Showing papers on "Valence (chemistry) published in 1980"

Preparation, structure, and magnetic properties of a dodecanuclear mixed-valence manganese carboxylate

Abstract: This new type of dodecanuclear crystalline complex was obtained by reaction of Mn 2÷ with MnO 4 in acetic and propionic acids. The reddish-black acetate complex has the formula [Mn12(CHaCOO)I6(H20)4OI2 ] .2CH 3COOH.4H20, established by chemical and singlecrystal X-ray diffraction methods. This complex is tetragonal, space group I~,, with a -- 17.319 (9), c = 12.388 (7)/t,, V = 3716 ,/k 3, Z = 2, M r = 2060.3, D c = 1.84, D,,, = 1.83 Mg m -a. The final R and R w were 0.045 and 0.034 for 1172 non-zero reflexions. The crystals are built up of [MnIE(CH3COO)16(H20)4012] molecules, waters of crystallization and disordered acetic acid molecules. In the dodecanuclear molecules, which have 4 ($4) crystallographic symmetry, the Mn atoms are linked by triply bridging oxo O atoms and by carboxylate bridges from acetate anions. The occurrence of a strong Jahn-Teller effect in Mn a+ ions differentiates the Mn 3+ and Mn 4+ ions. The interesting magnetic properties (the magnetic moment increases from 30-9 x 10 -24 J T -1 at 3-3 K to a maximum of 56.5 x 10 -24 J T -~ in the range 17-31 K and then decreases to 33.4 x 10 -24 J T -~ at 280K per Mn atom) may be interpreted in terms of the Mn-Mn distances and superexchange via bridge O atoms.

Core-level binding-energy shifts for the metallic elements

Abstract: A general treatment of core-level binding-energy shifts in metals relative to the free atom is introduced and applied to all elemental metals in the Periodic Table. The crucial ingredients of the theoretical description are (a) the assumption of a fully screened final state in the metallic case and (b) the ($Z+1$) approximation for the screening valence charge distribution around the core-ionized site. This core-ionized site is, furthermore, treated as an impurity in an otherwise perfect metal. The combination of the complete screening picture and the ($Z+1$) approximation makes it possible to introduce a Born-Haber cycle which connects the initial state with the final state of the core-ionization process. From this cycle it becomes evident that the main contributions to the core-level shift are the cohesive energy difference between the ($Z+1$) and $Z$ metal and an appropriate ionization energy of the ($Z+1$) atom (usually the first ionization potential). The appearance of the ionization potential in the shift originates from the assumption of a charge-neutral final state, while the contribution from the cohesive energies essentially describes the change of bonding properties between the initial and final state of the site. The calculated shifts show very good agreement with available experimental values (at present, for 19 elements). For the other elements we have made an effort to combine experimental ionization potentials with theoretical calculations in order to obtain accurate estimates of some of the atomic-core-level binding energies. Such energies together with measured metallic binding energies give "pseudoexperimental" shifts for many elements. Our calculated core-level shifts agree exceedingly well also with these data. For some of the transition elements the core-level shift shows a deviating behavior in comparison with that of neighboring elements. This is shown to be due to a difference in the atomic ground-state configuration, such as, for example, ${d}^{5}s$ in chromium relative to the ${d}^{n}{s}^{2}$ configuration in vanadium and manganese. When the core-level shift is referred to, the ${d}^{n}{s}^{2}$ (or ${d}^{n+1}s$) atomic configuration for all the elements in a transition series, a quite regular behavior of the shift is found. However, some structure can still be observed originating from a change of screening within the $d$ band from a bonding to an antibonding type as one proceeds through the series. For elements beyond the coin metals the screening of a core hole is performed by $p$ electrons, which provide a less effective screening mechanism than the $d$ electrons for the transition metals. The coin metals are intermediate cases, partly due to a dominating $s$-electron screening and partly due to $d$-electron bonding in the initial state. The effect of the electron-density redistribution between the free atom and the solid on the core-level shift is particularly striking in the case of the rare-earth elements Pr-Sm and Tb-Tm. Here the remarkable situation is that a deep core electron is less bound in the atom than in the solid. Also for the actinides the electronic redistribution upon condensation gives rise to pronounced effects on the core-level shifts. Further, it is shown that the measured $6{p}_{\frac{3}{2}}$ binding energy in metallic uranium provides a clear demonstration of the occupation of the $5f$ level in this metal. The present treatment of the core-level shift for bulk metallic atoms can easily be generalized to surface atoms. From an empirical relation for the surface energy a simple expression for the shift of the surface core-level relative to the bulk can be derived. For the earlier transition metals, it is found that the core electrons are more bound at the surface than in the bulk, while for the heavier ones the opposite situation exists. This change of sign of the surface shift depends on the bonding-antibonding division of the $d$ band. To illustrate how the present approach can be applied to alloy systems, a treatment of core-level shifts for rare-gas atoms implanted in noble metals is undertaken.

