Showing papers on "Ionic bonding published in 1977"

Initial and final state effects in the ESCA spectra of cadmium and silver oxides

Abstract: The factors which influence chemical shifts are examined in order to elucidate the cause of the anomalous chemical shifts for Cd and Ag oxides. The effects of extra‐atomic relaxation are accounted for using a procedure employing experimental Auger and binding energies. Atomic partial ionic charges for some simple Cd, Ag, and Zn compounds are calculated from experimental binding energies using a model which includes the effects of lattice potentials and extra‐atomic relaxation. Inclusion of extra‐atomic relaxation effects did not have a drastic effect on the relative ionicities computed for these selected compounds. However, for CdO, a large extra‐atomic relaxation energy contribution reduces the binding energy by 0.5 eV more than is predicted from nearest neighbor electronegativity arguments.

Bond ionicity and structural stability of some average-valence-five materials studied by x-ray photoemission

Abstract: Core-level and valence-band spectra have been obtained by means of x-ray photoemission spectroscopy for the group IV, V, and VI elements Ge, Sn, Pb, As, Sb, Bi, S, Se, and Te and the group IV-VI compounds GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, and PbTe. These results, taken under ultrahigh-vacuum conditions with unmonochromatized x rays, are presented and discussed in terms of the bonding in materials with an average valence of 5. The effects of relaxation on the chemical shifts are found to be relatively small. The chemical shifts are found to vary in a manner similar to that expected from the magnitude of the elemental electronegativities except for the ordering of the shifts of GeS and GeSe. Relative charge transfers are calculated from the chemical shifts and are found to be in general agreement with ionicities calculated using the Phillips-Van Vechten theory although there is some disagreement as to their magnitudes. A consideration of the structures of the compounds relative to the charge transfers demonstrates the importance of metallic as well as covalent and ionic bonding in determining the most stable structure. The value of critical ionicity carried over from the average-valence-4 materials does not apply to the average-valence-5 materials and this concept does not appear useful in understanding their bonding because of the increased importance of nondirectional metallic bonding.

A neutron diffraction study of hydration effects in aqueous solutions

Abstract: The nature of ionic hydration for the cations in NiCl2 solution and the anions in NaCl solution has been investigated by neutron diffraction experiments on samples which have been isotropically enriched. Well defined hydration numbers are found for both the Ni2+ ion and Cl- ion.

The metal-semiconductor interface: Si (111) and zincblende (110) junctions

Abstract: The metal-semiconductor interface is analysed by means of a simple method, which includes both the effects of virtual surface states and the many-electron interaction. This model is used to obtain the barrier height phi Bn for the junctions of Si (111) with Al and Na; the results are in good agreement with experimental data. The trend on going from covalent to increasingly ionic semiconductors is also studied for junctions of zincblende (110) compounds for different metals. The results do not show the transition from Barden-like to Schottky-like behaviour displayed by experimental data.

Molecular dynamics computer simulation of surface properties of crystalline potassium chloride

Abstract: Computer simulation of the (100) face of crystalline potassium chloride has been achieved using the method of molecular dynamics and a lamina model of thickness 10 ionic layers. The basic cell contained 360 ions, interacting via a Tosi–Fumi potential. A solution has been obtained for the long range correction to the Coulomb potential for an ionic lamina. This differs significantly from the solution for a 3-dimensional lattice.Surface distortion, surface energy and stress have been evaluated and agree well with experiment and previous model calculations. The ratios of surface mean square amplitudes of vibration perpendicular to the surface divided by the bulk values show an approximately 25 % anharmonic enhancement over previous calculations. The layerwise velocity autocorrelation functions yield power spectra featuring a peak for the surface mono-layer at 75 cm–1 which compares favourably with the frequency of surface acoustic modes predicted by previous calculations.

Metal-non-metal transition in silver chalcogenides

Abstract: The paper reviews the crystallographic, optical, magnetic and transport properties of silver chalcogenides Ag2S and Ag2Se. In the low temperature β-phases, the influence of stoichiometry on the respective magnitude of ionic and electronic conductivities is important. In the disordered high-temperature α-phases, the characteristic feature is the statistic distribution of lattice silver ions. This situation is analysed according to the theory established by Mott (e.g. 1974, and Mott and Davis 1971) for amorphous semiconductors. Owing to the disorder, tails appear both in valence and conduction band, and in α-Ag2Se, the bands overlap.

