the difference of enthalpies H ^o(T)-H ^o(0 K), and the reduced Gibbs energy Φ^o(T) = S ^o(T)–[H ^o(T)–H ^o(0)]/T were calculated on the basis of smoothed dependence C _p (T) in the 0–320 K range. An assessment was made for the heat capacities and the absolute entropies of solid solutions Nd_1+x Ba_2?x Cu₃O _y . The obtained set of thermodynamic parameters can be used for the calculation of phase equilibria in the Nd-Ba-Cu-O system.     

Vibrationally Excited Ozone Relaxation by CO

Time profiles of ozone concentration after pulsed UV laser photolysis in the O₂- O₃-Ar-CO mixture, measured using time-resolved absorption spectroscopy, are presented. The experimental results show the dominance of the stabilization channel over the reactive one for the interaction between the vibrationally excited ozone molecule O₃(υ) and carbon monoxide CO. The rate constant of the process O₃(υ)+CO→O₃ + CO, obtained by processing experimental data by the kinetic modeling method is (1.5 ± 0.2) · 10^?13 cm³/s.     

Effect of oxygen impurity on the efficiency of the formation of complexes with H-bond and aggregation of color centers in lithium fluoride

The effect of impurities on the efficiency of the formation of color centers and hydrogen-bonded molecular complexes upon exposure to various radiations in lithium fluoride crystals grown in air is studied. The results of experiments for measuring optical properties, IR vibrational spectra, luminescence, and thermally stimulated luminescence are presented. The fact that the band in the range of 1800–2300 cm^–1 corresponds to stretching vibrations of a complex with strong hydrogen bond is proved based on the Fermi-resonance perturbation in the region of 2080 cm^–1, shaped as the Evans hole and bands A, B, and C. It is shown that the composition of these complexes includes an OH^– ion and an HF molecule. The crucial role of O^2? V _a ⁺ oxygen dipoles in the aggregation efficiency and gradient distribution of color centers and radiation resistance of hydroxyl ions is revealed. It is shown that products of radiation decomposition of OH^– ions stimulate, while decay of O^2? V _a ⁺ dipoles suppress, the formation of positively charged color centers.     

Development of the method for low-temperature investigations of HTSC systems on an X-ray electron magnetic spectrometer

Peculiarities of the chemical structure of bulk polycrystalline samples of the high-temperature superconductors Bi₂Sr₂CaCu₂O₈, BiSrCaCu₂O_5.5, BiSrCaCu₃O₈, and YBa₂Cu₃O_{7 ? δ} have been investigated in detail at room and superconducting temperatures on an X-ray electron magnetic spectrometer equipped with an attachment for low-temperature studies. It is shown that covalent bonding is formed at a superconducting temperature between copper and oxygen due to Cu²⁺ ions. Due to the enhancement of the d(Cu)–p(O) hybridization of copper and oxygen electrons in the superconducting state, the d-electron density increases near E _F. The occurrence of additional peaks in the O1s and Sr3d (Ba3d) spectra after transition of the system to the superconducting state indicates changes in the nearest environment of O and Sr (Ba) atoms, in particular, the transition of Sr atoms to a higher oxidation state.     

Ferromagnetism of metallic compounds with the perovskite structure

On the basis of the concept of strong interaction in one unit cell, it is stated that ferromagnetic instability can occur in a system with hops between oxygen anions and transition metal (Me) cations in A_1?xB_xMeO₃ compounds. A phase diagram is constructed to describe the occurrence of ferromagnetic ordering as a function of the average number of holes (h_{t, d}) and (h_p) in the low-spin or high-spin t _2g ⁶ or high-spin 3d¹⁰ shell of the transition metal and in the 2p⁶ shells of O^2?.     

Reactions of initiation of the autoignition of H<Subscript>2</Subscript>–O<Subscript>2</Subscript> mixtures in shock waves

The initiation of the autoignition of hydrogen–oxygen–argon mixtures behind reflected shock waves is studied by absorption and emission spectrophotometry in the temperature range of 960 < T < 1670 K at pressures of ~0.1 MPa. Introduction of Mo(CO)₆ additive in an amount of ~80 ppm made it possible to study the effect of O atoms on the shortening of the ignition delay time of H₂–O₂–Ar mixtures. A kinetic modeling of our own and published experimental data at temperatures of 930 < T < 2500 K and pressures of 0.05 < P < 8.7 MPa enabled to establish how the initiation reactions influence the process of self-ignition and to evaluate the rate constant for one of the initiation reactions: k(H₂ + O₂ → 2OH) = (3 ± 1) × 10¹¹exp(–E _a/RT), cm³ mol^–1 s^–1, where E _a = (40 ± 2) kcal/mol.     

Structural and magnetic phase transformations in the BiFeO<Subscript>3</Subscript>-CaFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Bi_{1 ? x} Ca _x Fe_{1 ? x/2}Nb _x/2O₃ system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinski?-Moriya interaction. In CaFe_0.5Nb_0.5O₃, the Fe³⁺ and Nb⁵⁺ ions are partially ordered and the unit cell is monoclinic (space group P2₁/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed.     

Cluster <Emphasis Type="Italic">ab initio</Emphasis> modeling of local lattice instability in relaxor ferroelectrics

The possibility of a zigzag-type instability occurring for oxygen atoms in B-O-B, B-O-Nb, and Nb-O-Nb linear chains is examined in disordered mixed perovskite compounds Pb(B_1/3, Nb_2/3)O₃ (B=Mg, Zn, Cd). Local adiabatic potentials for oxygen atoms are studied using total energy calculations by the ab initio Hartree-Fock + MP2 method for many-atomic clusters with different oxygen surroundings of lead atoms. The effect of lattice relaxation along the chain on the shape of the local potential in the transverse direction for the central oxygen atom is considered.     

