Defect structure of Cu2+ center in K2SO4‒Na2SO4‒ZnSO4 glasses

The spin Hamiltonian parameters (g factors g _//, g _⊥ and hyperfine structure constants A _//, A _⊥) of Cu²⁺ in K₂SO₄?Na₂SO₄?ZnSO₄ glasses are calculated from the high-order perturbation formulas of 3d⁹ ion in tetragonal octahedral sites. The calculated results are in agreement with the observed values. Since the EPR parameters are sensitive to the local structure of a paramagnetic impurity center, the defect structure of Cu²⁺ center in K₂SO₄?Na₂SO₄?ZnSO₄ glasses is estimated. The validity of results is discussed.     

Electrical conductivity of superionic solid solutions of Na2SO4 with Mx (XO4)y -[M=Na,K, Rb,Cd, GdandX=W,Mo, S,Si;x=1, 2, 4 andy=1, 3]

Electrical conductivity data are reported for solid solutions of Na₂SO₄, K₂WO₄, Na₂WO₄, Na₂MoO₄, Rb₂SO₄, Na₄SiO₄ and Gd₂ (SO₄)₃. In all cases, except K₂SO₄, we observed an increase in Na⁺ conductivity effected by lattice expansion and/or incorporation of ion vacancies in addition to a structural transformation. Boundary conditions were shown to exist for these factors to yield a limiting Na⁺ conductivity with a constant fraction of Na⁺ based on a percolation model of transport. The higher conductivity data observed for the larger radius isovalent WO^2-₄ and aliovalent SiO^4-₄ doped Na₂SO₄ show conclusively that the anion-rotation ”cogwheel” mechanism does not contribute to the cationic conductivity in Na₂SO₄.     

Energy levels of rare-earth ions in Gd<Subscript>2</Subscript>O<Subscript>2</Subscript>S

The energies of the ground 4f ⁿ levels of tri- and divalent rare-earth ions with respect to the conduction and valence bands of Gd₂O₂S crystal has been determined. It is shown that the Pr³⁺, Tb³⁺, and Eu³⁺ ions can be luminescence centers in Gd₂O₂S. The levels of the Nd³⁺, Dy³⁺, Er³⁺, Tm³⁺, Sm³⁺, and Ho³⁺ ions lie in the valence band; therefore, these ions cannot play the role of activators. The ground 4f level of the Ce³⁺ ion is near the midgap, due to which Ce³⁺ effectively captures holes from the valence band and electrons from the conduction band and significantly decreases the afterglow level of the Gd₂O₂S:Pr and Gd₂O₂S:Tb phosphors.     

Phase transitions and energy storage properties of some compositions in the (1−x)Li2SO4–xNa2SO4 system

In the binary system (1?x)Li₂SO₄–xNa₂SO₄, the solid–solid phase transitions and energy storage properties of Li₂SO₄, Na₂SO₄, the binary compound LiNaSO₄ and two eutectoids (E₁: 0.726Li₂SO₄–0.274Na₂SO₄; E₂: 0.03Li₂SO₄–0.97Na₂SO₄) were investigated by X-ray diffraction and differential scanning calorimetry. Li₂SO₄ has a solid–solid phase transition at 578 °C with the transition enthalpy 252 J g^?1. The binary compound LiNaSO₄ gives a slightly lower enthalpy value, 214 J g^?1 and its transition temperature is clearly reduced to 514 °C. The transition enthalpy of the eutectoid E₁ is maintained to 177 J g^?1 and its transition temperature is further reduced to 474 °C. Li₂SO₄, LiNaSO₄ and the eutectoid E₁ are applicable phase transition materials because of their large transition enthalpies. The enthalpies of Na₂SO₄ and the eutectoid E₂ are not very high (～45 J g^?1), but their transition temperatures are quite low (～250 °C); thus their transition properties may be applied at such low temperatures.     

Electronic structure of crystalline sulfites

The band spectra, densities of states, and distributions of the valence and difference densities in Na₂SO₃ and K₂SO₃ crystals with a hexagonal lattice are calculated in terms of the local-density functional theory. It is found that the absorption edge of these crystals is curvilinear. The partial compositions of the bundles of valence bands are determined. It is shown that the polarizing effect of the cations on the anions substantially depends on the symmetry and the number of metal sublattices.     

