Showing papers on "Ionic bonding published in 1976"

Molecular dynamics studies of the vitreous state: Simple ionic systems and silica

Abstract: Some applications of molecular dynamics calculations to the vitreous state have been examined for simple ionic MX and MX2 type glasses. The MX system which corresponds to a hypothetical vitreous and supercooled amorphous KCl is found to undergo an isobaric transition in the region of 0.3 Tf and an isothermal transition at around 35 kbar (T=Tf). The transition is evidenced by a discontinuity in second order thermodynamic properties and may be associated with a virtual disappearance of translational diffusion. By employing a variety of irreversible stress histories, the vitreous state in this case is shown to be insensitive to the thermodynamic stress history up to a maximum computationally permissible relaxation time of around 10−10 sec. Simulations are also reported for some ionic liquids of MX2 stoichiometry which, in contrast with KCl, have glass‐forming ability (BeF2, ZnCl2, and SiO2). Although the relaxation times involved in laboratory glass formation far exceed the long‐time limit of computer ’’expe...

Mixed Electrical Conduction in the Fluorite‐Type Ce1 − x Gd x O 2 − x / 2

Abstract: The solid solutions exhibit high ionic conduction due to the migration of O2− ions via oxygen ion vacancies. The emf of a galvanic cell,, is measured within the temperature range between 700° and 800°C as a function of . At such high oxygen pressures as in air, the emf changes according to indicating the ionic transference number in the solid solution is unity. With decreasing oxygen pressure, however, the observed emf becomes smaller than the theoretical value. This indicates the onset of electronic conduction generated by the reduction of the solid solution. The dependence of the emf on is described bywhere is a constant corresponding to the oxygen partial pressure at which the ionic transference number of the solid solution becomes 0.5. From the temperature dependence of , partial molar enthalpy changes for the reduction of are calculated.

Modified electron-gas study of the stability, elastic properties, and high-pressure behavior of MgO and CaO crystals

Abstract: A theoretical study of the crystal structure and bonding in the simple inorganic oxide crystals magnesium oxide and calcium oxide is reported. An a priori method based on an electron-gas approximation is applied to an ionic description of these crystals. The quantities calculated are the equilibrium interionic separation, the elastic constants and their pressure derivatives, the transition pressures and volume changes in high-pressure-induced phase transitions, and the thermodynamic stability of the crystal structures. Comparison with available experimental data shows good agreement, without any adjustable parameters in the theory. We conclude that the ionic description of these crystals is rather accurate for energy calculations, provided that a suitably stabilized oxide ion is used in the theory.

Studies of amorphous GeSeTe alloys (I): Preparation and calorimetric observations

Abstract: We have used differential scanning calorimetry (DSC) to examine the thermally induced transformations of bulk and thin-film amorphous alloys within a large portion of the GeSeTe system. Most chalcogen-rich compositions showed a discontinuous increase of heat capacity when heated through the glass transition temperature T G . The Ge-rich compositions, which could only be prepared as sputtered amorphous films, were invariably characterized by an irreversible exothermic crystallization process on heating, beginning at the crystallization temperature T X . Values of T g and T X have been tabulated for all alloys investigated and the compositional dependence of T g has been examined in the light of recent models for viscous flow in glass-forming chalcogenide systems. In addition, a region of liquid immiscibility has been observed in the vicinity of Ge 20 Se 40 Te 40 in which a GeSe 2 -rich liquid phase segregates from a tellurium-rich liquid phase. The existence and limits of this immiscibility region have been rationalized on the basis of ionic perturbations to the covalent bonding. The segregation of a GeSe 2 -rich liquid increases the concentration of GeSe bonds which are the strongest and most ionic of the six angle-bond types which can occur in this system.

