Showing papers on "Valence (chemistry) published in 1981"
TL;DR: In this article, the authors review the experimental results observed in the subset of rare-earth systems for which the 4f ions form a lattice with identical valence on each site, and discuss key thermodynamic experiments, such as susceptibility and lattice constant, and spectroscopic experiments such as XPS and neutron scattering.
Abstract: Valence fluctuation phenomena occur in rare-earth compounds in which the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration (valence) and/or of the magnetic moment. The authors review the experimental results observed in the subset of such systems for which the 4f ions form a lattice with identical valence on each site. The discussion includes key thermodynamic experiments, such as susceptibility and lattice constant, and spectroscopic experiments such as XPS and neutron scattering. This is followed by a review of existing theoretical work concerning both the ground states and the isomorphic phase transitions which occur in such compounds; the emphasis is on those aspects which make valence fluctuation phenomena such a challenging many-body problem.
707 citations
459 citations
TL;DR: In this article, the 4f and valence states of all metallic rare earths have been studied using X-ray photoelectron spectroscopy (XPS) for the occupied part and bremsstrahlung isochromat (BIS) for unoccupied part, and it is found that the bandwidth increases from Gd to Lu, and that the valence band spectra are in fair agreement with APW calculations.
Abstract: For pt.I see ibid., vol.11, p.113 (1980). The 4f and valence states of all metallic rare earths have been studied using X-ray photoelectron spectroscopy (XPS) for the occupied part and bremsstrahlung isochromat spectroscopy (BIS) for the unoccupied part. It is found that the bandwidth increases from Gd to Lu, and that the valence band spectra are in fair agreement with APW calculations. The intensities of the 4f final-state multiplets are well described in terms of the coefficients of fractional parentage. There is a symmetry between the fn XPS and f14-n BIS spectra. The observed energies of the 4f excitations correspond to transitions to completely screened final states. These values enable one to predict the elements which are liable to interconfiguration fluctuation when their 4f levels are shifted to EF by the chemical environment or by compression. The large Coulomb correlation energies, U, which prevent the formation of 4f bands in these elements, are directly obtained from the authors' spectra, and are found to be in good agreement with recent calculations. The linewidths and singularity indices of the XPS and BIS 4f lines are determined and discussed.
388 citations
TL;DR: In this paper, an exact formula for the short-circuit photovoltaic current in a pure crystal in terms of its Bloch states and energy bands was derived, and the authors showed that the anomalous bulk PV effect can occur even in pure non-pyroelectric piezoelectric crystals, e.g., Te and GaP.
Abstract: A theory is presented for the intrinisic anomalous bulk photovoltaic effect observed in noncentrosymmetric crystals, e.g., BaTi${\mathrm{O}}_{3}$. An exact formula is derived for the calculation of the short-circuit photovoltaic current in a pure crystal in terms of its Bloch states and energy bands. Unlike a conventional field or diffusion current, the photovoltaic current is essentially determined by the change of wave functions upon photoexcitation of an electron from the valence to the conduction band. Our theory also reveals that the bulk photovoltaic effect can occur even in pure nonpyroelectric piezoelectric crystals, e.g., Te and GaP, which have no polar axis and therefore no a priori direction for the photovoltaic current.
308 citations
306 citations
TL;DR: In this paper, the ionic fraction in the reflected beam when an atom or ion is scattered from a metal surface at finite temperature is considered, and three regimes are distinguished: low temperature, high temperature and low temperature.
256 citations
TL;DR: In this paper, a perturbational approach to relativistic calculations on molecules has been applied to a number of compounds containing heavy elements and the results show that the method is a viable alternative to more involved treatments.
Abstract: A recently proposed perturbational approach to relativistic calculations on molecules has been applied to a number of compounds containing heavy elements. Comparison of calculated spectroscopic constants with experiment and results from other theoretical models shows that the method is a viable alternative to more involved treatments. Relativistic corrections are essential for compounds containing heavy elements and result in a contraction of bond distances and a substantial change in bond energies. The perturbational approach lends itself readily to a straightforward interpretation of both effects. It appears that the bond contraction is not due to the relativistic contraction of valence atomic orbitals. It is a direct relativistic effect, the repulsion due to the rise in kinetic energy when bonds are shortened being diminished by the mass–velocity correction.
