Effective Number Of Electrons Research Articles

Influence of Covalence on the Metal-Insulator Temperature in the CuIr 2 (S 1− x Se x ) 4 Spinels

Influence of the covalence on the metal-insulator (M-I) temperature in the CuIr 2 (S 1 m x Se x ) 4 spinel series is considered. For this purpose a vacancy model and the effective number of valence electrons per molecule are used. Calculations of the vacancy model parameters, i.e., the ion packing coefficients, the filling coefficient of the unit cell, the difference of the Pauling electronegativities and the vacancy parameter suggest that the covalence increases with increasing selenium concentration x , leading to a decrease of the metal-insulator temperature. It was observed that the metal-insulator temperature is related to the effective number of valence electrons per molecule, suggesting that with increasing covalence, less and less valence electrons take part in the metal-insulator transition. These effects are explained within the framework of the mixed valence of the iridium ions including structural defects.

Comparison of Voltammetric Responses of Toluene and Xylenes at Iron(III)-Doped, Bismuth(V)-Doped, and Undoped β-Lead Dioxide Film Electrodes in 0.50 M H 2 SO 4

Voltammetric activities are compared for the designated compounds in 0.50 M at the specified film electrodes configured as rotated disks. The effective number of electrons equiv mol−1) calculated from the Koutecky-Levich plot is for toluene oxidation at the electrode, compared to and at the and electrodes, respectively. Gas chromatography-mass spectrometry data confirm that the primary product of toluene oxidation is benzyl alcohol (2 equiv mol−1) at the electrode with production of small amounts of benzaldehyde (4 equiv mol−1) and benzoic acid (6 equiv mol−1). X-ray photoelectron spectroscopy data indicate the ratio of Fe:Pb is ca. 1:100 (atom:atom) in the films. By comparison, a Bi:Pb ratio of 33:100 (atom:atom) is easily attained in films. Nevertheless, the apparent heterogeneous rate constant for toluene oxidation is larger at the electrode compared to the electrode X-ray absorption near-edge structure data confirm speculation that iron exists in the 3+ oxidation state with octahedral coordination by O atoms in films. The large activity of the electrode is attributed to the benefit of adsorption of aromatic molecules at Fe(III) sites. © 2001 The Electrochemical Society. All rights reserved.

Voltammetric Sensing of Thiols. The Electrocatalytic Oxidation of 4-Acetamidophenol in the Presence of Cysteine: a Mechanistic Rotating Disk Electrode Study

A numerical method is illustrated for the modeling of an important electrocatalytic reaction at the rotating disk electrode (RDE). This reaction, labeled EC2XE, occurs via the following route: A − 2e- ⇌ B, B + X → B−X (k2), B−X − 2e- ⇌ [B−X]2+. The numerical method is based on a finite difference formulation of coupled mass transport and kinetic equations and makes use of a Hale coordinate transformation. Iterative solutions are performed using a Gauss−Newton scheme to solve the nonlinear differential equations. Numerical solutions are calculated to generate working surfaces in the effective number of electrons transferred, Neff. Cyclic voltammetry is reported in hydrostatic conditions with a 4-acetamidophenol (paracetamol)/cysteine system, which is shown to proceed via the EC2XE reaction pathway. The resulting data is modeled using the commercial package DIGISIM, and a result of k2 = 1.25 ± 0.25 × 107 mol-1 cm3 s-1 is obtained. RDE studies of the same system using steady-state linear sweep hydrodynamic ...

First principles study of electronic structure and optical properties of CaTiO3

The electronic-energy band structure, site and angular momentum decomposed density of states (DOS) and charge-density contours of perovskite CaTiO3 are calculated by the first principles tight-binding linear muffin-tin orbitals method with atomic sphere approximation using density functional theory in its local density approximation. The calculated band structure shows an indirect (R-Γ) band gap of 1.5 eV. The total DOS as well as the partial density of states (PDOS) are compared with the experimental photoemission spectra. The calculated DOS are in reasonable agreement with the experimental energy spectra and the features in the spectra are interpreted by a comparison of the spectra with the PDOS. The origin of the various experimentally observed bands have been explained. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Ca and TiO3 is mainly ionic and that the TiO3 entities bond covalently. Using the projected DOS and band structure we have analyzed the interband contribution to the optical properties of CaTiO3. The real and imaginary parts of the dielectric function and hence the optical constants such as refractive index and extinction coefficient are calculated. The calculated spectra are compared with the experimental results for CaTiO3 and are found to be in good agreement with the experimental results. The effective number of electrons per unit cell participating in the interband transitions are calculated. The role of band structure calculation as regards the optical properties of CaTiO3 is discussed.