Aggregation effect on the infrared absorption spectrum of small ionic crystals

Abstract: The frequency spectrum of clusters of small ionic particles is calculated when the particle mutual interaction is described by a dipole-dipole interaction Hamiltonian. The crystallites are spheres of known bulk dielectric properties. The actual geometry of the cluster is taken into account. Clusters of increasing size (pair, triplet, quadruplet, etc.) have been solved analytically. As expected, the dipolar broadening of the frequency spectrum of (optically active or inactive) surface modes increases with cluster size and compactness. From the spectrum of few-particle clusters, one obtains the long-wavelength modes of large linear or planar aggregates of such clusters by application of the Bloch-Floquet theorem.Application is made to ir optical absorption of NiO. The present approach is intermediate between the continuum model of a structureless powder on the one hand and microscopic calculations for single microcrystals of prescribed shape on the other hand.

X‐ray diffraction study of the average solute species in CaCl2 aqueous solutions

Abstract: The structure of several CaCl2 aqueous solutions was studied by means of x‐ray diffraction. It is demonstrated that the solutions can be treated as being composed of independent hydrated ions, and for this reason the structural problem is limited to the study of close hydration. This description allows the calculation of structure functions in very good agreement with experimental data when using coordination number six and octahedral geometry for both the ionic species present.

Chemistry of soil aluminum

Abstract: Better understanding of soil aluminum has had dramatic effects on the interpretation of many aspects of soil chemistry. Aluminum is a Group III element, metallic in nature, and exhibits both ionic and cuvaient bonding. It is the most plentiful of all metallic cations of the earth's crust. It is released from octahedral coordination with oxygen in minerals by weathering processes. Once released, the trivalent Al ion assumes octahedral coordination with six OH2 groups each of which dissociates a H ion in sequence as pH increases. The resulting hydroxy‐Al ions are absorbed to the cation exchange capacity of the soil. Here they polymerize on charged surfaces and in the interlayers of the clay minerals obstructing both the contraction of the clay lattice and the exchange of cations. Soluble Al is toxic to most plants, and reacts readily with soluble phosphates converting them to relatively insoluble and plant‐unavailable forms. Adsorbed and polymerized aluminum affects actual lime requirements of soil...

E.S.R. spectra of matrix isolated alkali atom clusters

Abstract: E.S.R. spectra assigned to Na3 molecules have been obtained by codepositing sodium atoms and diluent argon on a sapphire surface loosely coupled to a liquid helium cryostat and warmed by a heat leak, to allow aggregation before the alkali atoms are frozen into the argon matrix. The trimer spectra under these conditions were much more intense than the residual atom spectra. The trimer spectra indicate the unpaired electron in these molecules has predominantly s rather than p character. Approximately 95 per cent of the 3s spin density is equally distributed between two of the alkali atoms, with only about 7 per cent of the spin density on the third atom. The trimers thus are not merely van der Waals adducts but involve chemical bonding. Both a covalent molecular orbital model for linear or obtuse isosceles geometry and an ionic charge-transfer model giving M2 +M- or M+M2 - appear qualitatively consistent with the spectra.

Solvent structure in aqueous mixtures. III. Ionic conductances in ethanol-water mixtures at 10 and 25°C

Abstract: Viscosities at 10 and 25°C and dielectric constants at 10°C are reported for ethanol-water mixtures. Precise transference numbers for KBr at 10°C were determined by the moving-boundary method for five solvent mixtures containing up to 20 mole % ethanol. Conductance measurements for the alkali halides and tetraalkylammonium bromides were carried out across the complete range of solvent composition at 25°C, but for the water-rich region only at 10°C. The resulting limiting ionic Walden products confirmed the conclusions arrived at from earlier measurements. The maxima occurring in the water-rich region cannot be attributed solely to a structure-breaking effect owing to the ionic size dependence which is in the wrong direction. The tetraalkylammonium ions in these mixtures do not exhibit an enhanced hydrophobic effect, nor do they appear to enhance structure of any kind. Rather, the addition of alcohol to an aqueous Bu4N+ solution merely reduces the magnitude of the hydrophobic effect, as illustrated by a steadily increasing Walden product in the water-rich region. A decrease in temperature has no effect on the Walden product for the Bu4N+ ion but increases the magnitude of the maxima for the alkali-metal and halide ions. These maxima are attributed to sorting of the solvent components by the ionic charge, due to an acid-base type interaction, so that the proportion of water in the ionic cospheres is greater than in the bulk solvents. The sorting effect is shown to be temperature independent. The oxyanions are shown to exhibit a behavior in these mixtures that cannot be identified at this time.