The valence of copper in sulphides and selenides: An X-ray photoelectron spectroscopy study

Abstract: A number of copper sulphides and selenides were studied using X-ray photoelectron spectroscopy. It was found that the copper in all the compounds is monovalent, while the average oxidation state of the chalcogen varies from —2 in Cu2S and Cu2Se, to − 3 2 in Cu3Se2, to −1 in CuS and CuSe and to built− 1 2 in CuS2 and CuSe2. The deficit of electrons relative to a closed-shell configuration is delocalized (holes in the S(3p) or Se(4p) valence band), leading to p-type metallic conduction. This is also found for the ternary compounds KCu4S3, CuV2S4 and TlCu2Se2. S2− and S− coexist in KCu4S3; the new compounds TlCu4S3 and KCu4Se3 are isostructural with KCu4S3.

A local Fermi liquid theory of intermediate valence systems

Abstract: A non-interacting local Fermi liquid theory applicable to Ce, Sm, Eu and Yb metallic intermediate valence materials is presented. The induced density of states of a given rare earth ion is taken as the Lorentzian appropriate for a rare earth ion impurity, for which the justification is tentatively discussed. The degeneracy of the resonance is that of the magnetic configuration of the ion. The results are applied to the f occupation, susceptibility and specific heat. A relationship existing in the literature between the zero temperature susceptibility and linear coefficient of specific heat follows exactly from the present very simple theory. The results are compared with a range of experimental data.

Interpretation of the Mössbauer Spectra of the High‐Potential Iron Protein from Chromatium

Abstract: The Mossbauer spectra of both reduced and oxidized high-potential iron protein (Hipip) from Chromatium have been analysed using computer fits to theoretical spectra derived from a spin Hamiltonian. Fits to spectra obtained over a range of temperatures between 4.2 and 195 K and in applied magnetic fields up to 10.0 T lead to a consistent set of hyperfine parameters. These results are interpreted in terms of a model of the four-iron four-sulphide active centre which is consistent with its electronic and magnetic properties in both redox states. In the model for the reduced centre all four iron atoms have essentially the same valence, intermediate between ferric and ferrous, with the spins being coupled antiferromagnetically to give the centre zero net spin. The oxidized centre has one less electron which at low temperatures appears to have come predominantly from one pair of iron atoms which thus become ferric with the other pair remaining substantially unchanged. It is clear from the Mossbauer hyperfine parameters obtained from the computer fits to the low-temperature spectra that a larger magnetic moment is associated with the ferric/ferrous pair of iron atoms than with the ferric pair of iron atoms. This also explains the g values with an average of greater than 2 which are observed in electron paramagnetic resonance (EPR) measurements. At higher temperatures the differences between the electron charge density at the different iron atoms in the oxidized centre appear to become smeared out.

Many-Body Effects in Valence and Core Photoionization of Molecules

Abstract: The spectral function of ionization is discussed for the whole energy range. It is found that different energy regions are likely to exhibit different types of many-body effects. For the ionization out of an outer valence orbital most of the intensity appears in one main line. The many-body effects explain the additional satellite lines and, in addition, can lead to an ordering of main lines which is different from the ordering determined from one-particle calculations. For the ionization out of an inner valence orbital the intensity may be distributed over several lines and in many cases it is not possible to identify any of these lines as the main line representing the orbital. Such a breakdown of the quasiparticle picture of ionization is stressed to be a common phenomenon. Ionization of core orbitals can usually be viewed within a quasiparticle picture, i.e., the process leads to a main line accompanied by satellite lines. In cases, however, where the creation of the core hole leads to a strong charge transfer, the shake-up energies can become small or even negative and the quasiparticle picture may break down. The origin of the above effects is discussed and typical examples are presented.

