Showing papers on "Valence (chemistry) published in 1989"

Superconductivity produced by electron doping in CuO 2 -layered compounds

Abstract: We have discovered that the Ce4+doping and subsequent annealing in reducing atmosphere give rise to 24-K superconductivity in the Nd2CuO4-type structure with sheets of Cu-O squares. In contrast to the previously reported high-Tccuprates, the charge carriers in the new superconductors are doped electrons, not holes; this was confirmed by the measurements of Hall and Seebeck coefficients as well as by chemical analysis of the effective copper valence. An anomalous dependence ofTcon the concentration of doped electrons is shown for these electron-doped superconducting cuprates.

Inorganic Crystal Structures

Abstract: Part 1: cubic-close-packed arrays (usually of anions) projected along the diagonal of an octahedron - a four-fold axis hexagonal-close-packed arrays (usually of anions) projected along the three-fold axis normal to the close-packed layers hexagonal-close-packed arrays projected along an axis parallel to a CP plane but normal to a CP row in that plane hexagonal-close-packed arrays projected along a CP row of atoms. Part 2: projection along the shortest axis of the FCC projection along the shortest axis of mixed (H+C) stackings primitive cubic arrays and derived structures primitive hexagonal arrays and some derived and related structures the stereochemistry of Valence and lone pair electrons topological transformations noncommensurate Vernier or Nonius structures structures that can be related to BCC or PC packing alloy structures with edge-capped tetrahedra, octahedra, square antiprisms, stellae quadrangulae and/or tetraedersterns silicate structures.

Calculation of bond energies in compounds of heavy elements by a quasi-relativistic approach

Abstract: A quasi-relativistic method, in which the valence density is optimized with respect to the first-order relativistic Hamiltonian, has been evaluated by calculations on systems containing heavy elements including third-row transition metals and actinides. The method adopts the statistical energy expression and employs in addition the frozen core approximation. The quasi-relativistic method has been applied in calculations on atomic orbital energies for the valence shells of heavy elements. It is concluded from these calculations that the quasi-relativistic scheme affords results in better accord with the fully relativistic Dirac-Slater method than the first-order relativistic method based on perturbation theory. Calculations on the M-X bond energies in MX{sub 4} (M = Th, U; X = F, Cl, Br, I) as well as the M-R bond energies in Cl{sub 3}MR (M = Th, U; R = H, CH{sub 3}) revealed in addition that bond energies based on the quasi-relativistic method (QR) were in better agreement with experimental data than bond energies based on the first-order perturbation theory (FO). The absolute mean derivations with respect to experimental values were 6.9 and 16.5 kcal mol{sup {minus}1} for QR and FO, respectively, the case of the MX{sub 4} systems. It is concluded that the quasi-relativistic method, inmore » which changes in the electron density induced by relativity ({Delta}{rho}{sup R}) are approximately taken into account in the energy expression, should be used for compounds containing actinides. Both QR and FO (in which contributions from {Delta}{rho}{sup R} to the total energy are absent, even though they are present in the orbital energies) are appropriate for elements up to Z = 80, although QR represents a slight improvement for the elements in the third transition series.« less

New negative differential resistance device based on resonant interband tunneling

Abstract: We propose and demonstrate a novel negative differential resistance device based on resonant interband tunneling. Electrons in the InAs/AlSb/GaSb/AlSb/InAs structure tunnel from the InAs conduction band into a quantized state in the GaSb valence band, giving rise to a peak in the current-voltage characteristic. This heterostructure design virtually eliminates many of the competing transport mechanisms which limit the performance of conventional double-barrier structures. Peak-to-valley current ratios as high as 20 and 88 are observed at room temperature and liquid-nitrogen temperature, respectively. These are the highest values reported for any tunnel structure.

Mixed metal–silicon clusters formed by chemical reaction in a supersonic molecular beam: Implications for reactions at the metal/silicon interface

Abstract: We report observation of a reaction between a metal atom and silicon in a supersonic jet to form metal atom–silicon clusters. Using the technique of laser vaporization supersonic expansion with metal carbonyl seeded carrier gas, clusters of the form MSin have been detected by ArF and F2 laser photoionization time‐of‐flight mass spectrometry. Three group‐VIB transition metals and copper have been investigated. The dominant product cluster peaks observed in the mass spectra obtained for all three group VIB metals corresponds to identical but remarkable cluster stoichiometries. The dominant product peaks have formulas given by MSin where n=16. Copper results are different than the other three metals, indicating the importance of the metal valence electronic structure to the chemistry. The metal–semiconductor clusters are relatively more stable towards photofragmentation than the bare silicon cluster of the same size. The observation of these new species may be relevant to reactions which occur at the interfa...