240 citations
TL;DR: In this paper, the authors derived the asymmetry parameters necessary for determining the differential cross-sections of randomly-oriented atoms within the one-electron, central-potential model and the dipole approximation for all subshells of C, O, Al, Si, S, Ni, Cu, Ga, Ge, As, Se, In, Sb, Cs, Ba, Ce, Ta, W, Pt, Au, and Pb.
233 citations
TL;DR: In this article, the selfconsistent electronic structures of Si, Ge, and zinc-blende GaP, GaAs, ZnS, and ZnSe have been determined using the linear combination of Gaussian orbitals method with a local-density form of the exchange-correlation functional.
Abstract: The self-consistent electronic structures of Si, Ge, and zinc-blende GaP, GaAs, ZnS, and ZnSe have been determined using the linear combination of Gaussian orbitals method with a local-density form of the exchange-correlation functional. A completely general form of the spatial dependence of the potential has been used to describe accurately the bonding character in the tetrahedral environment. Results are presented for the valence- and conduction-band energies, densities of states, effective masses, and charge densities. Comparisons are made with previous calculations and with photoemission measurements. A striking result is that the local-density theory underestimates the optical band gaps by approximately 30% or more, although the general conduction-band topology is good. The theoretical valence-band energies, charge densities, and electron and hole effective masses are also in good agreement with experiment. The energies and wave functions presented here are used to determine the optical properties of these materials in the following paper.
216 citations
TL;DR: X-ray edge absorption of copper and extended fine structure studies of both copper and iron centers have been made of cytochrome oxidase from beef heart, Paracoccus dentrificans, and HB-8 thermophilic bacteria to afford the basis for an oxygen reduction mechanism involving oxy- and peroxy intermediates.
TL;DR: The physical basis for valence molecular connectivity was studied and a new way of estimating valence state electronegativity is proposed from a count of exterjacent electrons divided by the quantum number squared for at least the first three quantum levels.
TL;DR: The interpretation of molecular valence Auger spectra by means of ab initio computational methods is discussed in this article, where the feasibility of a procedure based on single ionization potentials and of CI calculations using frozen orbitals is tested.
Abstract: The interpretation of molecular valence Auger spectra by means of ab initio computational methods is discussed. As alternatives to self‐consistent‐field optimizations of the full Auger spectrum, the feasibility of a procedure based on single ionization potentials and of CI calculations using frozen orbitals is tested. Numerical calculations are performed for the CO, N2, NO, and CO2 molecules, which show especially structure‐rich Auger spectra. These spectra are analyzed in detail and they are found to contain three nonoverlapping regions of transitions corresponding to final state vacancies in outer–outer, outer–inner, and inner–inner valence orbitals. It is investigated whether this division of the transitions also is relevant with respect to the magnitude of dynamical relaxation errors in the single ionization potential procedure or with respect to the choice of molecular orbital basis in the CI calculations. CI effects are found to be important for intensities and energies of the Auger transitions in t...
TL;DR: In this paper, the acceptor-doped BaTiO3 with acceptors (Cr to Ni) was determined gravimetrically as a function of the O2 partial pressure during and after annealing at 700° to 1300°C.
Abstract: The oxygen vacancy concentration of BaTiO3 doped with acceptors (Cr to Ni) is determined gravimetrically as a function of the O2 partial pressure during and after annealing at 700° to 1300°C. The oxygen vacancy concentration of these materials is larger than that of undoped and donor-doped BaTiO3. The oxygen vacancies are doubly ionized and they compensate the acceptors of lower valence. Both the vacancy concentration and the valence of the acceptor dopants depend on the annealing conditions. The electronic energy levels of the acceptors within the BaTiO3 band gap are derived from the gravimetric measurements. The electrical properties of the acceptor-doped ceramics are favorable for base-metal-electrode multilayer capacitors, which require sintering in reducing atmospheres.
TL;DR: In the case of the reduced halides of the lanthanoids as discussed by the authors, the metal-metal bonds are linked together by metal clusters of the same type, which can be seen as a way of connecting the two types of structures.
