Showing papers on "Valence (chemistry) published in 1983"

X-ray K-edge absorption spectra of Fe minerals and model compounds: Near-edge structure

Abstract: Synchrotron radiation has been used to collect high-resolution Fe K absorption near-edge spectra of a suite of Fe minerals and compounds having a range of Fe environments. These spectra, along with those of previous workers, indicate that the number, position, and intensity of near-edge features are characteristic of Fe valence and general site geometry. For example, the crest of the K-edge for Fe2+ in a six-coordinated site in the oxides studied is about 3 eV lower in energy than that for Fe3+ in a similar site. The K-edge crest for Fe3+ in a four-coordinated site is 1 to 2 eV lower than for Fe3+ in a regular site. The shape of the edge crest is sensitive to the details of first-neighbor bonding distances, tending to be broader in species with irregular Fe sites and varying in energy according to the average bond length. Comparison with Ca2+ and Zn2+ spectra from the literature is made and the applicability and utility of edge measurements discussed.

Cluster expansion of the wavefunction, valence and rydberg excitations, ionizations, and inner-valence ionizations of CO2 and N2O studied by the sac and sac CI theories

Abstract: Valence and Rydberg excitations, ionizations, and inner-valence ionizations of CO 2 and N 2 O, which are isoelectronic, are studied by the SAC and SAC CI theories. We have given a systematic assignment of the electronic spectra of these molecules, though there were some controversial situations in the assignment of the spectra. The broad and overlapping features of the spectra of the inner-valence ionizations are due to the existence of a large number of ionization-excitation states mixing with the singly ionized states. The theoretical origins of the similarities and differences in the photoelectron spectra of CO 2 and N 2 O are clarified.

Investigation of TiO2SiO2 glasses by X-ray absorption spectroscopy

Abstract: The coordination and nearest-neighbor bond distances of Ti in a series of TiO2SiO2 glasses have been quantitatively determined using a combination of XANES and EXAFS measurements about the Ti K-edge at 4966 eV. The glasses covering the range 0.012 to 14.7 wt% TiO2 were prepared by flame hydrolysis of predetermined mixtures of SiCl4 and TiCl4 vapors. At TiO2 concentrations below ∼0.05 wt%, Ti is found in a rutile-like octahedral coordination. With increased TiO2 content in the glass, a two-site model applies, in which Ti is found predominately in a fourfold site. About ∼9 wt% TiO2, t6he sixfold/fourfold ratio increases appreciably and eventually at ∼15 wt% TiO2, crystalline TiO2 segregates out as a second phase. The average TiOSi bond angle in these glasses was estimated to be ∼159 ° which is slightly larger than the most probable value of 152 degrees for SiOSi in vitreous SiO2. Within the accuracy of the edge shift measurements all Ti in the glass is in 4+ valence. Finally, various physical properties such as density, optical transparency and thermal expansion are correlated in light of the new structural data for this interesting binary silicate glass system.

Electronic structure of Ni and Pd alloys. I. X-ray photoelectron spectroscopy of the valence bands

Abstract: X-ray photoelectron (XPS) spectra of the valence bands of approximately 60 Ni and Pd alloys and intermetallic compounds with 20 different elements are presented. In alloys with electropositive elements the Ni and Pd $d$-band centroids move to larger binding energies and the density of $d$ states at the Fermi level (${E}_{F}$) is greatly decreased, indicating that the Ni and Pd $d$ bands are being filled. The Ni and Pd $d$ bands become narrower in alloys with electropositive elements. It is shown that in such alloys, the Ni-$M$ or Pd-$M$ interactions give a significant contribution to the Ni and Pd bandwidths. This contribution is larger when the second element $M$ has a large density of states at the energy of the Ni or Pd $d$ bands in the alloy. As this contribution is small in systems that form complicated structural types and glasses we speculate that its absence helps stabilize such structures. The satellite at \ensuremath{\sim}6 eV in the XPS spectrum of Ni is found to weaken and shifts to higher binding energy in alloys with electropositive metals. In the twelve alloys where its intensity is large enough to allow us to identify its energy, the binding energy of the satellite agrees with the two-electron binding energy of the Ni $^{1}G {d}^{8}$ term derived from Auger spectroscopy. The site- and symmetry-selected densities-of-states curves were calculated for 14 of the alloys to show that this filling is largely due to changes in the hybridization of the Ni or Pd $d$ bands with the partner element bands. The actual transfer of Ni and Pd $d$ electrons is probably small. These observations are used to rationalize published results of specific-heat and magnetic measurements on such alloys.

