Showing papers on "Valence (chemistry) published in 1973"
TL;DR: In this article, it was shown that all ions with an isoelectronic core can be fitted by a single pair of parameters, R0 and N, that are independent of the ionic character of the bond and the coordination number of the cation.
Abstract: Bond-strength-bond-length relationships for bonds between oxygen and H+, Li+, Be2+, B3+, Na+, Mg2+, Al3+, Si4+, P5+, S6+, K+, Ca2+, Sc3+, Ti4+, V5+, Cr6+, Mn2+, Fe3+, Fe2+, Co2+, Cu2+, Zn2+, Ga3+, Ge4+ and As5+ have been derived by requiring that the sums of the bond strengths around the cations be equal to their valence in 417 crystals whose structures have been accurately determined. The relationship is of the form s = (R/R0)-N where s = bond strength, R = bond length and R0 and N are fitted constants. It is further shown that all ions with an isoelectronic core can be fitted by a single pair of parameters, R0 and N, that are independent of the ionic character of the bond and the coordination number of the cation. The resulting bond strengths have the property that they are directly related to the covalent character of the bond and that their sum around each atom is, on average, within about 5% of its valence. The bond-strength-bond-length curves are particularly useful in accounting for bonding in cases where the coordination is very distorted (e.g. Na+, Cu2+ and V5+). They can also be used to predict the positions of hydrogen atoms, to analyze for different oxidation states and site occupancies, to calculate ionic radii and to provide an indication of the correctness of crystal structure determinations.
1,170 citations
446 citations
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
TL;DR: In this article, a model based on tight-binding combinations of bonding hybrid orbitals is explored, which attempts to relate a wide range of properties to a few parameters of covalent and polar solids.
Abstract: A model electronic structure is explored which attempts to relate a wide range of properties to a few parameters of covalent and polar solids. The model is based upon tight-binding combinations of bonding hybrid orbitals. Of the many overlap matrix elements which enter, only three are retained; the three are associated with covalency, polarity, and metallicity. Many properties may be computed quite simply in terms of the parameters of the model, and measured values of the properties can then be used to determine the parameters. In this study the matrix elements associated with metallicity are obtained directly from the atomic-term values; those associated with covalency and polarity are obtained from the static dielectric constant using essentially the approach of Phillips, but in terms of the formula for the dielectric constant appropriate to this model. Also calculated in terms of the model were the valence energy bands themselves, obtained explicitly for silicon and for gallium arsenide. In treating other properties the unitarity of the final diagonalization was utilized to avoid carrying it out explicitly. The dipole moment of the individual bonds was defined and calculated as was an effective ionic charge and the macroscopic transverse charge. The cohesive energy was also obtained for ionic and metallic structures as well as for the covalent tetrahedral structures. Criteria for the stability of each structure were thereby obtained. The model also explains why some properties scale approximately linearly with the ionicity defined by Phillips.
197 citations
191 citations
TL;DR: In this article, the authors compared the valence-band spectra of Cu, Ni, and Ag, obtained using x-ray photoemission, with one-electron densities of states.
Abstract: Valence-band spectra of Cu, Ni, and Ag, obtained using x-ray photoemission, are compared with one-electron densities of states. Similar measurements on the CuNi and AgPd alloy systems are shown to be compatible with the predictions of the coherent-potential approximation.
174 citations
TL;DR: In this article, it was shown that WO 3 γ-Al 2 O 3 cannot be appreciably reduced in hydrogen at 550 °C, whereas WO3 SiO 2 is reduced to lower valence states and unsupported wO 3 is completely reduced to tungsten metal under these conditions.
171 citations
TL;DR: The present results strongly support the assignment of a low spin ferric structure to the iron ion in oxyhemoglobin.
167 citations
TL;DR: In this article, the authors derived selection rules for the direct optical valence-to-conduction band transitions in GaSe near the fundamental gap using pseudopotential band calculations.
Abstract: Absorption, and reflexion spectra of GaSe near the fundamental gap are interpreted in terms of pseudopotential band calculations. Selection rules for the direct optical valence-to-conduction band transitions are derived. Valence band mixing induced by spin-orbit coupling is invoked to explain the low observed probability for transitions in light polarized perpendicular to the crystal c-axis. The spectra of the excitons associated with the direct gap are discussed in the ellipsoidal effectivemass approximation. Corrective terms are added to account for the observed exchange splitting of the exciton ground state. Field-free spectra as well as spectra modified by the presence of magnetic fields parallel and perpendicular to c are considered. The magneto-Stark effect which gives rise to a mixing of the 2s and2py states and thus renders the2py state visible affords determination of the anisotropy parameter. The value of this parameter as well as those of the components parallel and perpendicular to c of the reduced effective masses show that the electronic states in GaSe are nearly isotropic. This is in good agreement with the results of the pseudopotential band calculations which clearly demonstrate the three-dimensional character of valence and conduction bands.