Theory of core photoemission spectra in CeO2

By using the Anderson model with a filled valence band, we calculate the core photoemission spectra of CeO₂, and compare them with experimental results. It is shown that in the ground state of CeO₂ the 4?⁰ and 4?¹ configurations are mixed strongly due to the large hybridization between the 4? states and the valence band. In the final state of the photoemission, the 4?¹ and 4?² configurations are also mixed strongly due to the final state interaction coming from a core hole potential and the large hybridization. The fractional intensity of the 4?⁰ photoemission peak is considerably different from the weight of the 4?⁰ configuration in the ground state because of the strong final state interaction.     

Photoelectrochemical properties of FTO/m-BiVO4 electrode in different electrolytes solutions under visible light irradiation

Photoelectrochemical properties of FTO/BiVO₄ electrode were investigated in different electrolytic solutions, potassium chloride (KCl) and sodium sulphate (Na₂SO₄), and under visible light irradiation condition. In order to accomplish that, an FTO/BiVO₄ electrode was built by combining the solution combustion synthesis technique with the dip-coating deposition process. The morphology and structure of the BiVO₄ electrode were investigated through X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. Photoelectrochemical properties were analyzed through chronoamperometry measurements. Results have shown that the FTO/BiVO₄ electrode presents higher electroactivity in the electrolyte Na₂SO₄, leading to better current stabilization, response time, and photoinduced current density, when compared to KCl electrolyte. Besides, this electrode shows excellent performance for methylene blue degradation under visible light irradiation condition. In Na₂SO₄, the electrode has shown higher degradation rate, 51 %, in contrast to 44 % in KCl, plus higher rate constant, 174?×?10^?4 min^?1 compared to 150?×?10^?4 min^?1 in KCl. Results presented in this communication leads to the indication of BiVO₄ thin films as alternate materials to use in heterogeneous photoelectrocatalysis, more specifically in decontamination of surface water.     

Specific features of the electronic structure of the CeCu<Subscript>6</Subscript>, CePd<Subscript>3</Subscript>, CeSi<Subscript>2</Subscript>, and CeF<Subscript>3</Subscript> systems

The electronic structure of cerium systems, the hybridization of 4 f and outer-shell electrons, and the influence of the position of the localized 4 f level with respect to the Fermi level E _F in the conduction band have been investigated. The CeCu₆, CePd₃, CeSi₂, and CeF₃ systems have been studied using X-ray photoelectron spectroscopy. The densities of states have been calculated by the tight-binding linearized muffin-tin orbital method within the atomic sphere approximation, which takes into account the covalent character of bonds and the nonspherical distribution of the electron density. The results obtained from the calculations of the total density of states are in good agreement with the valence band X-ray photoelectron data for the systems under investigation. It has been shown that the differences in the properties of the cerium systems are determined by the specific features of their electronic structure. A strong interatomic interaction is characteristic of heavy-fermion systems.     

Raman study of cation effect on sulfate vibration modes in solid state and in aqueous solutions

Raman spectra of potassium, sodium, and ammonium sulfates (K₂SO₄, Na₂SO₄, and (NH₄)₂SO₄) are reported and analyzed. These sulfates have been investigated under two states: solid (anhydrous and hydrated) salts and aqueous solutions. The effects of monovalent ions (K⁺, Na⁺, and NH₄⁺) and hydration on the position of Raman lines assigned to internal vibrations of sulfate anion SO₄²⁻ are discussed. In solid salts, the line position of each Raman peak is shown to decrease with increasing radius of the cation. The main ν₁ mode of sulfate molecule is particularly affected. It is emphasized that this sensitivity in solid sulfates vanishes in aqueous solutions. As a consequence, this mode can be probed by Raman spectroscopy as the main signature of SO₄²⁻ to determine its concentration within a single calibration. Copyright © 2013 John Wiley & Sons, Ltd.     

Fast Intrinsic Emissions of Wide-Gap Oxides Under Electron Irradiation

The emission spectra have been measured in the range of 1.6–9.0eV under irradiation of wide-gap oxides by single electron pulses (3 ns, 300kV). A fast (τ < 3 ns) continuous and temperature-independent emission, connected mainly with the transitions of hot holes between the levels of the valence band of oxides, can be separated in these spectra at 300–600 K, when the inertial emissions (5–7eV) of localized excitations undergo a strong thermal quenching. It is suggested that a drastic decrease of the intensity of this so-called hole intraband luminescence (IBL) in a short-wavelength spectral region is caused by the lowering of the density of states at the edges of the valence band and, therefore, supplies information on the width of an anion valence band E^v. The drastic decrease of the IBL intensity takes place at 6.4–8.6eV in BaMgAl₁₀O₁₇, SrAl₂O₄, MgAl₄O₇, MgO and BeO, that agrees satisfactorily with the values of E^v in these systems obtained by other methods.     