Magneto-optical investigations of a novel superlattice: HgTe-CdTe

Abstract: Far-infrared magnetoabsorption experiments done in a HgTe-CdTe superlattice are presented. From the results, which are interpreted in terms of interband transitions from valence to conduction subbands, the superlattice band structure has been deduced. These investigations show, in particular, that this superlattice is a quasi zero-energy-gap semiconductor, and yield the first determination of the offset between the HgTe and CdTe valence bands.

Glass formation and properties of chalcogenide systems XXVI: Permittivity and the structure of glasses AsxSe1−x and GexSe1−x

Abstract: Correlation between the real part of the dielectric constant and the structure of glasses in the system AsxSe1−x and GexSe1−x is reported. The mole polarization is calculated using the Sellmeyer approximation neglecting the Lorentz field. The vibrationally caused part of the permittivity which is obtained by subtraction of the mole refraction reflects ordered states in the investigated series. Besides the known crystalline compounds As2Se3 and As4Se4 the formation of the vitreous AsSe3 and As3Se2 from the liquid state has to be supposed. In the system GexSe1−x the formation of the compounds GeSe2 and Ge2Se3 is completed by GeSe4 which as Ge2Se3 obviously only exists in the non-crystalline state. GeTe4 has been reported as a metastable crystalline phase. The temperature dependence of ϵr of vitreous As2Se3 is tentatively interpreted in terms of the dipole orientation caused by conversion of the charge in valence alternation pairs.

Electronic structure of α-alumina and its defect states

Abstract: We have investigated the electronic structure of $\ensuremath{\alpha}$-alumina and its defect states by using a semiempirical method. Our results show that the fundamental gap is indirect, and the charge transfer enhances the hybridization of aluminum and oxygen orbitals, which in turn leads to the broad upper and lower valence bands. Using the calculated total and orbital density of states, we have provided a consistent interpretation of the existing spectra. For the ideal $\ensuremath{\alpha}$-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$(0001) surface we have found that the Al---O bonds at the surface have a considerable covalent character and the Al---dangling bonds produce the major surface-state band. Energy positions of the oxygen-vacancy states, and of the surface states, strongly suggest that the oxygen deficiency is responsible for some of the lowenergy structures in the excitation spectra.

Ground-state diatomic potentials

Abstract: Extended-Rydberg potentials have been derived for the ground states of all diatomic molecules obtained from atoms in the periodic list Li to Cl for which accurate spectroscopic data are available. From a comparison with RKR data where these are known we conclude that this simple function is an excellent representation of the potential in the valence region.

Electron transfer between magnetic ions in mixed valence binuclear systems

Abstract: This paper describes how the Hubbard model in the atomic limit implemented by taking into account molecular vibrations can give a description of mixed valence binuclear systems with both metallic ions simultaneously magnetic. A FeIII (high spin) FeII (high spin) binuclear complex would constitute an example. In such systems electron transfer and electron exchange are expected. If the compound belongs to class II (Robin and Day classification) we find that the activation energy of the thermal electron transfer and the intensity of the intervalence band are spin dependent but that the energies of spin states are given by the Heisenberg Hamiltonian with a new expression for the exchange parameter. For class III binuclear complexes a quite different behavior is found. The energy of the intervalence band is spin dependent but energies of spin states are no longer given by the Heisenberg Hamiltonian.

Electron delocalization in mixed-valence tungsten polyanions

Abstract: Etude du transfert d'electrons dans les heteropolysels contenant du tungstene a partir des variations des profils de raies en fonction de la temperature, dans les spectres RPE de ces composes

Cu and Ag as one‐valence‐electron atoms: Pseudopotential results for Cu2, Ag2, CuH, AgH, and the corresponding cations

Abstract: Semiempirical pseudopotentials, representing singly ionized X+ cores, have been adjusted for the Cu and Ag atoms. Polarization of the X+ cores (including core‐valence correlation) is explicitly taken into account. Deviations of (unpolarized) core interactions, X+–X+ and X+–H+, from point‐charge behavior are treated at the SCF level. Valence correlation is included within the framework of the spin‐density functional formalism. Results are given (i) for atomic dipole polarizabilities and electron affinities, (ii) for bond lengths and dissociation and ionization energies of the diatomics XH m+ and X m+ 2 (X=Cu, Ag; m=0, 1). The accuracy of our results for the dimers is 0.1 eV for binding energies and 0.05 A for bond lengths. In contrast to the situation for the alkali dimers, Cu+ 2 and Ag+ 2 prove to be less strongly bound than the corresponding neutral dimers.