Electron impact study of the energy levels of trans‐1,3‐butadiene: II. Detailed analysis of valence and Rydberg transitions

Abstract: In this electron energy loss investigation of the electronic states of trans‐1,3‐butadiene, high energy resolution (25 meV) and a wide range of incident energies (9.5 to 49 eV) have been employed to locate and assign valence and Rydberg transitions in the typically congested 5–9 eV (2500 to 1400 A) region of the spectrum of a polyatomic molecule. Comparisons of the observed relative differential scattering cross sections (DCS) of various features in the lower energy (5.0 to 6.8 eV) region with the unambiguous assignments of these features from optical absorption and multiphoton ionization spectra allowed certain conclusions to be drawn regarding the typical DCS of Rydberg and valence singlet–singlet transitions. These ’’rules’’ have been applied to the congested 7–9 eV energy loss region of the butadiene spectrum. A diffuse process near 7.3 eV has been assigned as a forbidden valence transition, probably 1Ag←1Ag, on the basis of its relative DCS. The identification of the 7.07 and 8.00 eV transitions as successive members of the same Rydberg series is supported, and both transitions are demonstrated to contain considerable valence component.

Electronic structure of lanthanum perovskites with 3d transition elements

Abstract: The systematics of the electronic structure of $\mathrm{La}B{\mathrm{O}}_{3}$ perovskite oxides, where the $B$ element scans the $3d$ transition-metal series from Ti to Co, are examined. X-ray photoelectron spectra of valence bands and shallow core states are presented and compared with theoretical molecular cluster and free-ion multiplet models. Self-consistent ionic configurations are obtained from the embedded cluster calculations, which differ from the assumptions of crystal-field theory due to metal-oxygen covalency. The prospect for more rigorous many-electron models is discussed.

Surface mixed valence in Sm and SmB/sub 6/

Abstract: Surface-sensitive photoelectron measurements reveal bulk-to-surface shifts of the Sm 4f/sup 6/ level which imply inhomogeneous valence mixing on the surface of Sm and SmB/sub 6/. The surface valence fraction is estimated to be the same for both materials. The measurements take advantage of a large resonant enhancement of 4f electron emission due to 4d ..-->.. 4f photon absorption, and detailed spectra showing this phenomena are presented. It is shown that the 4d hole in the 4d ..-->.. 4f absorption process stabilizes the 4f state by approx. 4 --7 eV. Exposure of Sm films to oxygen is found to eliminate, rather than increase, the emission from the surface 4f/sup 6/ state, showing that the 4f/sup 6/ state does not arise from oxygen contamination. Observed variations in Sm film spectra are described, including the finding in some films of an unexplained photoemission peak 2.4 eV below the Fermi level. SmB/sub 6/ also displays a broad band of Auger emission when a boron 1s core hole is created, and this is ascribed to electrons in the boron 2p bonding band. Various trends in 4d and 4f binding energies for Sm and SmB/sub 6/ are pointed out and discussed.

Manganese hexacyanomanganate: Magnetic interactions via cyanide in a mixed valence Prussian blue type compound

Abstract: A Prussian blue type compound of the stoichiometric composition Mn(CN)3⋅xH2O (x=0.57) is formed as the final product of hydrolysis of Na3[Mn(CN)6] in perchloric acid. IR and visible spectra as well as structural and magnetic data show that it is a mixed valence compound consisting of sixfold N‐coordinated Mn(II) and sixfold C‐coordinated Mn(IV) in a cubic face‐centered lattice. The compound exhibits ferrimagnetism with a Curie temperature of 48.7 K. A molecular field treatment assuming two collinear spin sublattices provides an adequate description of the observed magnetic behavior. The calculation of exchange integrals, which was based on a Heisenberg model, reveals that the magnetic ordering is almost exclusively induced by an antiferromagnetic interaction between Mn(II) and Mn(IV) ions. This interaction is described in terms of a super exchange involving cyanide as bridging ligand.

Gallium Nitride Studied by Electron Spectroscopy

Abstract: The electron structure of hexagonal GaN has been investigated by means of ESCA. In particular the valence region has been studied. Comparisons with existing pseudopotential band structures are made. The value of an ionicity parameter for GaN is derived from the electron spectrum.