The electronic states of KrF

Abstract: Ab initio configuration interaction calculations for the electronic states of KrF are presented. States dissociating to Kr+F(2P), Kr+(2P)+F− and Kr* (3P)+F(2P) are investigated. Spin–orbit coupling is included in a semiempirical manner for the covalent and ionic states. The covalent curves are essentially repulsive in character, while the attractive ionic curves have wells 5.2–5.3 eV deep with calculated Re′s of 2.4–2.5 A. The lowest 2Σ+ Rydberg state also forms a bound state with calculated values of Re=1.83 and De=1.33 eV. The intense laser emission at 248 nm is found to arise from the transition between the lowest ionic Ω=1/2 to covalent Ω=1/2 states (III1/2–I1/2 in our notation) calculated to occur at 241 nm with a spontaneous lifetime of 7 nsec. The observed emissions at 220 nm and 275 nm are assigned to the IV1/2–I1/2 and II3/2–I3/2 transitions, respectively.

The basic atomic processes of corrosion I. Electronic conduction in MnO, CoO and NiO

Abstract: We have used simple ionic models to study the lattice distortion and polarization about localized holes in MnO, CoO and NiO. Our results demonstrate a rather fine balance between factors favouring localized small polarons and band-state large polarons, but we find the correct trend consistent with the observation of small polarons in MnO and large polarons in CoO and NiO. Our calculations of the activation energy for hole hopping give results in good general agreement with the measured free hole mobility in MnO and the trapped small polaron motion in CoO and NiO. We can also make reasonable estimates of the binding of the hole to a monovalent dopant. These satisfactory conclusions seem to justify the neglect of the complications of Jahn-Teller, crystal field and covalency contributions in our models.

Electron Transfer in Monolayer Assemblies with Incorporated Ruthenium (II) Complexes

Abstract: An arrangement for light induced vectorial charge separation is discussed in which the electron of the excited dye is transferred to an acceptor and the dye is recovered by electron tunneling through a high and narrow potential barrier. The possible relevance of the model in connection with photosynthesis is considered. Monolayers of Ru(II)-bipyridine complexes with long chain hydrocarbon substituents were investigated which may be useful as component in such arrangements. The surface pressure area isotherms of the monolayers were measured for various ionic compositions of the subphase, and the results demonstrate a strong effect of these ions on the structure of the layers. The layers were deposited on different substrates. The luminescence and its change by contacting the sample with water and by subsequent drying were found to be strongly dependent on the architecture of the layer assembly. Attempts of a photochemical cleavage of water with these assemblies failed. The pH-dependence of the absorption and the luminescence of a Ru(II) bipyridine-dicarboxylic acid complex in solution is interpreted by assuming that the electron in the excited state is localized in one pyridine part of the substituted ligand, the conjugation with the second half of the bipyridine carboxylic acid being negligible. Monolayer assemblies for measuring the energy transfer from the ruthenium complex to an adequate energy acceptor and from an adequate energy donor to the ruthenium complex were investigated. The results demonstrate that the deactivation of the excited ruthenium complex occurs mainly by passing the luminescent state. Assemblies were investigated for measuring the electron transfer from the excited ruthenium complex to an appropriate electron acceptor positioned in the carboxylate portion at the same interface as the electron donor. With bipyridinium ions as acceptor the ruthenium complex luminescence is quenched at average distances between acceptor molecules of about 10 A, while this distance is 30, 60 und 75 A for different cyanine dyes used instead of the ruthenium complex. A correlation between this distance and the ionization energy in the excited state of the donor is observed.

Defect Structure of α-Al2O3 Doped with Titanium

Abstract: Titanium-doped α-Al2O3 exhibits a high-temperature conductivity which is ionic at high oxygen pressures and electronic at low oxygen pressures. Both are isotropic. The temperature dependence of conductivity under conditions where equilibrium with the atmosphere is not maintained indicates both the position of the energy level of titanium (TiAlx) in the forbidden gap and the temperature dependence of the mobility of the native ionic defects (Al vacancies, VAlm). Optical absorption responsible for the pink color of the reduced crystals is measured as a function of po2 and is used to determine concentrations of Ti3+ and Ti4+. Parameters for the equilibrium constants of the reactions involving electrons by which the composition of Al2O3:Ti and undoped Al2O3 is varied are determined. The chemical diffusion data by Jones et al. are described quantitatively.

Charge transfer and ionic bonding in organic solids with segregated stacks

Abstract: In ionic charge-transfer salts which have segregated stacks of organic donor ($D$) and acceptor ($A$) molecules, the major feature of the classical electrostatic interactions is the strong repulsion between like charges along the stacks. Two ways are described by which the magnitude of this repulsion may be decreased: (i) by the formation of a complex salt (i.e., one whose ratio $D:A\ensuremath{ e}1:1$); and (ii) by incomplete transfer of charge from $D$ to $A$. In both cases, some of the molecules in the stack are neutral, giving rise to a mixed-valence, partly ionic ground state. The first effect is quantitatively investigated by a Madelung energy calculation of tetrathiafulvalene-${\mathrm{Br}}_{0.79}$ as a function of (theoretical) bromine concentration. The results of this calculation show that the unusual composition of this salt occurs in order to reduce the Coulomb repulsion along the stacks. As an example of the second effect, a similar calculation is described for TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane), as a function of the amount of charge transferred from TTF to TCNQ. Although electrostatic interactions do not give a complete description in this case, they are clearly an important factor in determining the degree of charge transfer in such salts. In fact, in cases where the net electrostatic binding energy is small, incomplete charge transfer may be energetically favored. In this case, the charge distribution is shown to be modulated along the stack and charge density waves are formed, with wave vector "$4{k}_{F}$". These results are compared to the case of Rb-TCNQ, where the Madelung energy is shown to favor complete charge transfer.

