Valence (chemistry)

About: Valence (chemistry) is a research topic. Over the lifetime, 24937 publications have been published within this topic receiving 645252 citations. The topic is also known as: valency.

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.

Covalent Bond in Crystals. I. Elements of a Structural Theory

Abstract: An a posteriori theory is developed for the structural energy of covalent crystals. The microscopic theory is based on ionic pseudopotentials and valence dielectric screening. The theory explains the difference between empirical pseudopotential form factors derived from the optical spectra of semiconductors and the metallic form factors calculated from free-ion term values by Animalu and Heine. A byproduct of the theory, which utilizes Penn's model isotropic semiconductor dielectric function, is a relation between the covalent bonding charge and the macroscopic dielectric constant. In self-consistent form the theory is an example of the "bootstrap" approach, applied here to treat the effect of covalent bonding on the ground-state energy of the valence electron gas. It is argued that the axiomatic character of the covalent theory is to be expected on symmetry grounds, and it is shown that the theory is superior to a nonlinear multiple-scattering theory based on the free-electron dielectric function. The extension of the theory to III---V and II---VI semiconductors is described briefly. The theory may be used to calculate elastic and macroscopic dielectric properties of covalent crystals starting only from ionic pseudopotential form factors.

Superconducting Solid Solution Alloys of the Transition Elements

Abstract: The solid solution alloys formed by the incomplete dshell metals in groups 4, 5, 6, and 7 were tested for superconductivity down to 1 deg K. For alloys formed between neigh boring elements in a given row of the periodic table, two transition temperature maxima are observed with valence numbers approximately equal to 4.7 and 6.4, respectively, the only exception being the first long period, in which the upper maximum is absent. Similar maxima occur when the constituent elements are selected from different rows of the periodic table, thus confirming the dominant role of the d-shell electrons. It is known that the normal density-ofstates function, N(O), passes through a series of maxima as the d-shell is filled up, two of these peaks lying at about the same composition as the two transition temperature peaks observed in the present work. The relationship of T/sub c/ to N(O) for the transition metal alloys is discussed. Transition temperature data are also presented for alloys composed of neighboring elements in a given column of the periodic tabular. In this case, the form of the relationship between T/sub c/ and electronic or lattice properties is still obscure. (auth)

Relativistic energy‐consistent pseudopotentials—Recent developments

Abstract: The direct adjustment of two-component pseudopotentials (scalar-relativistic + spin-orbit potentials), to atomic total energy valence spectra derived from four-component multiconfiguration Dirac–Hartree–Fock all-electron calculations based on the Dirac–Coulomb–Breit Hamiltonian, has been made a routine tool for an efficient treatment of heavy main-group elements. Both large-core (nsp valence shell) and small-core ((n − 1)spd nsp valence shell) potentials have been generated for all the post-d elements of groups 13–17. At the example of lead and bismuth compounds (PbHal, BiH, BiO, BiHal (Hal = F, Cl, Br, I)), we show how small-core and large-core potentials can be combined in accurate, yet computationally economic, spin-free-state-shifted relativistic electronic structure calculations of molecular ground and excited states. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 767–778, 2002