Showing papers on "Valence (chemistry) published in 1970"
TL;DR: In this paper, a model for determining the density of states of pure stoichiometric NiO is proposed, taking into account the free-ion energy levels, and taking the Madelung potential, screening and covalency effects, crystalline-field stabilizations, and overlap effects.
Abstract: The electrical and optical properties of materials which are characterized by narrow bands in the vicinity of the Fermi energy are discussed. For such materials, electronic correlations and the electron-phonon coupling must be considered explicitly. Correlations in $f$ bands and in extremely narrow $d$ bands can be handled in the ionic limit of the Hubbard Hamiltonian. It is shown that free carriers in such bands form small polarons which contribute to conduction only by means of thermally activated hopping. Wider bands may also exist near the Fermi energy. Carriers in these bands may form large polarons and give a bandlike contribution to conductivity. A model is proposed for determining the density of states of pure stoichiometric crystals, beginning with the free-ion energy levels, and taking into account the Madelung potential, screening and covalency effects, crystalline-field stabilizations, and overlap effects. Exciton states are considered explicitly. The Franck-Condon principle necessitates the construction of different densities of states for electrical conductivity and optical absorption. Because of the bulk of experimental data presently available, the model is applied primarily to NiO. The many-particle density of states of pure stoichiometric NiO is calculated and is shown to be in agreement with the available experimental data. When impurities are present or nonstoichiometry exists, additional transitions must be discussed from first principles. The case of Li-doped NiO is discussed in detail. The calculations are consistent with the large mass of experimental information on this material. It is concluded that the predominant mechanism for conduction between 200 and 1000 \ifmmode^\circ\else\textdegree\fi{}K is the transport of hole-like large polarons in the oxygen $2p$ band. A method for representing the many-particle density of states on an effective one-electron diagram is discussed. It is shown that if correlations are important, donor or acceptor levels cannot be drawn as localized levels in the energy gap when multiple conduction or valence bands are present. This result comes about because extrinsic ionization energies of two correlated bands differ by an energy which bears no simple relation to the difference in energies of the intrinsic excitations, which are conventionally used to determine the relative positions of the bands.
738 citations
TL;DR: In this article, the authors used X-ray photoelectron spectroscopy (XPS) to measure splittings of metal-atom electron binding energies in both inorganic solids and gases.
Abstract: X-ray photoelectron spectroscopy (XPS) is used to measure splittings of metal-atom electron binding energies in both inorganic solids and gases. These splittings are due to the various possible multiplet states formed by coupling a hole in a metal-atom subshell to an unfilled valence subshell. Splittings are observed in various solids containing $3d$-series atoms. In particular, the $3s$ binding energy is split into a doublet with as much as 7.0-eV separation between the two components. The instrumental resolution is \ensuremath{\sim} 1.0 eV. $3s$ splittings are exhibited by inorganic compounds containing Mn and Fe, as well as by Fe metal, Co metal, and Ni metal. Theoretical predictions are in good agreement with experiment, provided that the effects of covalency in chemical bonding are taken into account. For Fe metal, the $3s$ splitting is identical both above and below the Curie point. The $3p$ binding energies of these solids also appear to show multiplet effects, but the interpretation of these results is less straight-forward. The $2p$ binding energies in Mn${\mathrm{F}}_{2}$ are broadened by at least 1.3 eV, and this is shown to be consistent with multiplet splitting. XPS results for gaseous monatomic Eu also indicate the presence of multiplet splittings. The two components in the $4d$ photoelectron spectrum are found to have an intensity ratio in disagreement with observed ratios for neighboring atoms with filled valence subshells. Also, the width of the $4f$ photoelectron peak above the instrumental contribution can be explained in terms of multiplet effects.
302 citations
196 citations
TL;DR: In this article, the effect of the spin exchange interaction between electron and hole is investigated for the case of excitons originating from one of the valence bands and an $s$-like conduction band, as is the case for IIb-VIb compounds.
Abstract: The effect of the spin-exchange interaction between electron and hole is investigated for the case of excitons originating from one of the $p$-like valence bands and an $s$-like conduction band, as is the case for IIb-VIb compounds. A general exciton matrix is constructed, starting from the work of Pikus. It includes spin-orbit, crystal-field, spin-exchange, and deformation-potential interactions. Use of this matrix then allows a theoretical fit to our experimental data which describes the shift of exciton levels under uniaxial pressure in ZnO, CdS, and CdSe. This fit results in the determination of six deformation potentials, two spin-orbit parameters, the crystal-field parameter, and the exchange parameter. The general theory, when adapted to the zinc-blende structure, allows us to fit our data on cubic ZnS and ZnSe, resulting in a determination of two deformation potentials and the spin-exchange parameter for each compound.