Ab initio effective valence shell Hamiltonian for the neutral and ionic valence states of N, O, F, Si, P, and S

Abstract: The effective valence shell Hamiltonian, Hv, which acts within a finite valence space and exactly describes all the valence state energies, is applied to several atomic systems. The n=2 (L shell) Hv of the first row atoms, N, O, and F and n=3 (M shell) s and p orbital Hv of the second row atoms, Si, P, and S, are evaluated through second order using STO 5s4p2d and 6s5p3d basis sets, respectively. The calculations are equivalent to a (perturbative) Bk approximation which incorporates all excited configurations and which chooses the primary (valence) space as all the valence K2(2s)m(2p)n and K2L6(3s)m(3p)n configurations, respectively. Using the calculated matrix elements of Hv, the energies of all the valence states of the neutrals and ions are simultaneously determined from a single ab initio calculation on only one charge state of each of these atomic systems. To understand the dependence of Hv on the choice of core and valence orbitals, several sets of orbitals, obtained within the same primitive orbita...

Solid-state structure and electronic properties of a mixed-valence two-dimensional metal, potassium copper sulfide (KCu4S3)

Abstract: Abstract : KCu4S3, prepared by the high temperature reaction of copper, potassium carbonate, and sulfur, crystallizes with a unique layered structure. Double layers of tetrahedrally - coordinated (by S) copper ions are separated by layers of potassium ions. Although formally a mixed-valence Cu(I,I,I,II) complex, all copper ions are crystallographically equivalent. Consistent with its Robin and Day Class III designation, KCu4S3 exhibits electrical conductivity characteristic of a metal. Room temperature compaction conductivities of ca. 4000/Ohm/cm increase to ca. 60000/Ohm/cm at 20K. The metallic nature of this two-dimensional material is supported by the temperature independent paramagnetism and the metallic reflectivity through the visible and near ultraviolet region of the spectrum. (Author)

Temperature dependence of the samarium oxidation state in SmB6 and Sm1-xLaxB6

Abstract: 2014 The cubic lattice parameter temperature dependence of SmB6 between 300 K and 4.2 K as well as the X-ray absorption at the LIII edge measured in the same temperature range, give direct evidence of the average samarium valence change which goes from 2.60 at 300 K to 2.53 at 4.2 K. Previous work claimed that the Sm2+ : Sm3+ ratio was temperature independent in SmB6. As for SmB6 a samarium valence change has been observed with decreasing temperature below 300 K in the Sm0.75La0.25B6 solid solution. J. Physique 41 (1980) 1141-1145 OCTOBRE 1980, Classification Physics Abstracts 78 . 70D Samarium hexaboride 5mB6 has attracted much experimental and theoretical attention in the last few years. SmB6 is a homogeneous mixed valent compound in which the Sm2+ : Sm3+ ratio at room temperature has been estimated to be about 4 : 6 from magnetic susceptibility, Mossbauer resonance measurements, LIII X-ray absorption, and X-ray photoelectron spectroscopy (XPS) [1-10]. Furthermore previous Mossbauer resonance and X-ray absorption experiments performed respectively down to l.l K and 150 K had not indicated significant variations of the SM2, : Sm 3 + ratio [3, 4, 5]. On the other hand, the lattice-parameter measurements carried out previously by us between 300 K and 4.2 K suggested a samarium valence change in SmB6 [5, 11]. In the dynamic (homogeneous) mixed valence state, each rare earth ion can be viewed as fluctuating between two configurations with the 4f shell occupation number differing by one. The charge fluctuation time ip is such that fast measurements like X-ray absorption or XPS detect both configurations separately, while slow measurements such as Mossbauer isomer shift detect only an average time, in the homogeneous mixed valent compounds. In order to determine quantitatively the samarium valence change in SmB6 as a function of temperature, X-ray absorption measurements have been carried out. The results obtained by direct observation will be compared with those deduced from lattice parameter temperature dependence. 1. Experimental. SmB6 has been prepared according to the reaction Density measurements, X-ray and chemical analysis indicate an atomic ratio B/Sm = 6. The cubic lattice parameter temperature dependence of SmB6 and SMO.7.5LaO.2.5B6 have been studied between 300 K and 4.2 K with a X-ray powder diffractometer using a monochromatic CoKa X-ray radiation [ 13]. The X-ray absorption experiments have been performed at the French synchrotron in Orsay (LURE). The X-ray beam is emitted by 1.72 GeV electrons in the ring D.C.I. The radiation was monochromatized with the help of a (220) silicon crystal. The (*) Department of Physics, Tohoku University, Sendai, Japan. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198000410100114100