Electronic structure of the Sr2MgSi2O7:Eu persistent luminescence material

The electronic structures of the distrontium magnesium disilicate (Sr₂MgSi₂O₇(:Eu²⁺)) materials were studied by a combined experimental and theoretical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structures were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands and the band gap energy of the material as well as the position of the Eu²⁺ 4f ground state were calculated. The calculated band gap energy (6.7 eV) agrees well with the experimental value of 7.1 eV. The valence band consists mainly of the oxygen states and the bottom of the conduction band of the Sr states. The calculated occupied 4f ground state of Eu²⁺ lies in the energy gap of the host though the position depends strongly on the Coulomb repulsion strength. The position of the 4f ground state with respect to the valence and conduction bands is discussed using the theoretical and experimental evidence available.     

A nuclear magnetic resonance study of the structural properties and molecular motions of Li2KH(SO4)2 and LiKSO4 single crystals

The structural properties and relaxation mechanisms of Li₂KH(SO₄)₂ crystals were determined using the temperature dependences of NMR spectra and the spin-lattice relaxation times (T₁) of their ¹H, ⁷Li, and ³⁹K nuclei. The results obtained were compared with the previously reported physical properties of LiKSO₄ crystals. The substitution of the potassium ions with protons in the LiKSO₄ crystals were variations in the phase transition temperatures, and the non-appearance of ferroelastic properties. The ⁷Li T₁ for the Li₂KH(SO₄)₂ crystals was much shorter than the ⁷Li T₁ for the LiKSO₄ crystals, and these findings indicate that the presence of the protons in Li₂KH(SO₄)₂ causes the Li ions to move with greater freedom.     

To the explanation of the “white” line in the X-ray <Emphasis Type="Italic">K</Emphasis>-absorption spectrum of sulfur in NaBiS<Subscript>2</Subscript>

The maximum in the K-absorption spectra of sulfur in LiBiS₂, NaBiS₂, and KBiS₂, which is located more than 12 eV above the absorption edge and is especially strong for NaBiS₂, cannot be explained within single-electron calculations. Apparently, it is not related to any ordering in the arrangement of alkali metal and bismuth atoms in the cation sublattice. A scheme is considered in which a Na₂S-Bi₂S₃ solid solution is formed with subsequent oxidation of Na₂S with the formation of sulfate Na₂SO₄ with sulfur in the valence state +6, as a result of which the absorption edge shifts by approximately 13 eV. Thus, the initial experimental spectrum is treated as a weighted sum of the K-absorption spectra of sulfur in NaBiS₂, Na₂S, Bi₂S₃, and Na₂SO₄.     

Band structure and optical spectra of RbNH4SO4 crystals

First-principal density functional theory (DFT) calculations of the band structure, density of states and dielectric functions ε(E) of the rubidium ammonium sulfate (RAS) crystal, RbNH₄SO₄, in the orthorhombic phase Pnma have been carried out using the CASTEP code. Valence electron bands of the crystal are flat in k-space, that responds to the relatively great effective mass, m*?5m_e. The top valence band of the crystal has been found to be the most flat, what might be an evidence of a weak chemical bonding of the sulfate complexes (SO₄) in the crystal and therefore for the predisposition to structural instability and phase transitions. The characteristic feature is that two top valence bands are originated almost entirely from p-electrons of oxygen. The bottom part of the conduction band is formed mainly by the hydrogen atoms, the higher parts of this band—by a mixed set of chemical elements and orbital moments. The calculated refractive indices in the range of crystal's transparency agree satisfactorily with the experiment considering that the infrared absorption is not taken into account in calculations.     

LiKSO4的无公度相

本文用多晶X射线衍射配合差热分析的方法研究了LiKSO₄室温以上的相变。发现当温度在熔点以下到675℃之间,晶体结沟与α-K₂SO₄的高温相同构,α相最可能的空间群为P6₃/mmc.在T_i=675℃以下出现调制结构,类似K₂WO₄,K₂MoO₄等的无公度相;参数κ的值由0.492(640℃)而随温度变化。在470℃出现整合相变κ=0.500,整合后的结构为室温相的超点阵。然后在439℃转变为室温相(空间群为P6₃)。 关键词：     

Photocatalytic Oxidation of Hydrosulfide Ions by Molecular Oxygen Over Cadmium Sulfide Nanoparticles