178 citations
TL;DR: In this article, the core lines and valence bands of LiF, BeO, BN and graphite have been studied by the ESCA technique and the energy differences between inner levels and the valence band are compared with X-ray transition energies.
Abstract: The core lines and valence bands of LiF, BeO, BN and graphite have been studied by the ESCA technique. The energy differences between inner levels and valence bands are compared with X-ray transition energies. The changes in binding energy for the Bels level when going from metal to oxide and fluoride are compared with X-ray spectroscopic data and with a study of the disintegration constant in electron capture of 7Be in the same compounds.
169 citations
TL;DR: In this paper, the authors discuss the method of optimized valence configurations, which is reasonable application of the multi-configuration self-consistent field technique to the quantitative description of chemical bonding.
Abstract: Publisher Summary This chapter discusses the method of optimized valence configurations, which is reasonable application of the multi-configuration self-consistent field technique to the quantitative description of chemical bonding. It is the purpose of this chapter to review and bring up to date the conceptual features, the analysis, and results obtained by the application of the method of Optimized Valence Configurations to diatomic molecules. Several significant conclusions can be drawn from the experience obtained in developing this method, and in assessing its relationship to other schemes. (1) It is possible to quantitatively separate the “molecular” aspects of the changing correlation energy of two approaching atoms from the remaining correlation. (2) The number of significant configurations representing extra molecular correlation is small and readily obtainable by a sequence of limited multi-configurational self-consistent-field computations, followed by a single configuration interaction involving all new orbitals thus obtained while those configurations representing atomic correlation are indeed numerous, but are easily accounted for by a suitable perturbation technique. (3) An excellent initial guess for excited starting orbitals can be obtained by maximization of the exchange integral between the orbital of the Hartree-Fock configuration being correlated and the excited orbital.
114 citations
TL;DR: It is shown that the superconducting transition temperature for compounds in seven different structure types exhibits a peak at about 3.7 to 3.9 valence electrons per atom.
Abstract: It is shown that the superconducting transition temperature for compounds in seven different structure types exhibits a peak at about 3.7 to 3.9 valence electrons per atom.
109 citations
TL;DR: In this article, a minimal basis set of Slater-type orbitals was used, and holding the atomic 1s orbitals doubly occupied, full configuration interaction calculations were carried out for all molecular states at no fewer than nine internuclear separations.
Abstract: Precise quantum mechanical calculations have been performed on the 72 states of carbon monoxide which dissociate to a 3P, 1D, 1S, or 5S carbon atom plus a 3P, 1D, or 1S oxygen atom. A minimal basis set of Slater‐type orbitals (optimized for the C and O atoms) was used, and holding the atomic 1s orbitals doubly occupied, full configuration interaction calculations were carried out for all molecular states at no fewer than nine internuclear separations. Seventeen bound states (De ≥ 0.27 eV) were obtained, eight of which have been observed experimentally. The theoretical ordering of known bound states agrees with experiment except for the a 3Π and A 1Π states. This fact is rationalized by noting that these two states have much smaller re values than do the other excited states. The most interesting of the unobserved predicted bound states are the 5Σ+ and 5Π states, which dissociate to 3PC+3PO, and the third 1Π state, with a relatively large calculated dissociation energy. A dominant molecular orbital configu...
102 citations
97 citations
94 citations
TL;DR: In this paper, two types of structure stability fields were constructed using effective ionic radii, which can be used to predict the radii of certain cations and the effects of high pressure on structure.
TL;DR: In this paper, a series of valence levels of the molecules CH and NH+ are estimated semi-empirically from corresponding atomic data, and a low-lying 4Σ− state in CH is calculated to lie 7500 cm−1 above the X 2Π state.
Abstract: Ab initio calculations (LCAO–MO–SCF) are performed on a series of valence levels of the molecules CH and NH+. Correlation energies are estimated semiempirically from corresponding atomic data. Close agreement with experiment is found for known states for a series of molecular properties such as equilibrium internuclear distances, vibration frequencies, term values, and dissociation energies. A low‐lying 4Σ− state in CH is calculated to lie 7500 cm−1 above the X 2Π state. No observable quartet transition could be found for CH, while for NH+ a 4Π–4Σ− transition should occur in the region of 1000 A. Other qualitative differences in the observed spectra of the two molecules are discussed. Finally a value of D00(NH+) = 3.4 eV is calculated.
TL;DR: In this article, it was shown that household bleach (sodium hypochlorite) generates ruthenium tetroxide from its lower valence states, which can be used as a source of acidity.