145 citations
TL;DR: In this paper, an experimental determination of the relative intensities of X-ray photoelectron lines corresponding to the inner levels of elements with Z ⩽ 20, and calculations of the total photo-ionization cross-sections for all shells of these elements with the Hartree-Fock-Slater potential are reported.
142 citations
TL;DR: In this article, core binding energies of a number of nitrogen bases and their adducts with BF3 are reported, and interpreted using both ab initio and semi-empirical molecular orbital calculations.
Abstract: Core binding energies of a number of nitrogen bases and their adducts with BF3 are reported, and interpreted using both ab initio and semi-empirical molecular orbital calculations. The binding energies are found to reflect the change in molecular charge distribution which occurs on formation of the B—N σ bond. The correlation of the valence molecular orbitals of the complexes with those of the bases and BF3 is discussed.
TL;DR: In this article, the experimental radii for all d and f transition metals are given, and the set of radii refers to twelve coordination and is based on the observed atomic volumes of the metals.
TL;DR: In this paper, the valence bands of crystalline and amorphous As, Sb, and Bi have been measured and compared with band-structure calculations for the crystalline materials.
Abstract: X-ray photoelectron spectra (XPS) of the valence bands of crystalline and amorphous As, Sb, and Bi have been measured The results are compared with band-structure calculations for the crystalline materials A splitting, $\ensuremath{\Delta}E$, of the "$s$-like" peak for the crystalline phase disappears in the amorphous phase and the "$p$-like" peak is shifted towards higher energies This similarity to tetrahedrally coordinated covalent semiconductors is explained by describing the semimetals as layers of distorted covalently bonded hexagonal rings This generalizes the "even-odd" ring effect previously observed in group-IV semiconductors to the $A7$ lattice of As, Sb, and Bi The relation of $\ensuremath{\Delta}E$ to the interatomic distance is described by a universal curve, which applies to the group-V semimetals as well as group-IV semiconductors
TL;DR: In this paper, a vibrational analysis has been carried out on cyclohexane, cycloencane-d12, cyclohenane-1, 1, 4, 4, 4, 5, 6, 8, 9, 10, 11, 12, cyclone-d13, cycloehexane-n12 and cyclonehexane d14 using the set of 60 internal coordinates compatible with the Snyder and Schachtschneider transferable valence force field for saturated hydrocarbons.
TL;DR: In this article, the valence band structure of representative MX2 layer structure compounds has been obtained by X-ray photoemission with monochromatized radiation, and the results were compared with u.v. and He II photo-emission and with recent band structure calculations.
TL;DR: In this article, the Fourier transform was used to measure the properties of As-Te 3/2 units, which are interconnected through As-As locks, and it was shown that these locks play a crucial role in the stability of the As-rich glasses in favoring strongly the covalent bonds.
Abstract: Diffraction measurements on bulk and thin films AsTe glasses are reported, throughout the whole range of stability. FNP distances and areas, average valence angles and compactness are determined from the Fourier transform. For As-rich glasses important differences are found between short-range order in both vitreous and crystalline states. An interpretation at the molecular scale is proposed. It takes account of the particular topological properties of the AsTe molecules, which are evidenced by hand-made models. The proposed structure consists of a network of AsTe 3/2 units which are interconnected through “AsAs locks”. It is shown that these “locks” play a crucial role in the stability of the As-rich glasses in favoring strongly the covalent bonds. On the other hand, in Te-rich glasses there is no similar locking mechanism. The presence of threefold-coordinated tellurium sites enhances the metallic character, and weakens the difference between crystalline and vitreous states. The structural models which are proposed enable us to explain the anomalies and the discontinuities of most thermodynamic properties which are described in paper I.
01 Jan 1973
TL;DR: In this article, the authors describe the situation by regarding the individual atoms in a molecule as spheres with different potentials, and by measuring these "chemical shifts" for individual atoms, a mapping can in principle be made of the charge or the potential distribution in the molecule.