Ionic Conductivity of Solid and Liquid LiAlCl4

Abstract: The electrical conductivity of lithium chloroaluminate has been investigated in the temperature range between room temperature and 180°C It is molten above 145°C The conduction is practically purely ionic and is attributed to the transport of lithium ions The experiments show high values for ionic conductivity in the solid state, only exceeded by Li‐β‐alumina and some LiI‐based materials At 25°C the conductivity was found to be , with an activation enthalpy for of 047 eV A‐C conductivity measurements were influenced by sample preparation and cell arrangement The ionic conductivities of this and several other newly found lithium ionic conductors, as well as those previously known, are compared in the temperature range between 20° and 250°C

Potential energy surfaces for simple chemical reactions:. Application of valence-bond techniques to the Li + HF → LiF + H reaction

Abstract: Ab initio multi-structure valence-bond calculations have been performed to determine the potential energy surface governing the reaction Li + HF → LiF + H. Results for both linear and non-linear nuclear geometries are presented. The system is a prototype for many heavier alkali metal plus hydrogen halide reactions which have been studied using the crossed molecular beams technique. The ab initio valence-bond results are improved by applying corrections, within the framework of the orthogonalized Moffitt (OM) method, for the atomic errors present. The orbital basis set used was of double zeta quality, and was augmented by some extra orbitals. Preliminary calculations on the neutral and ionic diatomic species were performed to ensure the adequancy of the valence-bond structure basis sets used and care was taken to ensure that the basis sets provided an adequate description of F–, HF– and LiF–. The endoergicity of the reaction, ignoring the zero-point vibrational energies, was predicted by the ab initio and OM methods to be 5.8 and 2.5 kcal mol–1 respectively, as compared with the experimental value of 2.6 kcal mol–1. Besides the ground state potential energy surface, several surfaces for excited electronic states have been calculated and are presented. The relationship of the ground state potential energy surface to the reactive cross section, and its variation with energy, is discussed. The ground and excited state potential energy surfaces are compared with previously proposed models and a mechanism for the production of alkali metal ions in hyperthermal alkali metal atom–hydrogen halide collisions is proposed.

Simple electrolytes in the mean spherical approximation. III. A workable model for aqueous solutions

Abstract: Some improvements in the primitive model of electrolyte solutions are discussed within the framework of the mean spherical approximation. Excellent agreement has been found between experimental and calculated osmotic and activity coefficients of 16 alkali halides in H2O at 25 °C, whenever a density dependent hard core ionic diameter is combined with an adjustable but not density dependent dielectric constant. Promising agreement is also found when Pauling crystallographic radii are used for the ions.

Proton transport by bacteriorhodopsin through an interface film.

Abstract: Interface films of purple membrane and lipid containing spectroscopically intact and oriented bacteriorhodopsin have been used as a model system to study the function of this protein. Small positive charges in surface potential (<1 mV) are detected upon illumination of these films at the air-water interface. These photopotentials, are not affected by overlaying the interface film with a thin layer (0.3 mm) of decane. However, they are dramatically increased when lipid soluble proton carriers FCCP or DNP are added to the decane. The polarity of the photopotential indicates that, in the light, positive charges are transported through the interface from the aqueous to the organic phase. The action spectrum of the photopotential is identical to the absorption spectrum of bacteriorhodopsin. Since bacteriorhodopsin molecules are oriented with their intracellular surface towards the aqueous subphase, the characteristics of the photopotential indicate that in the light bacteriorhodopsin translocates protons from its intracellular to its extracellular surface. The kinetics of the photopotential reveal that the rate and extent of proton transport are proportional both to the fraction of bacteriorhodopsin molecules excited and to the concentration of proton acceptor. The photopotentials result from changes in the ionic distribution across the decane-water interface and can be cancelled by lipid soluble anions.

Defect Structure of α‐Al2O3 Doph with Magnesium

Abstract: The conductivity of single crystals of Al2O3+ Mg and the ionic and electronic transference numbers were measured at high temperatures as a function of orientation, oxygen pressure, and temperature. Optical absorption in the visible range was measured on cooled annealed crystals. The results are interpreted on the basis of a model with either Ali3- or VO as the dominant native defects and lead to expressions for the ionic mobility as a function of T and orientation, and for the position of the MgAl’level in the forbidden gap.