Formation, Nature, and Mobility of Ions of Atmospheric Importance

Abstract: An understanding of the evolution of atmospheric small ions requires the knowledge of 1) the type of radiation, 2) the atmospheric trace gas composition as a function of altitude and the physical and chemical molecular parameters of these trace gas molecules such as ionization potential, electron affinity, proton affinity, polarizability, chemical reactivity, etc., 3) ion-molecule reaction rates for eventually every collisional interaction of ions with the neutral gaseous environment and 4) the theory of ionic mobility. Partial information on 1) –4) is available today and presented in this paper. Based upon laboratory and field measurement, we can now derive a preliminary picture of the nature of atmospheric ions as a function of altitude. The atmospheric ions are thought of being in a state of constant change, reflecting in their chemical nature the collisional interaction with atmospheric trace gas molecules. Molecular rearrangement of attached molecules can lead to the formation of new chemical species on the “surface” of small ions, which allows the ion to grow into larger ion clusters. The mobility spectrum of atmospheric small ions is thought to be pseudo-continuous in the lower part of the atmosphere due to the presence of numerous trace gases (on the level of less than 10–9 of all parts) interacting with the ions during their lifetime. However, direct mass spectrometric in situ measurements of tropospheric and stratospheric ions are still missing.

The interaction between Beta-lactoglobulin and sodium N-dodecyl sulphate.

Abstract: 1. The binding of sodium n-dodecyl sulphate to beta-lactoglobulin was studied in the pH range 3.5-7.0 by equilibrium dialysis, ultracentrifugation and microcalorimetry. 2. At low binding concentrations (less than 30 bound surfactants anions per protein molecule) the complexes formed aggregates in solution. 3. At higher binding concentrations aggregation does not occur at low ionic strength (0.01 mol/litre), but continues at high ionic strength (0.1 mol/litre). 4. At 25 degrees C the enthalpy of interaction of sodium n-dodecyl sulphate with beta-lactoglobulin can be interpreted as the sum of the enthalpies of formation of a complex with 2 bound surfactant anions, with an enthalpy change of -9.5 kJ-mol-1 of bound surfactant, and complexes containing at least 22 bound surfactant anions, with limiting enthalpies per bound surfactant anion of -12.4 kJ-mol-1 at pH 3.5 and -3.25 kJ-mol-1 at pH 5.5. 5. The binding of surfactant and the enthalpy of interaction at pH 3.5 ARE NOT SIGNIFICANTLY AFFECTED BY THE ADDITION Of 8 M-urea. 6. The data indicate that at low binding concentrations the interaction is of an ionic nature, and is accompanied by a conformational change in the protein.

Effects of polarization on equilibrium and dynamic properties of ionic systems

Abstract: Molecular-dynamics calculations are reported for systems of ions in which the contribution to the interionic forces arising from polarization is taken explicitly into account. Two models of polarization are considered: a modification of that used for alkali-halide molecules by Tosi and Doyama and a simplified version of the shell model of lattice dynamics. In each case the effects of polarization are superimposed on an underlying rigid-ion pair potential of the type proposed by Tosi and Fumi. Calculations of the phonon spectra show that use of either model leads to agreement with experimental data on solid NaCl, which is considerably better than that achieved with the Tosi-Fumi potential alone; good results are also obtained for solid KI. Calculations for molten KI based on the shell model show that the effect of polarization in the liquid state is to increase significantly the diffusion coefficients, particularly that of the positive ion, and decrease the shear viscosity, but the effect on electrical conductivity is almost negligible. These results are broadly consistent with the known adequacy of the rigid-ion-model to account for the transport coefficients of molten alkali halides. Polarization is also shown to lead to a reduction in the characteristic frequencies and a considerably increased damping of collective longitudinal-optic-type modes in the liquid. The effect on other collective modes is less pronounced.