Abstract: When atoms are brought close together to form molecules the orbitals of individual atoms are perturbed and replaced by molecular orbitals. Inner orbitals, i.e. with higher binding energies, may still be regarded as atomic and belonging to specified atoms within the molecule, whereas the external orbitals combine to form the valence level system of the entire molecule. These orbitals take a more or less active part in the chemical bonds which are formed between the atoms in the molecule and which specify the chemical properties. The chemical bonds affect the charge distribution so that the original neutral atoms can be regarded as charged to various degrees and with different signs with a net charge of zero for a neutral molecule. We may describe the situation by regarding the individual atoms in the molecule as spheres with different potentials. Inside each charged sphere the atomic potential, set up by the removal of a certain small charge from its surface to the neighbouring atoms taking part in the chemical bond, is constant according to classical electrostatics. The result of this atomic potential is to shift the whole inner level system of any atom by a small amount, each level being shifted an equal amount. Levels belonging to different atoms in the molecule are generally shifted differently, however, and by measuring these “chemical shifts” for individual atoms in the molecule a mapping can in principle be made of the charge or the potential distribution in the molecule.
TL;DR: In this article, high-resolution x-ray-photo-emission valence-band spectra (0-45 eV binding energy) of cleaved single-crystal PbS, PbSe, and PbTe are reported.
Abstract: High-resolution x-ray-photoemission valence-band spectra (0-45 eV binding energy) of cleaved single-crystal PbS, PbSe, and PbTe are reported. The spectra are compared with available band-theory results. Relativistic orthogonalized-plane-wave results exhibit the best over-all agreement with experiment. Empirical-pseudopotential-method (EPM) results show similar agreement for all but the most tightly bound valence band. The uppermost peak, corresponding to the three least tightly bound bands, shows detailed structure in good agreement with the EPM predictions. The PbTe valence-band spectrum can be synthesized from the x-ray-photoemission valence-band spectra of Pb and Te.
TL;DR: In this article, a normal coordinate analysis and a 34 parameter valence field has been computed by an iterative process for four isotopic species of propene in the gaseous and liquid states.
TL;DR: In this article, a metal is placed upon the surface of a semiconductor and the resulting changes in the electron interaction near the interface produce changes in exchange and correlation potential, and the most important aspect is that the potential changes differ with the state considered.
Abstract: For pt. I see abstr. A75119 of 1972. When a metal is placed upon the surface of a semiconductor the resulting changes in the electron interaction near the interface produce changes in the exchange and correlation potential. The most important aspect is that the potential changes differ with the state considered. In particular the reaction of the valence and conduction bands are very different causing changes in the band gap in the surface region. The image potential has opposite signs in the valence and conduction band corresponding to the semiclassical electron and hole picture; this is shown to be due to the screened exchange difference between the two bands. The screened exchange difference between the two bands is calculated numerically up to the surface for the two limiting cases of a covalent and ionic semiconductor. Finally the difference in behaviour of these two types is briefly discussed in the light of the results on Schottky barriers.
TL;DR: In this article, the empirical pseudopotential method is used to compute the band structure, density of states, and reflectivity of hexagonal zinc oxide, and the local approximation is assumed, on the basis of recent photoemission experiments which showed that the zinc core 3d band lies deep below the valence band.
Abstract: The empirical pseudopotential method is used to compute the band structure, density of states, and reflectivity of hexagonal zinc oxide. The local approximation is assumed, on the basis of recent photoemission experiments which showed that the zinc core 3d band lies deep below the valence band. The calculation succeeds in identifying the reflectivity peaks, but gives stronger amplitudes than are observed, and these identifications are shown to fit well into the systematics of the other three zinc chalcogenides. Reasonable agreement is also obtained with the experimental locations of the maxima in the density of states and with the large value of the valence bandwidth.
TL;DR: In this paper, the energy distribution curves of the photoelectrons emitted from naphthacene (C 18 H 12 ) crystal in the vacuum ultraviolet region were measured by an ac modulated retarding potential method.
TL;DR: In this paper, a comprehensive interpretation of the energy loss spectra is attempted with the use of photochemical and photoelectron data, as well as quantum chemical calculations in the literature.
Abstract: High resolution, inelastic electron scattering data can provide new spectroscopic information on the electronic structure of polyatomic molecules. Features in the acetone energy loss spectrum from 0 to 15 eV obtained for 100 eV incident electrons correspond to vibrational, electronic discrete, and electronic continuum excitations. These data are compared with optical measurements in a wide spectral region extending from the infrared to the vacuum ultraviolet. A comprehensive interpretation of the energy loss spectra is attempted with the use of photochemical and photoelectron data, as well as quantum‐chemical calculations in the literature. Three Rydberg series with quantum defects of 1.03, 0.81, and 0.315 join onto bands previously discussed in terms of transitions to valence orbitals. These series converge to an ionization limit of 9.705 eV in good agreement with previous optical determinations. Dissociative continua underlie the Rydberg region and give rise to a variety of neutral products observed in ...