Overlapping core to valence and core to Rydberg transitions and resonances in the XUV spectra of SiF4

Abstract: By comparison of the Si 2p absorption spectra of molecular and solid SiF4 the local character of the resonances above threshold and the valence or Rydberg character of the lines at threshold have been established After checking the reliability of the Z+1 model on the core spectra of SF6, an assignment of the SiF4 lines, based on SCF calculations for the Z+1 analogue PF4+, is presented which is consistent with the experimental data

Intermediate Valence on Dilute Europium Ions

Abstract: Dilute alloys of Eu in Sc${\mathrm{Al}}_{2}$ show single-ion anomalies in their transport properties. The susceptibility is intermediate between ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$. The M\"ossbauer isomer shift is intermediate as well as strongly temperature dependent. This is the first case of microscopically confirmed configurational instability in any dilute rare-earth alloy.

Trends in the electronic properties of substitutional 3 d transition-metal impurities in GaAs

Abstract: The $X\ensuremath{\alpha}$ scattered-wave method, in its spin-restricted version, has been used to calculate the electronic states associated with an ideal gallium vacancy as well as with the following substitutional transition-metal impurities in gallium arsenide: chromium, manganese, iron, cobalt, nickel, and copper. The gallium vacancy is found to behave as a simple acceptor with an ionization energy of approximately 0.50 eV. In addition, a level of ${a}_{1}$ symmetry with considerable amplitude in the vacancy sphere is found at an energy of approximately 0.75 eV below the valence edge. The transition-metal impurities can be broken up into two categories, depending on whether or not the $3d$ states of the transition metal play an active role in determining the active electronic states of the defect. Those impurities to the left of (and including) cobalt are found to behave in the "standard" way, as the active impurity states in the gap have substantial $d$ character and there is clear evidence of bond formation involving the impurity and the neighboring host ligands. By contrast, nickel and copper appear to behave as simple acceptors in the sense that the active defect levels have little $d$ character and are rather more characteristic of the broken bonds at the impurity site. The $d$ states of this latter group appear as "resonances" in the host valence band and play little direct role in determining the electronic properties of the material. The paper concludes with a discussion of these results and possible factors that might affect their validity.

Chemical Shifts of Auger Electron Lines and Electron Binding Energies in Free Molecules. Silicon Compounds

Abstract: The silicon KL2, 3L2, 3(1D2) Auger energies and the silicon 2p3/2 binding energies have been measured for more than twenty organic silicon compounds in the gas phase. The group shift model has been applied to the Auger energy data as well as the binding energy data. Approximate proportionality between these two types of chemical shifts was found for some sub-series of the type SiR4-nTn. The transition potential model for the transition from one to two holes in the silicon core has been developed to take into account the difference between the 1s and 2p binding energy shifts. Semi-empirical calculations have been performed for most of the compounds, CNDO/2 and EWMO (a modified iterative extended Huckel method, briefly presented in the text). It was found that EWMO provides a more satisfactory description of the valence charge densities and their changes upon core ionization than does CNDO/2. The correlation with the experimental data was also found to be better applying the transition potential models to the EWMO calculated charges than to the CNDO/2 charges.

Excited states of CH+: Potential curves and transition moments

Abstract: Wave functions and potential curves for the ground X 1Σ+ state and eight excited states of 1Σ+ and 1Π symmetry of CH+ have been obtained using ab initio configuration interaction (CI) methods. In order to take proper account of valence–Rydberg mixing, Rydberg functions were included in the basis set. The orbitals used in the set of reference configurations for the CI wave functions included both valence and Rydberg orbitals, determined from a multiconfiguration self‐consistent field and a natural orbital calculation, respectively. Transition moments between the ground electronic state and the 2 1Σ+, 3 1Σ+, A 1Π, and 2 1Π states have been computed, and the importance of these states in photodissociation of CH+ is discussed.