Studies of copper valence states with Cu L 3 x-ray-absorption spectroscopy

Abstract: We have used x-ray-absorption spectroscopy to study a series of compounds in which Cu assumes a formal valence between 0 and 3. We find that the shape, the threshold energy, and the intensity of the Cu ${L}_{3}$ absorption edge is strongly influenced by the chemical state of the Cu atoms. We use the Cu 2p x-ray-absorption spectra of a large number of Cu compounds, including sulfides, oxides, La-Sr-Cu-O compounds, a phthalocyanine complex, and various minerals to show that the presence of a strong 2p-3d excitonic transition is a fingerprint of the Cu(${d}^{9}$) contribution to the ground state. A simple ionic picture is generally inadequate to describe these compounds.

K-shell photoabsorption of the N2 molecule.

Abstract: The {ital K}-shell photoabsorption spectrum of the N{sub 2} molecule, recorded with unprecedented energy resolution and statistical accuracy, reveals several new Rydberg transitions and double-excitation states. The nearly identical term values, vibrational frequencies and internuclear separations of the 1{ital s}-excited states of N{sub 2} and of the 2{pi}-excited states of NO, allow for a complete peak assignment for the N{sub 2} Rydberg series and strongly support both the core-hole localization picture and equivalent-core model. Subtle differences are related to the final-state charge distribution in the vicinity of the nuclei and to the bonding character of the valence orbitals.

Electrochemical and Spectroscopic Evidence on the Participation of Quadrivalent Nickel in the Nickel Hydroxide Redox Reaction

Abstract: Electrochemical and iodometric measurements on anodically charged thin film nickel hydroxide electrodes indicated an average nickel valence of . In situ visible spectroscopy of nickel hydroxide deposited on optically transparent electrodes featured one strong and broad absorption band with an intensity proportional to the state of charge. The results are not consistent with the presence of separate trivalent and quadrivalent phases or the participation of peroxide radicals in the nickel hydroxide redox reaction, but are best explained by the formation of a single mixed valence phase such as which has a 3.67 average nickel valence. Full utilization of the proposed oxidized phase could increase the discharge capacity of nickel battery electrodes by 67% beyond the previously assumed one‐electron theoretical capacity. That the discharge is generally incomplete may be due to electronic isolation of portions of the oxidized material by a high resistivity layer of divalent nickel hydroxide produced at the surface during discharge.

The binding energies of one and two water molecules to the first transition‐row metal positive ions. II

Abstract: The bonding of water to the transition metal positive ions is electrostatic in origin. The electrostatic bonding is enhanced by a variety of mechanisms: mixing in 4p character, 4s–3d hybridization, and 4s promotion into the compact 3d orbital. The importance of these effects varies between the different metal ions due to changes in the separation of the metal ion atomic states. Furthermore the changes in the metal–water repulsion when a second water is added also changes the relative importance of the different metal asymptotes. The second water binding energy varies from being 11 kcal/mol smaller than the first for Mn+ to 3 kcal/mol larger for V+ and Fe+.

Correlation of molecular valence- and K-shell photoionization resonances with bond lengths

Abstract: The relationship between the interatomic distance and the positions of valence-shell and K-shell sigma(asterisk) photoionization resonances is investigated theoretically for the molecules C2, F2, N2, O2, CO, NO, C2H2, C2H4, C2H6, HCN, H2CO, N20, CO2, and C2N2. The results of molecular-orbital computations are presented in three-dimensional diagrams, which are shown to be similar to the wave functions of a particle in a cylindrical well, confirming the validity of free-electron molecular-orbital (FEMO) approximations for modeling the potential along the symmetry axis. FEMO orbital energies and resonance positions are found to be in good agreement with previous theoretical and experimental results. Also included is a Feshbach-Fano analysis of the relevance of virtual-valence orbitals to the appearance of single-channel resonances in molecular photoionization cross sections.