Photocatalytic activity of CdS nanoparticles in hydrosulfide-ions air oxidation was revealed and thoroughly investigated. HS⁻ photooxidation in the presence of CdS nanoparticles results predominantly in the formation of SO₃ ²⁻ and SO₄ ²⁻ ions. Photocatalytic activity of ultrasmall CdS crystallites in HS^– photooxidation is much more prononced as compared to bulk CdS crystals due to high surface area of nanoparticles, their negligible light scattering, improved separation of photogenerated charge carriers etc. It was shown that hydrosulfide ions can be oxidized in two ways. The first is HS⁻ oxidation by the CdS valence band holes. This process rate depends on the rate of comparatively slow reaction between molecular oxygen and CdS conduction band electrons. The second reaction route is the chain-radical HS⁻ oxidation induced by photoexcited CdS nanoparticles and propagating in the bulk of a solution. In conditions favourable to chain-radical oxidation of HS⁻(i.e. at low light intensities and CdS concentration and high oxygen and Na₂S concentrations) quantum yields of the photoreaction reach 2.5.     

Realization of a heavily doped and fully compensated semiconductor state in a crystalline semiconductor with a deep impurity band

We use the data on the pressure (up to P=1.5 GPa) and field (up to H=17 kOe) dependence of the Hall coefficient and the resistivity at 77.6 and 300 K in p-CdSnAs₂〈Cu〉 to calculate the effective kinetic characteristics of the charge carriers, the density and mobility of the conduction electrons and the holes of the deep acceptor and valence bands, in an interval of excess-acceptor densities N _ext ranging from 10¹⁰–10¹⁷ cm⁻³. We establish that in a heavily doped semiconductor with a deep impurity band at the tail of the density of states of the intrinsic band, with unequal donor and acceptor densities, a a heavily doped and fully compensated semiconductor state is realized under hydrostatic compression. The threshold value of the pressure that initiates the transition into such a state, P _c, depends on the extent to which the impurity band is populated. In p-CdSnAs₂〈Cu〉 at N _ext=N _A, where N _A is the density of deep acceptors, and T⩽77.6 K the value of P _c amounts to 10⁻⁴ GPa. As the population of the deep acceptor band grows, P _c increases and in the limit becomes infinite. We discuss the special features of the electrophysical properties of p-CdSnAs₂〈Cu〉 arising from the absence of an energy gap between the states of the conduction band and those of the deep acceptor band. Zh. éksp. Teor. Fiz. 111, 562–574 (February 1997)     

Electronic structure of F,-center in MgO

The embedded-cluster numerical variational method has been developed to calculate the electronic structure of perfect MgO, F and F⁺-centers in MgO. The energy band, bulk density of states has been calculated by cluster Mg₁₄O₁₃, Mg₁₄O₁₂F⁺ and Mg₁₄O₁₂F. The calculated absorption energy for F⁺ and F centers is in good agreement with experimental data. In our calculated defect energy levels, that the first excited state of F⁺-center is at CB-3.46 eV, indicates the necessity of a large photoelectron yielding energy. We also calculate the radius of color center electron, and plot the map of charge-density distribution of valence electrons in which the structure of the color center is shown directly. Received 22 May 1998     

Influence of the special features of atomic structure of Si24 and MeSi24 (Me = Na or K) nanoparticles on their electron properties

The influence of atomic structure of silicon nanoparticles on the distribution of total and partial state densities in the valence and conductivity bands as well as on the valence band and gap widths is analyzed based on the calculated geometrical and electron structure of the Si ₂₄ and MeSi ₂₄ (Me = Na or K) clusters. Semi-empirical AM1 and PM3 methods are used for calculations. The adequacy of calculations is confirmed by their comparison with the available experimental data. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 46–51, February, 2006.     

Ionic transport in the Na2SO4Na2WO4 and Na2SO4M2(SO4)3 (M=La,Dy, Sm,In) systems

Premelted, predried Na₂SO₄, premelted Na₂WO₄, Na₂SO₄Na₂WO₄ composites and Na₂SO₄M₂(SO₄)₃ (M = La, Dy, Sm, In) have been studied by means of X-ray diffraction, DTA and electrical conductivity measurements. The high temperature, highly conducting Na₂SO₄ phase I has been successfully stabilised at room temperature; the Na₂SO₄ containing 4 mole% La₂(SO₄)₃ exhibits the highest conductivity (σ) and lowest activation energy (E_a) (σ=1.08 × 10⁻³ω⁻¹ cm⁻¹ at 290°C and E_a=0.50 eV) and therefore this system appears promising for further development.