Crystal structure of a high-pressure phase in Bi-based alloys related to Si VI

The structures of high-pressure intermediate phases in the Bi-rich binary alloys with Pb and In have been determined from diffraction x-ray data as an orthorhombic $\mathrm{oC}16$ cell, space group Cmca, related to Si VI. These phases are formed in Bi-based alloys at 1.5--2 GPa and are retained at ambient pressure by quenching. The same $\mathrm{oC}16\ensuremath{-}Cmca$ structure is proposed for Bi IV from the published data. Specific features of this structure and its stability are discussed with respect to the effective number of valence electrons and are accounted for the electronic origin.

Variation of K?/K? x-ray intensity ratios in 3d elements

The Kβ/Kα x-ray intensity ratios for compounds of 3d elements were calculated with the discrete-variational Xα (DV-Xα) molecular orbital method. It was found that the calculated results for V and Cr compounds are strongly correlated with the effective number of 3d electrons. The Kβ/Kα ratio is expressed as a linear function of the number of 3d electrons, but the slope of the straight line is different for different crystal symmetries. The Kβ/Kα ratio for Td symmetry is generally larger than that for Oh. Copyright © 2000 John Wiley & Sons, Ltd.

Dielectric Spectra in Charge Ordering State of Pr1-xCaxMnO3

We measured dielectric spectra in the charge ordering state of Pr 1- x Ca x MnO 3 (0.30 ≤ x ≤0.50), for frequencies of 100 Hz∼15 MHz. Observed dielectric spectra were analyzed using the Debye model. The effective number of electrons ( n / m ) is nearly temperature independent. Dissipation, γ, drastically increases with cooling. Temperature dependence is almost the same as resistivity curve. This indicates that the hopping of polarons dominate both dc- and ac-electrical transport in the charge ordering phase. Based on the small-polaron-hopping model, x -dependent activation energy is estimated, which reaches a maximum at x ≃0.33. The γ-value decreases in magnetic fields, but does not scale with magnetization.

Magnetostriction of ytterbium-based Kondo compounds YbXCu4 (X = In, Ag and Au)

Magnetostrictions were measured for polycrystalline YbXCu4 (X = In, Ag and Au) by the three-terminal capacitance method up to the magnetic field of 8 T in the temperature range 5-100 K. The volume magnetostrictions of all of the compounds are negative in contrast to the case for Ce-based Kondo systems. The absolute values of the volume magnetostriction coefficient for YbInCu4 and YbAgCu4 are large at low temperatures but depressed in the high-temperature localized-moment regime, whereas those of YbAuCu4 with almost stable Yb3+ moments are small. The results are discussed in terms of a modified Kondo-lattice model, where we emphasize that the volume dependence of the effective 4f electron (hole) number plays a dominant role. The anisotropic magnetostrictions of YbAuCu4 and YbAgCu4 have a positive sign but that of YbInCu4 is negative, suggesting a difference in background crystal-field ground states of the Yb3+ multiplet.

Theoretical determination of the absolute electron impact ionization cross-section function for silver clusters Agn (n=2–7)

A recently developed semiclassical approach (DM formalism) was used to calculate the absolute total electron-impact single ionization cross sections for silver clusters Agn (n=2–7) from threshold to 1000 eV. Similar to other cluster properties, a clear odd–even effect has been observed for the calculated cross sections and is ascribed to the effective electron numbers in the valence shell. The only available experimental data for comparison are partial cross sections for the production of the singly charged parent ions for the monomer up to the tetramer. No experimental data are available as yet for any larger clusters nor for the total ionization cross sections. Nevertheless, a comparison of the results of the present calculations with calculations using additivity rules applied to molecules and clusters gives valuable insight into the ionization behavior of clusters as a function of cluster size and thus elucidates the transition from a molecular-type situation where the cross sections of the constituents are additive to a cluster-type situation where geometric effects lead to a n2/3-dependence in the limit of large cluster sizes n.