Abstract: Household bleach (sodium hypochlorite) generates ruthenium tetroxide from its lower valence states.
TL;DR: In this paper, the authors present a theoretical model which describes the phenomenon and then calculate this shift and broadening by using a perturbation theory for the adsorption of cesium on tungsten and of lithium on rhenium.
Abstract: The interaction of a metallic surface with an alkali atom causes both a shift and a broadening of the valence level of the atom. We first present the theoretical model which describes the phenomenon and then calculate this shift and broadening by using a perturbation theory. The results are computed for the adsorption of cesium on tungsten and of lithium on rhenium.
IBM1
TL;DR: In this paper, all-electrons calculations of the band structure of an ideal polyethylene chain are presented, and the valence bands are shown to be fairly independent of the choice of basis sets while the conduction states are very sensitive to the size of the basis sets.
TL;DR: Isomer shift and electric quadrupole splitting of the 77 keV γ-rays of Au197 were investigated for a large number of Au (I and Au (III) compounds at 4.2 °K by nuclear γresonance Spectroscopy as mentioned in this paper.
Abstract: Isomer shift (IS) and electric quadrupole splitting (QS) of the 77 keV γ-rays of Au197 were investigated for a large number of Au (I) and Au (III) compounds at 4.2 °K by nuclear γ-resonance Spectroscopy. A close correlation between the observed isomer shifts and the spectrochemical series of the ligands was observed. For each oxidation state, isomer shift and electric quadrupole splitting show approximately a linear relationship. On the basis of LCAO-MO theory, the experimental results are interpreted by covalency effects in the molecular orbitals, synergic coupling of σ- and π-bonds, and the empirically known donor and acceptor properties of the ligands.
TL;DR: In this paper, the bonding in permanganate and chromate ions is described by means of self-consistent field molecular orbital calculations employing a basis of Slater type orbitals expanded in Gaussian type functions.
Abstract: The bonding in the permanganate and chromate ions is described by means of self-consistent field molecular orbital calculations employing a basis of Slater type orbitals expanded in Gaussian type functions. A new procedure for the solution of the self-consistent field equations is described and applied to the ions studied here. Excited state wavefunctions are calculated using configuration interaction considering all singly excited configurations involving all virtual and valence orbitals. The calculated transition energies and transition moments are compared with those from the experimental electronic spectra.
TL;DR: In this article, the authors used the penetration part of the two-electron coulomb integral to estimate the exchange energy of two hydrogen atoms in 2s, 2p or hybrid valence states.
Abstract: Several methods of estimating exchange energies have been tested for the interaction of two hydrogen atoms in 2s, 2p or hybrid valence states. The simplest relationship previously used, X = KS 2/R, does not give an accurate picture of the dependence on internuclear distance. A two-term expression X = S 2(AR -1 + BR -2) is considerably better but fails for the case of exchange between orthogonal orbitals (S = 0). An alternative expression in which C 0 - C is the penetration part of the two-electron coulomb integral, is just as accurate, contains no more constants and holds equally well for orthogonal and non-orthogonal orbitals. C 0 - C is a better measure of the net overlap of two orbitals than is the overlap integral S, because the latter is zero for orbitals which are orthogonal but nevertheless overlap. We have found no method of deducing the constants K, A or B other than by calculating the exchange energy exactly at one or two values of R.
TL;DR: In this paper, generalized x-ray scattering factors have been applied to several organic molecular crystals, in a simplified form, for point-charge analysis, including s−triazine, cyanuric acid, and uracil.
Abstract: Generalized x‐ray scattering factors, in a simplified form, have been applied to several organic molecular crystals. Atomic charge parameters for L‐shell scattering factors have been determined from x‐ray structure factors by the method of least squares. L‐shell scattering factors have been computed for products of SCF AO's and of standard molecular STO's (Slater‐type orbitals). Standard STO's are more satisfactory for point‐charge analysis than are SCF AO's. Bias from several sets of thermal parameters is rather small and does not alter general conclusions on valence structure. The experimental x‐ray charges are compared with Mulliken gross atomic populations from INDO and STO‐3G calculations. Agreement with theory is good for the molecular crystals of s‐triazine, cyanuric acid, and uracil. Within the point‐charge approximation, the experimental x‐ray dipole moment for uracil is 4.0 ± 1.3 D. The direction is at an angle 71° ± 12° from the N(1)–C(4) interatomic vector towards atom N(3).
01 Jan 1970
TL;DR: In this paper, the authors surveyed the Ti-Al-Sn-Zr system with emphasis on 1000°F creep strength and retention of ductility (stability) after creep exposure, and found that Ti-6Al-2Sn-4Zr-2Mo is the creep/density optimized stable alloy.