Abstract: The chemistry of metals in low valence states is marked by the frequent occurrence of metal clusters, which are easily recognizable when they occur as molecular units. Many metal-rich compounds of transition metals with p-elements (3rd to the 6th main groups) are closely related to the corresponding halides, since they are built up from metal clusters of the same type. The clusters are however, linked together (condensed) by metal-metal bonds. This principle of construction holds particularly well in the case of the novel reduced halides of the lanthanoids.
TL;DR: An analysis of the geometries of one hundred O-H⋯O hydrogen bonds observed by neutron diffraction in 24 crystal structures showed the following results as discussed by the authors : Twenty-five of the hydrogen bonds can be described as bifurcated, indicating that this form of association is more common than previously supposed.
TL;DR: In this paper, the results are discussed referring to the Ca 1− x Sr x FeO 3 system with 0.1 ≦ y ≦ 0.6 and the extreme values are close to those expected for Fe 3+ and Fe 5+.
TL;DR: In this paper, a model for intermediate-valence compounds is considered that is based on correlated ionic states with ${n}_{0}$ and${n}{0}+1 f$ electrons and includes the hybridization with band electrons as a perturbation.
Abstract: A model for intermediate-valence compounds is considered that is based on correlated ionic states with ${n}_{0}$ and ${n}_{0}+1 f$ electrons and includes the hybridization with band electrons as a perturbation. The expansion is formulated diagrammatically with Goldstone diagrams for the strongly correlated on-site processes and Feynman propagators between different sites. Suitable infinite-order resummations of the on-site processes lead to Brillouin-Wigner-type self-consistency equations for the $f$ quasiparticle energy shifts. The imaginary part of the single-site $f$-level Green's function exhibits a spike above a continuum which vanishes at the Fermi level. Deviations from the Ruderman-Kittel-Kasuya-Yosida behavior are found for the intersite terms. The effect of the shift of the chemical potential due to the fixed number of electrons is discussed.
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01 Jan 1981
TL;DR: In this article, the valence and core-electron spectra have been investigated for a series of alloys by means of ESCA, and a subtraction method is applied to derive the contribution of the low-concentration components to the spectra.
Abstract: The valence- and core-electron spectra have been investigated for a series of ${\mathrm{Cu}}_{x}{\mathrm{Pd}}_{1\ensuremath{-}x}$ alloys by means of ESCA (electron spectroscopy for chemical analysis). In the dilute limits a subtraction method is applied to derive the contribution of the low-concentration components to the valence-electron spectra. The Pd core-line asymmetry scales with the local palladium density of states at the Fermi level, and it is heavily reduced in the dilute palladium limit. For the Pd ${M}_{45}{N}_{45}{N}_{45}$ Auger spectrum a bandlike contribution is identified in addition to the dominating quasiatomic part. Special attention is paid to the chemical shifts of the Pd and Cu core levels as a function of alloy concentration. From these and the two-hole Auger final-state energy shifts various thermochemical data are derived and compared with experiment. In general, a good agreement is obtained, suggesting that ESCA calorimetry might become a very rapid and useful method for thermochemical investigations.
TL;DR: In this paper, the results of studies on a new and a very interesting intermediate valence (IV) material EuPd2Si2, which exhibits some unique features not observed so far in any Eu-based IV system.
Abstract: Reports the results of studies on a new and a very interesting intermediate valence (IV) material EuPd2Si2, which exhibits some unique features not observed so far in any Eu-based IV system. The magnetic susceptibility exhibits a very clear maximum and the isomer shift shows an unusually large temperature dependence in the interval 120-200K.
TL;DR: In this article, the experimental core level shifts in terms of chemical, relaxation and Fermi level shifts, from a combination of core level Auger and photoelectron energies, are described.
TL;DR: In this paper, the dependence of the properties of small copper clusters on their size and their relationships to the physical properties of the bulk metal have been studied through ab initio SCF calculations (with the emphasis on the clusters Cu8 and Cu13).