Atomic geometry of cleavage surfaces of tetrahedrally coordinated compound semiconductors

Abstract: Analyses of low‐energy electron diffraction intensities are utilized to determine the surface atomic geometries of GaAs(110), ZnO(1010), and ZnO(1120). The geometry of GaAs(110) is rippled, with the As atoms protruding upwards and the Ga atoms downwards relative to the truncated bulk (zincblende) geometry in a fashion such that no nearest‐neighbor bond lengths are altered. The geometries of ZnO(1010) and ZnO(1120) are nearly those of the truncated bulk (wurtzite) lattice. Small contractions of the upper‐layer spacings of both the oxygen (Δd=−0.1 A) and the zinc (Δd=−0.3 A) sublattices appear probable, however, for ZnO(1010). The qualitatively different nature of the surface reconstruction on GaAs(110) from those which may occur on ZnO(1010) is interpreted in terms of the covalent character of the bonding in GaAs as contrasted with a more ionic bonding in ZnO. Both types of surface reconstruction are consistent with a model in which the cation‐to‐anion charge transfer in the surface layer is less than in the bulk.

Mobility and Conductivity of Ions in and into Polymeric Solids

Abstract: There often has been great uncertainty as to whether typical measurements allow one to conclude that conduction in a given polymer is due predominately to ions or to electrons. Many researchers have assumed that relatively pure polymers can be treated as wide band gap semi-conductors but other scientists have believed that most non-conjugated polymers conduct by the movement of ions. Part of the trouble is that many of the theories predict dependences of current on voltage and sample thickness too similar to easily distinguish between the assumed mechanisms unless extensive and very good data are available. Some comparisons are discussed. More recent approaches that use pressure as an added variable, or those that give careful consideration to such things as gas evolution often allow one to deduce the predominant type of conduction. The concept of local structure of a polymer in the neighborhood of an ion is considered along with the relation of ionic conduction to the entropy correlation theory recently developed by Barker and applied to the case of diffusion in oriented polymers by Barker, Tsai, and Willency. According to the evidence, the activation entropy increases under the action of an organizing effect such as uniaxial elongation. Moisture has two important effects on conduction in polymers. At low concentrations it contributes charge carriers by its own dissociation and at higher concentrations it enhances the dissociation of other ionic species present. Some data for the correlation of conductivity and dielectric constant of solid polymers are discussed in terms of the weak electrolyte model developed by Sharbaugh and Barker for organic liquids. The existence of the correlation seems to imply that many polymers, under most circumstances, are predominantly ionic conductors.

Chemisorption-Site Geometry from Polarized Photoemission - Si(111)Cl and Ge(111)Cl

Abstract: The chemisorption-site geometries for chlorine on cleaved Si(111) 2\ifmmode\times\else\texttimes\fi{}1 and Ge(111) 2\ifmmode\times\else\texttimes\fi{}1 surfaces have been determined by comparing detailed pseudopotential calculations with energy distribution spectra and polarization selection rules of photoemission spectroscopy. Strong $p$-symmetry polarization effects observed in photoemission allow us to determine that on Si(111) the correct geometry is a onefold covalent site while on Ge(111) the Cl is in a threefold coordinated ionic site.

An infra-red spectroscopic study of cement formation between metal oxides and aqueous solutions of poly(acrylic acid)

Abstract: An infra-red spectroscopic study of the formation of cements between metal oxides and aqueous solutions of poly (acrylic acid) is reported. Oxides of divalent metals form cement gels more readily than oxides of trivalent metals. An attempt was made to gain structural information from the observed frequency shifts of the absorption bands of the carboxylate groups in the metal poly(acrylate) gels relative to those recorded for the purely ionic form. The form of cation binding to the polyanion chain is discussed.