Effect of Ce-doped MnO<Subscript>2</Subscript> catalyst on the lithium-air batteryperformance in the ambient atmosphere

MnO₂ doped with Ce was hydrothermally synthesized and the as-made breathable waterproof membrane used outside the cathode was prepared for improving the lithium-air battery performance in air. The samples were characterized by scanning electron microscopy (SEM), energy dispersive spectrum analysis (EDS), charge–discharge cycle tests, charge–discharge cycle tests of limited capacity, and electrochemical impedance spectroscopy (EIS) tests. The result showed that Ce _x Mn_1-x O₂ can effectively reduce the charge overpotential of the cathode. The charge and discharge electrical potential difference of Ce_0.1Mn_0.9O₂ was only 700 mV while MnO₂’s was 2100 mV. And Ce_0.1Mn_0.9O₂ that exhibited high discharge capacity of 400 mAh g^?1 in air had a stable discharge platform of 2.5 V and then the more obvious charge phenomenon appeared after 3.5 V. The excellent catalysis, the effect of cathode catalytic materials named Ce _x Mn_1-x O₂, may attribute to the decrease of reaction potential energy of oxygen reduction reaction and oxygen evolution reaction.     

Effect of the nuclear charge of a fast structural ion on its internal effective stopping in collisions with atoms

The energy losses of fast structural ions in collisions with atoms have been considered in the eikonal approximation. The structural ions are ions consisting of a nucleus and a certain number of electrons bound to it. The effect of nuclear charge Z of the ion on its effective deceleration κ^(p) (energy losses associated with excitation of only intrinsic ion shells) has been analyzed. It is shown that the allowance for the interaction of an atom with the ion nucleus for Z _a Z/v > 1, where Z _a is the charge of the atomic nucleus and v is the velocity of collisions in atomic units, considerably affects the value of κ^(p), which generally necessitates taking into account nonperturbatively the effect of both charges Z _a and Z on κ^(p).     

Spectroscopy of optical centers of Eu<Superscript>3+</Superscript> ions in partially stabilized and stabilized zirconium crystals

The structure of the optical centers of Eu³⁺ ions in tetragonal (ZrO₂)_1–x–y (Y₂O₃) _x (Eu₂O₃) _y (х = 2.7–3.6; y = 0.1) and cubic (ZrO₂)_1–x–y (Y₂O₃) _x (Eu₂O₃) _y (х = 8–38; y = 0.1–0.5) crystals of solid solutions on the basis of zirconium dioxide is studied using the methods of optical and Raman-scattering spectroscopy. Characteristic optical centers of Eu³⁺ ions with different crystalline environments are revealed in the above compounds.     

Ordered phases of lithium nickelite Li<Subscript>1−<Emphasis Type="Italic">x</Emphasis>−<Emphasis Type="Italic">z</Emphasis></Subscript>Ni<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript>

A symmetry analysis of ordering in lithium nickelite Li_1?x?zNi_1+xO₂ (Li_1?x?z□_yNi_1+xO₂) was performed with regard to the substitution of Li and Ni atoms and the occurrence of structural vacancies □ in the metal sublattice. For all the ordered phases, the k ₉ ⁽³⁾ ray of the Lifshitz {k₉} star is present in the order-disorder transition channel. This ray determines the consecutive alternation of atomic planes filled with only Ni atoms or only Li atoms and vacancies in the \([1\bar 11]_{B1} \) direction. It was shown that the rhombohedral ordered LiNiO₂ phase is formed in the defect-free lithium nickelite, whereas a family of three monoclinic Li₃□Ni₄O₈ (C2/m space group) and Li₂□Ni₃O₆ (C2/m and C2 space groups) superstructures arises as the concentration of structural vacancies increases. For all the superstructures, the order-disorder phase-transition channels were determined and the distribution functions of Li and Ni atoms have been calculated. The long-range order parameters describing each superstructure were found as functions of the Li_{1?x? z}Ni_1+xO₂ composition.     

Adsorption of oxygen on Mo (112) surface precovered with beryllium: formation of overlayer and electronic properties

Adsorption of oxygen on the Mo?(112) surface precovered with a pseudomorphic monolayer of beryllium has been investigated at room temperature by AES, LEED and contact potential difference methods. Such a Be/Mo?(112) substrate is actually a bimetallic surface where closely-packed atomic Mo ridges alternate with rows of Be atoms. It has been found that at small oxygen exposures (Q < 0.3 Langmuir), the initial sticking coefficient for oxygen S _O on Be/Mo?(112) is lower by a factor of ~1/15 than on the clean Mo?(112) surface where S _O is close to unity. However, with increasing the oxygen coverage above θ _O ≈ 0.1, the sticking coefficient showed a nonlinear growth, and oxygen saturation of the surface was achieved at Q = 1.6–1.7 L. Oxygen adsorption decreases the work function of the Be/Mo?(112) surface and gives rise to appearance of some Auger peaks specific to beryllium oxide, which indicates a change in the chemical nature of the surface. The formation of a polar-covalent BeO compound may be responsible for a self-activation of the surface with respect to oxygen which is reflected in the increase of the sticking coefficient observed under growth of oxygen coverage (a kind of autocatalytic reaction). Annealing of the O/Be/Mo?(112) system to T _an  = 1100 K resulted in an additional decrease of the work function and a growth of the ratio between the Auger signals of Be in the oxide and metallic Be adsorbed phases. The presence of BeO molecules was detected up to T _an  = 1600 K, above which they dissociated with desorption of Be.