TL;DR: In this paper, a review of the analysis of the energy band structure of I-III-VI 2 semiconductors is presented, where the presence of noble metal d levels in the valence band has been confirmed directly by the observation of electroreflectance structure due to transitions from the d levels themselves to the lowest conduction band minimum.
TL;DR: In this paper, Coulomb screening was used to analyze charge-flow and valence-d$-band behavior in the Au-Sn alloy system by x-ray photoemission measurement of the valence bands and of core-level energy shifts for both atomic species.
Abstract: Charge-flow and valence-$d$-band behavior in Au alloys have been studied in the Au-Sn alloy system by x-ray photoemission measurement of the valence bands and of core-level energy shifts for both atomic species. Published M\"ossbauer isomer-shift data for these alloys indicate that there is significant valence $s$ charge increase at Au sites and a somewhat smaller decrease at Sn sites upon alloying. An analysis, which includes screening in the Coulomb interactions between valence and core electrons and which combines the M\"ossbauer and core-level photoemission data, has been carried out. The results indicate that there is little net charge flow between Au and Sn, i.e., that the increase of non-$d$ conduction-electron count at Au sites is almost entirely compensated for by a decrease in $5d$ count; a similar $s\ensuremath{-}p$ compensation occurs at Sn sites. Coulomb screening is important to this analysis and we have applied it to similar results reported by us previously in the AuAg alloy system. Substantial hybridization of the Au $d$ bands with the Au "$s$" and with Sn valence-electron states is exhibited by the valence-band spectra; and the extent of this hybridization with final states above the Fermi level is indicated by the observed depletion in $d$ count.
TL;DR: He(I), He(II) and valence region X-ray photoelectron spectra are reported for the Group VIA hexacarbonyls and are assigned on the basis of published molecular orbital calculations.
Abstract: He(I), He(II) and valence region X-ray photoelectron spectra are reported for the Group VIA hexacarbonyls and are assigned on the basis of published molecular orbital calculations A pronounced increase in intensity of the first photoelectron band is observed on descending the group, and multiplet structure is clearly discernible on this band in the case of W(CO)6
TL;DR: In this article, the density of valence states of the II-VI compounds ZnTe, CdSe and CdTe were determined with far uv (h v = 21.2 and 40.8 eV) photoemission.
Abstract: The density of valence states of the II–VI compounds ZnTe, CdSe, CdTe, HgSe, and HgTe have been determined with far uv (h v = 21.2 and 40.8 eV) photoemission (UPS). Although the upper portions of the valence bands are found to have shapes in qualitative agreement with theory, they are consistently 1 to 2 eV wider than predicted. With the high resolution obtainable in uv photoemission, the shapes of the d-bands of the cation can be resolved to permit an accurate determination of their positions relative to the top of the valence band and their spin-orbit splittings. The “apparent” spin-orbit splittings of the uppermost d-orbitals of Zn and Cd are found to be 50% larger in the metallic form then in the compounds.
Die Dichte der Valenzzustande der II-VI-Verbindungen ZnTe, CdSe, CdTe, HgSe und HgTe wurden mittels W-Photoemission (UPS)(h v = 21,2 und 40,8 eV) bestimmt. Obwohl gefunden wird, das die oberen Teile der Valenzbander sich in qualitativer Ubereinstimmung mit der Theorie befinden, Bind sie einheitlich 1 bis 2 eV breiter als vorhergesagt. Mit der hohen Auflosung in der UV-Photoemission lassen sich die Form der d-Bander des Kations auflosen und erlauben eine genaue Bestimmung ihrer Lage relativ zum oberen Rand des Valenzbandes und ihrer Spin-Bahn-Aufspaltungen. Die „acheinbaren” Spin-Bahn-Aufspaltungen der hochsten d-Orbitale von Zn und Cd sind in der metallischen Form 50% groser als in den Verbindungen.
TL;DR: In this paper, the SCF Xα method and the transition-state procedure were used to calculate photoionization, X-ray emission and UV excitation energies for SiO 2 by the SC FXα method.
TL;DR: In this paper, an x-ray photoemission spectroscopy study of the rare earths La, Ce, Pr, and Nd was performed and it was shown that the spectra of these metals in the vicinity of the Fermi energy account for the $4f$ levels as well as for the valence states.
Abstract: We present an x-ray photoemission spectroscopy study of the rare earths La, Ce, Pr, and Nd. We show that the spectra of these metals in the vicinity of the Fermi energy account for the $4f$ levels as well as for the valence states. The energy location of the $4f$ electrons is in fairly good agreement with an existing computation performed in the framework of the renormalized-atom method.