Electron densities and chemical bonding in TiC, TiN, and TiO derived from energy band calculations

Abstract: The chemical bonding in the interesting class of refractory transition metal compounds is illustrated for TiC, TiN, and TiO. Self-consistent augmented plane wave (APW) calculations are already available for these compounds. Using the respective potentials we have repeated the band calculations on a finer k grid with the linearized APW method to obtain accurate densities of states (DOS). These DOS can be divided into local partial contributions to characterize the bonding. Further information can be obtained from a decomposition of the metal dDOS into t2g and eg symmetry components. These partial local DOS are compared with the LCAO counterpart and give a first picture of the chemical bonding in these compounds. The electron densities corresponding to the occupied valence states are obtained from the LAPW calculations. They provide further insight into characteristic trends in the series from TiC to TiO: around the nonmetal site the density shows increasing localization; around the metal site the deviation from spherical symmetry changes from eg to t2g. These effects can be traced back to the three types of valence bands. Electron density plots of characteristic band states (all energies of a selected k point in the Brillouin zone) will be shown. These plots can describe the different types of bonding occurring in these systems.

Intermediate valence state of cerium in CeNi

Abstract: Lattice parameter analysis and studies of thermal expansion show that the orthorhombic CeNi compound is, like CeSn 3 , an intermediate valence compound in which the cerium valence state varies from 3.5 to 3.3 between 4 and 300 K. This compound behaves as an enhanced Pauli paramagnet in which the magnetic susceptibility along the c axis passes through a maximum at around 140 K. Magnetic susceptibility, resistivity and heat capacity measurements are characteristic of an almost magnetic Fermi liquid in which spin fluctuations are present. Unlike the other cerium intermediate valence compounds which are generally cubic, large anisotropic effects due to the local surroundings are observed in CeNi because of its orthorhombic symmetry. All these properties can be understood in the light of the band structure of this type of alloy in which a large hybridization occurs between the Ni 3d electrons and the Ce 5d electrons.

Polyhedral skeletal electron pair approach. A generalised principle for condensed polyhedra

Abstract: A general procedure for evaluating the number of cluster valence molecular orbitals in molecules with condensed polyhedral geometries has been derived from molecular orbital calculations and shown to be widely applicable.

Inclusion of core-valence correlation effects in pseudopotential calculations. II. K2 and KH lowest Σ+ potential curves from valence-correlated wavefunctions

Abstract: For pt.I see J. Chem. Phys., vol.77, p.3571 (1982). The recently proposed perturbative treatment of core-valence correlation effects is extended to valence correlated wavefunctions. As expected, the valence correlation slightly diminishes the core-valence correlation for the ground state. This effect is illustrated for the atomic electron affinity of K. The core-valence correlation plays a very important role in the spectroscopy of K2 and KH, where it increases the lowest X1 Sigma + to A1 Sigma + transition energies by 2300 and 1100 cm-1, respectively.

Theoretical aspects of the photochemistry of methanol, methylamine, and related materials

Abstract: The photochemistries of methanol and methylamine are computationally rationalized using ab initio methods. It is shown that the lowest excited singlet states of these and related materials are n,3s Rydberg in character. These states are computationally shown to evolve adiabatically to the valence ground states of the various radical products along the NH, CN, CO, and OH bond rupture pathways in methylamine and methanol, respectively. The NH and CN n,3s bond rupture surfaces display minima in the region of the Franck-Condon excitation geometry. The NH bond ruptures in n,3s singlet ammonia and methylamine are shown to be identical in having small activation energies. The CN excited state bond rupture shows a much larger activation energy, indicating that trialkylamines should display some photostability in the region of the 0-0 transition. In methanol, neither CO nor OH excited-state bond rupture coordinates show minima. The observed preference for OH bond rupture in the UV photochemistry of methanol is rationalized as resulting from the lighter mass of the H atom as well as the computed more repulsive nature of the OH bond rupture. In methanol, both 1,2- and 1,1-H/sub 2/ molecular elimination excited-state pathways are examined. 1,2-H/sub 2/ elimination is found to have amore » small activation energy while the 1,1-elimination is difficult. The concept of de-Rydbergization is fully developed in order to rationalize the change in electronic character occurring along these various excited state pathways.« less

An ab initio study of core–valence correlation

Abstract: Especially in the cases of the first two columns of the periodic table, the inclusion of core-valence correlation in ab initio CI calculations yields a contraction of the atomic valence shell and improves both calculated atomic ionization potentials and atomic energy separations. For the alkali dimers Na2, K2, and Rb2, the presently calculated bond lengths are in excellent agreement with experiments when core-valence is included. In addition, the valence dissociation energies are accurate.