Energy‐dependent cross sections for quenching of Li(2p 2P) by several gases

Abstract: The lifetime of Na(3p 2P) atoms, in the presence of various quenching gases (H2, D2, N2, CO, CO2, and C2H4), has been directly determined by the single‐photon time‐correlation method. Na(2P) atoms were produced with varying amounts of translational energy by photodissociation of NaI with light of wavelengths 215–240 nm. The large cross sections obtained, and their inverse dependence on relative collisional energy, are best explained on the basis of a ’’harpooning’’ mechanism, in which a strongly attractive ionic state provides the coupling between upper and lower covalent states.

Metal-metal bond lengths in complexes of transition metals.

Abstract: In complexes of the transition metals containing clusters of metal atoms the cobalt-cobalt bond lengths are almost always within 1 pm of the single-bond value 246 pm given by the enneacovalent radius of cobalt, whereas most of the observed iron-iron bond lengths are significantly larger than the single-bond value 248 pm, the mean being 264 pm, which corresponds to a half-bond A simple discussion of the structures of these complexes based on spd hybrid orbitals, the electroneutrality principle, and the partial ionic character of bonds between unlike atoms leads to the conclusion that resonance between single bonds and no-bonds would occur for iron and its congeners but not for cobalt and its congeners, explaining the difference in the bond lengths

Ionic conduction in As2S3Ag2S, GeS2GeSAg2S and P2S5Ag2S glasses

Abstract: The glass-forming region in the system PSAg was determined and density, thermal expansion, dc conductivity and the transport number of Ag ions were measured for P 2 S 5 Ag 2 S glasses found in the PSAg system. The results for the transport number measurement show that P 2 S 5 Ag 2 S glasses are purely ionic conductors owing to the Ag ion migration, like most of the As 2 S 3 Ag 2 S and GeS 2 GeSAg 2 S glasses reported previously. Glass structure and ionic conduction processes in As 2 S 3 Ag 2 S, GeS 2 GeSAg 2 S and P 2 S 5 Ag 2 S glasses are discussed, based on their ionic conductivity and density data. The structural concept of α-Ag 2 S was applied to these glasses, which suggests that the Ag ions in the glasses are distributed in the available Ag ion sites in the non-conducting framework composed of both S anions and As, Ge or P cations. In each system the ionic conductivity increases linearly with increasing Ag + /total cation (%) in glass composition, the determining factor being the activation energy for ionic conduction alone. Thus, the activation energy in these glasses depends predominantly upon the molar ratio of Ag ions to total cation in the glass, irrespective of the kind of system. Small differences in the activation energy among the three systems can be interpreted as arising from differences in the field strength of As, Ge and P cations.

Mass Identified Mobility Spectra of p-Nitrophenol and Reactant Ions in Plasma Chromatography

Abstract: An interfaced plasma chromatograph/mass spectrometer (ALPHA-II) permits Identification of the positlve and negative ionic specles associated with ionlc peaks In the plasma chromatographic mobility spectra of pnitrophenol. The posltive ions observed and their reduced mobility values (KO) are MNO+ (1.67), MH+ (1.80) and the negative ion is [M - HI- at (1.87). These are essentially the ions that are predicted from chemical ionization mass spectrometry princlples and mobility data alone. An ion observed at mass 108 (KO 1.91) results from a thermal decomposition product of pnitrophenol. The major reactant ions were found at m/e 18, 30, 37, and 65, with probable structures of NH4+, NO', (H20)*H+and ( H20)2N2H+. The 37 and 65 ions occur at the same mobility. Comparison of reduced mobllity values, KO, between two independent laboratories gives an agreement within experimental reproducibility of f0.02 for these spectra. The technique of plasma chromatography (PC) allows observation of the characteristic positive and negative ionic mobility spectra of nanogram and picogram quantities of organic compounds at atmospheric pressure. The mobility spectra give qualitative and quantitative information for organic compounds much like that from infrared spectra. The method involves reaction of organic molecules with ions and electrons generated by a 63Ni source in a nitrogen carrier gas, followed by mobility separation of the products in an ion-drift spectrometer. A recent review and its references describe the technique and summarize its capabilities (I).