Valence- and conduction-band densities of states for tetrahedral semiconductors: Theory and experiment

Abstract: The theoretical and experimental electronic densities of states for both the valence and conduction bands are presented for the tetrahedral semiconductors Si, Ge, GaAs, and ZnSe. The theoretical densities of states were calculated with the empirical pseudopotential method and extend earlier pseudopotential work to 20 eV above the valence-band maximum. X-ray photoemission and inverse-photoemission results make it possible to compare critical-point features in the band structure with experimental structures.

Potential-induced variations in properties for carbon monoxide adsorbed on a platinum electrode

Abstract: A brief summary of the various theoretical models to explain the potential-induced vibrational frequency shifts of molecules adsorbed on electrode surfaces is presented Possible implications of studies involving immersed electrodes for enhancing this understanding are discussed The results of the variations of CO properties as a function of potential on a platinum electrode are given, based on the atom superposition and electron delocalization molecular orbital theory, the valence band shift model, and a large cluster model of Pt(111)

Possibility of a common origin to ferroelectricity and superconductivity in oxides.

Abstract: We describe an extension of a lattice dynamical model for displacive-type ferroelectrics which might be applied to high-temperature superconductors. The model is based on the instability of the oxygen ion O/sup 2-/, dynamically described by an on-site nonlinear electron-ion coupling, and on the valence instability of the cation (e.g., Cu(I), Cu(III)), represented by a nonlinear electron-electron interaction which provides a pairwise coupling of the electrons. The equivalent quantum-mechanical Hamiltonian describes an extended Peierls-Hubbard system. The most important outcome of the model is that a ferroelectric instability is a necessary condition for paired electron states which indicate the possibility of superconductivity. Depending on the strength of the coupling constants, either a ferroelectric or a superconducting state results.

The abinitio model potential method. First series transition metal elements

Abstract: In the a b i n i t i o core model potential method the Coulomb and exchange core operators are represented accurately and the valence basis set is the only component of the method which is optimized (following the variational principle) in an atomic valence restricted Hartree–Fock (RHF) calculation. In this paper we present the nonrelativistic Ar‐like and Mg‐like core model potentials and valence Gaussian basis sets for the first series transition metal elements. The pilot RHF molecular calculations on ScO, MnO, CuO, and ScS show that freezing the 3s orbital is safe all along the transition series but the same is true for the 3p orbital only towards the end of the series. A good agreement exists between the all‐electron and model potential results if the same type of valence part of the basis set is used: 0.01 A in R e and 25 cm− 1 in ν e .

Electronic structure of the quenched superconductivity materials Y1−xPrxBa2Cu3O7−δ☆

Abstract: The superconductivity of the Y1−xPrxBa2Cu3O7−δ system is quenched as x increases. It has been speculated that praseodymium has a valence of 4+, which results in extra charge in the Cu-O planes, and causes quenching of Tc. To study the electronic state of praseodymium, the valence band resonant photoemission of the Y1−xPrxBa2Cu3O7−δ system was measured for x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0. It was found that the praseodymium valence is close to 3+ for all x values and that the extracted Pr 4f spectral weight has a complex line shape. This line shape implies extensive Pr 4f-O 2p/Cu 3d hybridization which probably causes the quenching of Tc by disrupting the electronic or magnetic structure of the x = 0 material, perhaps via Pr-Cu and Pr-O superexchange interactions.

The prediction and interpretation of bond lengths in crystals

Abstract: The concept of bond valence and its correlation with bond length are reviewed. It is shown how to augment bond valence sums at an atom with other constraints so that bond lengths can be predicted for a given topology. These constraints can be read directly from the connectivity matrix for the structure. The physical reasons for the constraints are analysed, and this analysis leads in turn to a reformulation of Pauling's electrostatic valence sum rule. The apparent valences of atoms, calculated as a sum of bond valences derived from bond lengths, are often significantly different from the actual valences. It is demonstrated that such observations are often diagnostic of non-bonded repulsions. Limitations and possible extensions of the method are outlined.

