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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.


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Journal ArticleDOI
TL;DR: In this article, the authors show that SnTe can be optimized to be a high performance thermoelectric material for power generation by controlling the hole concentration and significantly improving the Seebeck coefficient.
Abstract: SnTe, a lead-free rock-salt analogue of PbTe, having valence band structure similar to PbTe, recently has attracted attention for thermoelectric heat to electricity generation. However, pristine SnTe is a poor thermoelectric material because of very high hole concentration resulting from intrinsic Sn vacancies, which give rise to low Seebeck coefficient and high electrical thermal conductivity. In this report, we show that SnTe can be optimized to be a high performance thermoelectric material for power generation by controlling the hole concentration and significantly improving the Seebeck coefficient. Mg (2–10 mol %) alloying in SnTe modulates its electronic band structure by increasing the band gap of SnTe and results in decrease in the energy separation between its light and heavy hole valence bands. Thus, solid solution alloying with Mg enhances the contribution of the heavy hole valence band, leading to significant improvement in the Seebeck coefficient in Mg alloyed SnTe, which in turn results in re...

364 citations

Journal ArticleDOI
TL;DR: In this paper, the dielectric properties of A2+B6+O4 (A2+: Ca, Pb, Ba; B6+: Mo, W) ceramics were investigated as a function of packing fraction and bond valence.
Abstract: Microwave dielectric properties of A2+B6+O4 (A2+: Ca, Pb, Ba; B6+: Mo, W) ceramics were investigated as a function of packing fraction and bond valence. For A2+B6+O4 specimens sintered at 800–1100 °C for 3 h, a single phase with a tetragonal scheelite structure was detected, and the theoretical density was higher than 93% throughout the composition. Although the ionic polarizability of Ba2+ ion was larger than that of Ca2+ ion, the dielectric constant (K) of BaB6+O4 showed a smaller value than that of CaB6+O4. These results could be attributed to changes of the packing fraction due to the effective ionic size. The Q·f value was largely dependent on the packing fraction, as well as the percentages of theoretical density. The temperature coefficients of the resonant frequencies (TCFs) of the specimens were affected by the bond valence of oxygen. The specimens of CaMoO4 sintered at 1000 °C for 3 h showed the K of 10.8, Q·f of 76,990 GHz and TCF of −22.8 ppm/°C, respectively.

362 citations

Journal ArticleDOI
Martin C. Gutzwiller1
TL;DR: In this paper, the wave function for the electrons is investigated when a set of narrow bands (valence states) has its energies within a wide band (conduction states), where the valence states are linear combinations of localized states which are attached to each lattice site.
Abstract: The wave function for the electrons is investigated when a set of narrow bands (valence states) has its energies within a wide band (conduction states). The valence states are linear combinations of localized states which are attached to each lattice site. The intra-atomic Coulomb and exchange integrals for the localized states are much larger than the bandwidths of the valence states. Some of the narrow bands are neither completely empty nor completely filled. The wave function is therefore expected to be correlated, because it is disadvantageous for the electrons to crowd into the same lattice site, or take up some configuration contrary to Hund's rule. This correlation is important in trasition metals, where it is considered to be the cause of ferromagnetism. The correlated wave function is obtained by applying to the uncorrelated antisymmetrized product of Bloch functions an operator which provides each configuration of localized valence states with an appropriate amplitude and phase factor. The procedure is worked out in detail for the case of few particles (electrons or holes) in the narrow bands with the help of a diagram analysis. The localized orbits of different lattice sites do not have to be orthogonal to on another, and the computational rules are actually simplified thereby. The example of a twofold degenerate band such as the upper part of the $3d$ band in Ni is treated, and the conditions for the occurrence of ferromagnetism are stated in the case of few $3d$ holes per lattice site.

362 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used a 6 μm diameter liquid-jet free vacuum surface at the MBI undulator beamline of the synchrotron radiation facility BESSY to study the valence band photoelectron spectra of liquid water.
Abstract: The valence band photoelectron spectra of liquid water (H2O and D2O) are studied in the photon energy range from hν = 60 to 120 eV. The experiments use a 6 μm diameter liquid-jet free vacuum surface at the MBI undulator beamline of the synchrotron radiation facility BESSY. Photoelectron emission from all four valence molecular orbitals (MOs) is observed. In comparison to those of the gas phase, the peaks are significantly broadened and shifted to lower binding energies by about 1.5 eV. This is attributed primarily to the electronic polarization of the solvent molecules around an ionized water molecule. Energy shifts, peak broadening, and relative peak intensities for the four MOs differ because of their specific participation in the hydrogen bonding in liquid water. Relative photoionization cross sections for MOs were measured for hν = 60, 80, and 100 eV. The main difference for liquid water, as compared to the gas phase, is the relative intensity decrease of the 1b2 and 3a1 orbitals, reflecting changes o...

359 citations

Journal ArticleDOI
J. A. Van Vechten1
TL;DR: In this paper, a two-band model was proposed to describe and predict the ionization potentials and electronic interband gaps of binary compounds and their alloys, and a revised method of calculating the excess heat of mixing of a substitutional alloy was presented.
Abstract: Electronegativity difference was redefined in Paper I of this series as a scaling parameter which combines the concepts of valence and size differences. A procedure has been developed for its evaluation in terms of a two-band model. In Paper II of this series it was shown that this model describes and predicts the ionization potentials and electronic interband gaps of binary ${A}^{N}{B}^{8\ensuremath{-}N}$ compounds and their alloys. Here the energy of this model semiconducting-insulating solid is evaluated relative to a free-electron gas, i.e., an idealized metal, as a function of composition, pressure, and temperature. Using this highly simplified scaling approach, we obtain suprisingly accurate predictions for the heat of fusion, melting point, and pressure-temperature phase diagrams of these materials. A revised method of calculating the excess heat of mixing of a substitutional alloy is presented. This calculation is extended to the case of an arbitrary dilute impurity in an arbitrary semiconducting host; the distribution coefficient at the melting point of the host is obtained.

358 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
20231,521
20222,997
2021616
2020611
2019584
2018577