Low-energy quenching of positronium by helium

Very low-energy scattering of orthopositronium by helium has been investigated for simultaneous study of elastic cross section and pick-off quenching rate using a model exchange potential. The present calculational scheme, while agrees with the measured cross section of Skalsey et al, reproduces successfully the parameter $^ 1Z_{\makebox{eff}}$, the effective number of electrons per atom in a singlet state relative to the positron. Together with the fact that this model potential also leads to an agreement with measured medium energy cross sections of this system, this study seems to resolve the long-standing discrepancy at low energies among different theoretical calculations and experimental measurements.

A simple method for calculating dispersion coefficients for isolated and condensed-phase species

A simple method is developed for the derivation of two-body and three-body dispersion coefficients for isolated and in-crystal atoms from a knowledge of their dipole polarizability and effective number of electrons. The method is checked by comparison with a large body of published theoretical results.

Application of the Modified Additivity Rule to the Calculation of Electron-Impact Ionization Cross Sections of Complex Molecules

This paper describes the application of the modified additivity rule (MAR) to the calculation of total (counting) electron-impact ionization cross sections of complex molecules with sum formulas of the form AxBy, AxByCz, and ApBsCtDu. The MAR incorporates weighting factors for the contributions to the molecular ionization cross section from the ionization cross sections of the constituent atoms, which depend explicitly on the atomic radii and the effective number of atomic electrons except for a few special cases (hydrides where the other constituent atom has a radius smaller than the radius of the H atom and species where both constituent atoms have radii smaller than the radius of the H atom), where the weighting factors depend only on the atomic radii, i.e., on geometric effects. A comprehensive comparison of the predictions of the modified additivity rule with available experimental data and with other theoretical predictions is presented.

Dynamic screening of fast ions moving in solids

The electron charge polarization induced by a fast ion moving on a solid medium is considered in a linear response model. An inhomogeneous electron gas model is introduced to describe the contribution to this dynamic screening by the bound target electrons. The action of the bound electrons is to increase the effective number of electrons of the uniform electron gas that reproduces features of the induced potential. We find that the value of the induced potential at the ion position is equivalent to that produced by a jellium of a given density, smaller than that required for the jellium to reproduce the value of the induced electric field at the ion. For the case of fast bare charges we find the precise value of jellium densities required to describe the experimental values of the stopping power in several solid targets. For the case of hydrogenic Kr projectiles in a Cu target, the mixing of its n53 states by the induced potential is just that required to explain recent measurements of radiative decay. @S0163-1829~98!05212-6#

Dielectric polarizability of ions and the corresponding effective number of electrons

In a previous paper it was shown that in-crystal ion polarizabilities derived from low-frequency precision measurements of the dielectric constants of single-crystal oxides can be given a quantitative interpretation using a quantum mechanically based result for a single, completely filled, electron subshell. In this paper, by exploiting the fact that the radii of related subshells differ only slightly, we propose an equation amended to include the possibility of contributions from incomplete or several subshells. Using the new result, effective electron numbers are derived for all the ions for which experimental polarizabilities have recently been published. It is shown that there is generally a good correspondence between these numbers and the known electronic configurations of the ions concerned. The new calculations show that the concept of a polarizability determined by the contribution of a single electron subshell is certainly valid with ions corresponding to elements of low atomic number, but that in the case of transition and rare earth ions there is probably more than one contributing electron subshell.

A modified additivity rule for the calculation of electron impact ionization cross-section of molecules AB n

This paper describes a modified additivity rule for the calculation of electron impact ionization cross-sections of molecules and radicals of the form AB n ( n = 1–6). This additivity rule incorporates weighting factors for the contributions to the molecular ionization cross-sections from the ionization cross-sections of the constituent atoms, which depend explicitly on the atomic radii and the effective number of atomic electrons. In a few special cases (hydrides where the other constituent atom has a radius smaller than the radius of the H atom and species where both constituent atoms have radii smaller than the radius of the H atom), the weighting factors can be simplified, so that they depend only on the atomic radii, i.e. on geometric effects. A comprehensive comparison of the predictions of this new modified additivity rule with available experimental data and with other theoretical predictions is presented.