Abstract: The Ti-Al-Sn-Zr system is surveyed with emphasis on 1000°F creep strength and retention of ductility (“stability”) after creep exposure. “Stability” is assured if Ti-6Al-2Sn-2Zr is found to be the creep/density optimized stable alloy. The alloying behavior of Ti is consistent with a “metallic bonding valence” of four and an “alloy structure valence” of 1.5. Covalent bonding between Ti and Al is confirmed. This substantially precludes blocking Al embrittlement through dilution of alpha soluble additions of elements from groups IV B , V B and VI B . Ti-6Al-2Sn-4Zr-2Mo is shown to have good hot strength and creep resistance. Between about 850° and 1050°F, Ti-6Al-2Sn-4Zr-2Mo obeys a creep rate equation of the general form for small strains and strain rates.
TL;DR: In this paper, two crystal structures in the system TlTe have been analyzed and probabilistic spatial correlations of the valence and core electrons reveal sensible core electron interactions.
Abstract: Two crystal structures in the system TlTe have been analysed. Probable spatial correlations of the valence and core electrons reveal sensible core electron interactions.
TL;DR: In this paper, the authors measured the energy and shape of the Kα-doublet of Al in numerous compounds with a high resolution Johann-type spectrograph, using secondary excitation and photoelectric detection.
TL;DR: In this article, Hartree-Fock energies have been calculated for 333 valence states of atoms and ions of the second row of the periodic table corresponding to 1s22sm2pn (m=0.1; 0
Abstract: Hartree-Fock energies have been calculated for 333 valence states of atoms and ions of the second row of the periodic table corresponding to 1s22sm2pn (m=0.1; 0
TL;DR: In this article, the authors used spin tickling to determine the 14N chemical shift anisotropy (ΔσN) on C1H314NC partially oriented in a nematic liquid-crystal solvent, and the result is + 360 ± 73 ppm.
Abstract: H–{14N} spin tickling has been used to determine for the first time a 14N chemical‐shift anisotropy (ΔσN). The measurement was made on C1H314NC partially oriented in a nematic liquid‐crystal solvent, and the result is + 360 ± 73 ppm. The 14N nuclear quadrupole coupling constant was also measured and is + 272 ± 2 kHz, only 57% of the gas‐phase value. Using a simple valence model of CH3NC to supply the orbital parameters, Pople's chemical‐shift theory yields a value of ΔσN = + 675 ppm. e2qNQN / h was calculated to be + 880 kHz using the results of an all‐electron SCF calculation in a Townes–Dailey treatment modified to reflect expansion of the nitrogen atomic orbitals in the molecule. Considering the level of approximation used in both calculations, the agreement with experiment is satisfactory. The problem of knowing the molecular geometry of CH3NC in the nematic phase is discussed along with the related difficulty of determining a reliable value for the anisotropy in the NH indirect coupling tensor.
TL;DR: In this paper, a general correlation between valence state and superconductivity in covalent compounds was deduced, and it was shown that PbNbS3 and PbTaS3 are superconducting below 2.62° and 3.07°K respectively.
TL;DR: In this article, a new model is presented to explain both the magnetic susceptibility data and the Mossbauer results, and the observed isomer shift of −0.4 mm/sec lies approximately halfway between the values (∼−0.9 and ∼0.1 µm/sec−1 respectively) obtained for well characterized ionic di− and trivalent Sm compounds, and is not observed to vary between room temperature and 1.1°K.
Abstract: The unusual magnetic and electrical properties of the semiconductor SmB6 have been closely investigated recently. Primarily from magnetic susceptibility measurements, it has been hypothesized that the Sm valence changes from 3 + (4f5, 6H5/2) at high temperatures to 2 + (4f6, 7F0) at low temperatures. We have used the Mossbauer effect in 149Sm to make a direct determination of the valence state by means of the ``isomer shift,'' which measures the s‐electron density at the Sm nucleus. This density changes with the 4f configuration because of coulombic shielding effects. Unfortunately, the change between 4f5 and 4f6 is too small to resolve resonance lines from the two charge states. The observed isomer shift of −0.4 mm/sec lies approximately halfway between the values (∼−0.9 and ∼0. mm/sec−1 respectively) obtained for well characterized ionic di‐ and trivalent Sm compounds, and is not observed to vary between room temperature and 1.1°K. A new model is presented to explain both the magnetic susceptibility data and the Mossbauer results.
TL;DR: The density of occupied states in the valence bands of the following transition metals have been studied by ESCA: Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au as discussed by the authors.