Valence-electron concentration parameter

Two kinds of valence electron concentration parameters are used VECgiugfer and VECmetai- According to Chevrel Sergent (1986), the first parameter can be calculated by means of ... 

For example, consider the TIC and TiN pair. Their lattice parameters are 4.32 A, and 4.23 A, respectively the difference is only two percent. Together with their mutual solubility (Schwarzkopf and Kieffer, 1953) this suggests that they have the same number of bonding valence electrons, although atomic carbon has four valence electrons, and atomic nitrogen has five. The extra nitrogen electron must be in a non-bonding state. This contradicts the valence electron concentrations assumed by Jhi et al., 1999. 

In several compounds, the so-called Valence-Electron Concentration (VEC) proved to be a parameter relevant to their composition, structure and stability. 

The extent and type of solid solution formation depends on several parameters. If both component elements are isostructural and, in addition, similar in size, valence electron concentration, and electronegativity, a series of complete substitutional solid solutions may form across the diagram, as for V-Cr, Ni-Cu, Cu-Au, and Sb-Bi otherwise, limited terminal solid solutions form which are substitutional for elements with a solute-solvent size difference less than 15%, but may be interstitial for element pairs with size ratios of more than 20%, for example, for Fe(C) or Pb(Au) (here, the bracketed element is the solute). 

ABSTRACT. For compounds with tetrahedral structure or anionic tetrahedron complex two valence electron concentration rules can be formulated which correlate the number of available valence electrons with particular features of the crystal structure. These two rules are known as the tetrahedral structure equation where the total valence electron concentration, VEC, is used as parameter and the generalized 8 - N rule where the parameter of interest is the partial valence electron concentration in respect to the anion, VEC. From the tetrahedral structure equation one can calculate the average number of non-bonding orbitals per atom and, in the case of non-cyclic molecular tetrahedral structures, the number of atoms In the molecule. An application of the generalized 8 - N rule allows the derivation of the average number of anion -anion bonds per anion or the number of valence electrons which remain with the cation to be used for cation - cation bonds and/or lone electron pairs. These rules have been used not only to predict probable structural features of unknown compounds but also to point out possible errors in composition or structure of known compounds. 

Introducing as parameter the total valence electron concentration, VEC, defined as... 

The drawings are complemented with text blocks detailing the numerical values of the different parameters which can be calculated from the valence electron equations discussed above. On top is given the total valence electron concentration, VEC. If VEC < 4 a tetrahedral structure involving all atoms is impossible. The parameters listed below VEC are derived from the valence electron concentration of the charged anion partial structure, VEC, and the next one from the partial valence electron concentration in respect to the anion, VECa- parameter C AC, to be discussed in the next paragraph, refers to the sharing of the anions and can be calculated from the composition of the compound. Finally, on the last row one finds a classification code for the base tetrahedron, also to be discussed later on. 

For DBES data three main factors contribute to the S parameter in polymers (1) free-volume content, (2) free-volume size, and (3) chemical composition. First, larger free-volume content contributes to a larger S value. DBES measures radiation near 511 keV where a major contribution comes from p-Ps. This p-Ps contribution is only 1/3 the o-Ps intensity as that in I3 of PAL data. Second, when p-Ps is localized in a defect with a dimension fix, the momentum Ap has a dispersion according to the Heisenburg uncertainty principle AxAp > h/4n. The S parameter from DBES spectra is a direct measure of the quantity of momentum dispersion. In a larger size hole where Ps is localized, there will be a larger S parameter due to smaller momentum uncertainty. Therefore, in a system with defects or voids, such as polymers, the S parameter is a qualitative measure of the defect size and defect concentration. The value of the S parameter also depends on the momentum of the valence electrons, which annihilate with the positrons. The absolute value of the S parameter therefore, may differ from polymer to polymer. Third, the S parameter depends on the electron momentum of the elements. As the atomic number of the elements increases, the electron momentum increases, and thus the S parameter decreases. Fortunately, in chemicals of... 

A mathematical model of metal-semiconductor contacts has been employed to estimate the quantity of charge transferred through the interface, based on parameter values that pertain to the M/Ti02 system [88]. The direction of electron flux in a metal-semiconductor contact depends on the relative values of the work function of the two materials. The work function of the semiconductor is a function of the kind (valence) and concentration of the dopant and of temperature. Doping of... 

The general subject of metal-support interactions is discussed in detail in another chapter of this volume. In the present article we concentrate on interactions of the electronic type and their influence on chemisorptive and kinetic parameters. More specifically, we concentrate on electronic metal-support interactions induced by doping of semiconductive support materials with cations of valence lower or higher than that of the host cation. 

Of the three types of electron spectroscopy, ESCA has perhaps been the most widely used for chemical studies. For this reason, this chapter will concentrate most heavily on ESCA. Although simple quantitative chemical analysis can be performed by ESCA, this probably represents the least effective use of this powerful tool, which provides quantitative information about such basic parameters as binding energies, charges, valence states, etc., which involve the atom as a function of its chemical environment. 

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