Correlation of an infrared absorption with carriers in rare-earth monoantimonides

Dielectric constants spectra were obtained in the single crystals LaSb, PrSb, GdSb and DySb at several temperatures. The spectra for these crystals except for LaSb show Drude's behavior with a hump due to an anomalous absorption lying at about 0.25 eV. The inverse of effective electron number ( N IA) of the absorption is linear in temperature, and the N IA at each temperature is dependent on the square of the effective Bohr magneton of each rare-earth ion. The sum of the number of effective electrons due to Drude adsorption and that due to infrared absorption agree well with the number of carriers obtained from their band calculations or their dHvAs. Therefore, this absorption seems to be due to the intraband transition induced by the scattering between the spin of carriers and the localized magnetic moments at each site of rare-earth ion.

The Hard and Soft Acids and Bases Principle

Making use of the approximate expression for the total energy E of a system, in terms of the chemical potential μ and the hardness η, E[ρ] = Neμ − 1/2 η + Ecore[ρ] , where Ne is an effective number of valence electrons and Ecore[ρ] represents the core contribution to the total energy, it is shown that the interaction between species whose softnesses are approximately equal is energetically favored, relative to the interaction between species whose softnesses are very different from each other.

Influence of covalence on the critical temperature in the spinel superconductors

Influence of covalence on the critical temperature in spinel superconductors of the type AB2X4 (where A = Li, Cu; B = Ti, V, Rh; × = O, S, Se) is considered. For this purpose the vacancy model is used and an effective number of valence electrons per molecule is proposed. Calculations of the vacancy model parameters, i.e. the ion packing coefficients, the fulfilment coefficient of the unit cell, the difference of the Pauling electronegativities and the vacancy parameter suggest that the covalence increases with increasing anion radius, leading to a decrease of the critical temperature. The occurrence of the superconducting state in the spinel compounds with a large covalence is still possible if they possess a large vacancy concentration, i.e. a large value of the vacancy parameter. It was observed that the critical temperature is related to the effective number of valence electrons per molecule, suggesting that with increasing covalence, less and less valence electrons take part in the superconducting process. These effects are explained with the aid of a charge transfer mechanism using the Kondo-like X-X hopping terms.

The Characteristics of SrS:Ce Thin Film Electroluminescent Devices

SrS:Ce thin films were fabricated by electron beam evaporation under several conditions such as substrate temperature, deposition rate and sulphur pressure during evaporation. The relationship between the quality of phosphor layers and the luminance of EL (electroluminescent) devices investigated. The bigger dead layer of SrS:Ce phosphor layer produced the larger spike current when bipolar trapezoidal voltage pulses were loaded on the SrS:Ce TFEL (thin film electroluminescent) devices. The low crystallinity of the phosphor bulk layer reduced the spike current. The magnitude of spike current is limitted by a balance between the number of hot electrons injected into the phosphor layer from the phosphor-insulator interface and the number of hot electrons captured by defect levels in phosphor layer. The luminance of EL devices is determined by the effective number of hot electrons and the crystallinity of phosphor layer.

Range and strength of interatomic forces: dispersion and induction contributions to the bonds of dications and of ionic molecules

An application of previously introduced correlation formulas in terms of polarizabilities of interacting species permits an assessment of the relative role of dispersion and induction contributions to describe bond lengths and energies in some doubly charged ion-neutral systems (the dications of alkaline earth atoms and rare gases). An extension to ion-ion systems is also presented, on the basis of an empirical analysis of available data on alkali halides, in order to establish the role of both dispersion and induction interactions as compared to Coulomb attraction. An improved version of the Slater-Kirkwood formulation with an appropriate choice for the effective electron numbers is suggested to estimate the long range effective dispersion coefficients. The present approach provides a unifying framework for the characterization of bonds for systems which greatly vary in size and strength and recipes are suggested for interatomic potential energy curves. It is concluded that this approach is also relevant as a starting point for extensions to include anisotropies due both to orbital overlap for open shell partners and to mutual orientation when molecules are considered.