Hopping transport of localized π electrons in amorphous carbon films

Abstract: The temperature dependence of the dc and ac conductivities has been studied in sputtered arnorphous carbon films. The dc conductivity is proportional to T" with n =15— 17 below room temperature. The mechanism is discussed in terms of the mobility-edge conduction, variable-range hopping, and small-polaron hopping. Quantitatively, these models could not interpret the observed behaviors. Instead, the multiphonon tunneling of localized electrons with weak electron-lattice coupling is suggested to be the dominant transport mechanism. This may be attributed to the ~-bonded nature of sputtered amorphous carbon. The ac conductivity is strongly correlated to the dc conductivity, which has never been explained by the current theories based on the pair approximation. Alternatively, the continuous-time random-walk approximation is shown to be a useful approach. I. INTRODUCTION It is believed that amorphous carbon (a-C) prepared by evaporation or sputtering is predominantly sp bonded with an optical gap of 0.4— 0.7 eV. ' At a carbon sp site, there are three strong o. bonds and one weak m bond lying normal to the o. bonding plane. The ~ states will form both the valence- and conduction-band states. ' The sp sites must be clustered together in aromatic units. The large size of such clusters could produce localized states near the Fermi level E+. The eFect of disorder in a ~-electron system is therefore of particular interest. The present authors have shown that the temperature dependence of the dc conductivity in sputtered a-C is empirically described by o. d, =o.oT", with n =15— 17, which may suggest that the nonpolaronic multiphonon tunneling of localized m electrons (weak coupling with lattice) dominates charge transport. Measurement of ac conductivity could provide direct information about the hopping rate of localized electrons. In the present study, the temperature dependences of both dc and ac conductivities are reported. Possible transport mechanisms, mobility-edge conduction, variable-range hopping (single-phonon process), and multiphonon hopping with strongly (small-polaron) and weakly coupled states, are discussed. It is suggested that the conventional mobility-edge conduction or small-polaron conduction could not explain the overall features of transport in a-C. It is also found that the ac conductivity is strongly correlated with the dc conductivity. This feature, which has never been explained by the current theory based on the pair approximation, is interpreted well in terms of the continuous-time random-walk (CTRW) approximation.

Stable, Two-Coordinate, Open Shell (d0—d9) Transition Metal Complexes

Abstract: Well characterized, stable, two-coordinate, open shell transition metal complexes are currently very rare species. The small number of known examples are limited to the first row metals Cr → Ni and the majority are derivatives of amide or modified amide ligands. A characteristic feature of the compounds is the small number of electrons in their valence shells, as low as eight in the case of the Cr complexes. The compounds are very reactive toward protic reagents and nucleophiles. However, this and many other aspects of their chemistry have not been well developed. Current data available are limited to structural (X-ray and electron diffraction) and some spectroscopic and magnetic studies.

Lattice dynamics of intermediate valence semiconductor SmB6

Abstract: All acoustic and the lowest optic branches in three main symmetry directions have been determined by inelastic neutron scattering at room temperature using a double isotope monocrystal 154Sm11B6. In comparison with the integer valence system LaB6 the phonons are softer, and the LA branches exhibit remarkable anomalies. These features are interpreted on the basis of a specific coupling of the lattice vibrations with the dipole (f-d) and monopole (f-f) excitations of the Sm shells, the latter being characteristic for the mixed valence semiconductor. A weak broad signal has been observed in the energy gap between acoustic and optic modes, which can be ascribed to an additional mode appearing due to the nonadiabatic interaction of phonons with valence fluctuations.

The novel mixed-valence, exchange-coupled, class III dimer [L2Fe2(µ-OH)3]2+(L =N,N′,N″-trimethyl-1,4,7-triazacyclononane)

Abstract: The reaction of Fe(CIO4)2·6H2O with N,N′,N,″-trimethyl-1,4,7-triazacyclononane (L) in methanol affords, in the presence of a small amount of oxygen, the deep blue binuclear complex [L2Fe2(µ-OH)3](CIO4)2·2MeOH·2H2O which was characterized by EXAFS, e.s.r., u.v.-visible and Mossbauer spectroscopy to be a mixed valence iron (II/III) species of class III with an Stot= 9/2 ground state.

Fermi energy dependence of surface desorption and diffusion of hydrogen in a-Si:H

Abstract: Using hydrogen evolution and SIMS profiling, the surface desorption energy ΔG and the diffusion energy ED of hydrogen (deuterium) in a-Si:H was determined as a function of the Fermi level EF Both ΔG and ED show a maximum for EF somewhat above midgap and decrease towards the valence and conduction bands The effects are explained by a Fermi energy dependent SiH bond rupture energy