Abstract: The dependence of the properties of small copper clusters on their size, and their relationships to the properties of the bulk metal have been studied through ab initio SCF calculations (with the emphasis on the clusters Cu8 and Cu13). The basis set used is of double‐zeta quality for the valence shells 3d and 4s. The analysis focuses on the following properties of the clusters: geometrical structure, binding energy, and distribution of energy levels. For the 13‐atom cluster, the icosahedron is found more stable than the cubo‐octahedron corresponding to the fcc structure of the bulk metal. The binding energy per atom increases almost linearly with the number of atoms of the cluster. From the orbital energy values, the sets of 3d and 4s levels are well separated for Cu8 and just begin to overlap for Cu13. This situation looks rather different from the one for the bulk metal where the s band totally overlaps the d band. The relationship between the orbital energies from the ab initio SCF calculations and the...
TL;DR: In this article, the major differential valence correlation effects of the lowest lying states arising from the s2dn, sdn+1, and dn+2 configurations of the first row transition metal atoms have been characterized using MCSCF and CI procedures.
Abstract: The major differential valence correlation effects of the lowest lying states arising from the s2dn, sdn+1, and dn+2 configurations of the first‐row transition metal atoms have been characterized using MCSCF and CI procedures. The important correlation effects are found to be, first, angular correlation of the 4s2 pair arising because of the near degeneracy of the 4s and 4p orbitals and, second, radial correlation of the 3d electron pairs. This large differential radial correlation of the 3d electrons can be interpreted as being due to nonequivalent d orbitals in the sdn+1 and dn+2 excited states. Both of these effects can be incorporated into a simple MCSCF wave function that reduces the error in the excited state atomic dissociation limits (∼0.2 eV in Sc–Cr and ∼0.5 eV in Mn–Cu for the sdn+1–s2dn excitation energy), yet still is of a form which lends itself easily to molecular calculations.
TL;DR: In this article, Hartree-Fock calculations using an ab inito effective potential and a Gaussian basis set of triple zeta quality followed by extensive CI's have been carried out for several states of the copper atom and for the ground state of diatomic copper.
Abstract: Valence Hartree–Fock calculations using an ab inito effective potential and a Gaussian basis set of triple zeta quality followed by extensive CI’s have been carried out for several states of the copper atom and for the ground state of diatomic copper. Correlation effects are determined to obtain a satisfactory agreement with experimental data for Cu2(re = 2.25 vs 2.22 A; ωe = 265 vs 266 cm−1; De = 15 550 vs 16 500 cm−1). The most striking effect is the 0.1 A shortening of the bond length induced by the d correlation energy. Important basis set superposition errors are shown to be possible (especially in all electron calculations) and the use of six Gaussian primitives for the d orbitals was required to avoid them.
TL;DR: In this paper, the spin-orbit splitting of the valence bands of the uniaxial and cubic polytypes of 6H, 15R and 3C SiC was measured.
TL;DR: Oxygen potential-composition relations have been determined at temperatures from 1000 to 1200°C on mixed uraniaplutonia fuels containing 10 and 40 mol% plutonia as discussed by the authors.
TL;DR: In this paper, the optical and electron spin resonance (ESR) spectra of barium borate glasses, containing the oxides of V, Fe and Cu separately and in mixed proportions, have been studied.
Abstract: The optical and electron spin resonance (ESR) spectra of barium borate glasses, containing the oxides of V, Fe and Cu separately and in mixed proportions, have been studied. The optical spectra of the single transition metal (TM) oxide glasses showed the usual features, while those for the mixed glasses showed single bands without showing individual features of the single TM oxide glasses. However, the linear plots of optical density against composition revealed the presence of two valence states for each TM element, and this was confirmed by ESR results as well. The ESR spectra of the mixed glasses showed a complicated interaction pattern for two different TM ions, in comparison with those of the glasses containing a single TM ion. For the Fe-V glasses, the progressively vanishing hyperfine structure of the VO2+ complex with increasing addition of iron oxide is discussed in terms of nuclear spin relaxation, cross-relaxation between two spin systems and spin diffusion within the vanadium spin system. The covalency of the VO2+ complex and the number of distorted Fe3+ ions were found to decrease with increasing addition of Fe2O3 replacing V2O5. Similar features were noted for the Cu-V glasses; the spectra of Cu-Fe glass also showed a strong interaction between two different TM ions. It has been suggested that all the possible four valence states (for a given mixed glass) from two different TM elements are present, and that pairing of two different TM ions from two dissimilar TM elements occurs, facilitating the formation of “associates” (e.g. V4+-O-Fe3+).