Showing papers on "Ionic bonding published in 1982"

Computer Simulation of Solids

Abstract: Theory of simulation methods for lattice and defect energy calculations in crystals.- Theory and calculation of defect entropies.- Characteristic volumes of point defects in tonic crystals.- Free energy calculations for crystals.- Molecular dynamics simulations of crystalline ionic materials.- Ab initio cluster calculations for defects in the solid state.- Computational methods for the electronic structure of defects in insulators.- Interatomic potentials in solids.- Potentials in metals.- Interionic potentials in ionic solids.- Interatomic potentials in covalent and semi-covalent solids.- Defect calculations for ionic materials.- Point defect calculations in metals.- Defect calculations in semiconductors.- Computer modelling of complex and massively disordered crystalline solids.- Aspects of the chemistry of phyllosilicates and intercalation in vermiculites.- Aggregation and precipitation in alkali halides.- Computer simulation of fast ton conductors.- Computer simulation of ionic crystal surfaces.- Long range order in non-stoichiometric oxides.

Salt effects on the phase transition of ionic gels

Abstract: An ionized acrylamide gel is found to undergo a discrete phase transition in equilibrium volume upon varying the salt concentration in the solution. The salt concentration required for the transition depends strongly on the valency of the positive salt ion added to the solution. In certain cases the concentration at the transition is many thousand times larger for monovalent ions than for divalent ions. A simple theoretical consideration of the osmotic pressure of the ions can explain the phenomenon.

Vibrational Spectroscopic Determination of Structure and Ion Pairing in Complexes of Poly(ethylene oxide) with Lithium Salts

Abstract: A structural model for crystalline complexes of poly(ethylene oxide) (PEO) with various lithium salts is presented, based on vibrational spectroscopic studies; these complexes are known to exhibit ionic conductivities of > 10 -/sup 5/ (/OMEGA/-cm)-/sup 1/ at 100/sup 0/C. Cation-dependent vibrational bands observed in the Raman indicate that significant cation-oxygen atom interactions occur and suggest that the polyether chain may wrap around the lithium cations. Spectroscopic studies indicate extensive contact ion pairing occurs in the PEO*LiNO/sub 3/ complex, and this may contribute to the somewhat lower ionic conductivity of this complex as compared to other lithium salt complexes with similar structures but weaker cation-anion interactions. 27 refs.

Angle-resolved photoemission from polar and nonpolar zinc oxide surfaces

Abstract: We report the first angle-resolved photoemission spectra (ARPES) of polar and nonpolar surfaces of an ionic wurtzite-type compound semiconductor. We present experimental results obtained at photon energies between 20 and 80 eV on the polar zinc and oxygen and the nonpolar ($10\overline{1}0$) surfaces of ZnO and identify surface-induced features by comparison with recent theoretical calculations. Comparison between ARPES experiments and theory confirms the determination of the surface-atom geometry at $\mathrm{ZnO}(10\overline{1}0)$ by low-energy electron diffraction studies.

A neutron diffraction investigation of the structure of vitreous zinc chloride

Abstract: A survey is presented of the AX 2 glasses and the corresponding crystalline polymorphs. The relationship between ZnCl 2 and the other members of the series is discussed and it is predicted that the number of shortest path seven and higher-membered rings in the non-chalcogenide AX 2 glasses is likely to be small. A neutron diffraction investigation of vitreous ZnCl 2 using the D4 diffractometer at ILL Grenoble shows that the structure of vitreous ZnCl 2 comprises a distorted random close packed array of Cl − ions with the Zn 2+ ions in tetrahedral holes, arranged in such a way as to maximise corner sharing of the resulting ZnCl 4 2− tetrahedra at the expense of edge and face sharing. The inability of molecular dynamics simulations, using purely ionic potentials, to predict the ZnZn component correlation function is taken as an indication of covalent character to the ZnCl bond. The crystalline polymorph formed on devitrification of dry ZnCl 2 glass is δ-ZnCl 2 and not the α-form as previously reported.

Effect of Chemical Modifications on Some Physicochemical Properties and Heat Coagulation of Egg Albumen

Abstract: Egg albumen was modified with succinic anhydride and a water-soluble carbodiimide, 1-ethyl-3(3-dimethylaminopropyl) carbodiimide. The chemically modified proteins exhibited only minor conformational changes, but were altered in their responses to heat treatment. Heat-induced coagulation was retarded by both modifications suggesting that thermocoagulation requires a balanced electrostatic attraction between protein molecules. Hydrophobicity measurements and pH-titration data suggest the involvement of hydrophobic and ionic interactions in heat aggregation and gelation of egg white proteins. The soluble fraction of the heat-coagulated proteins contained mainly monomers, while under conditions where a gel or coagulum was not formed, the heat-treated egg albumen solutions contained high M.W. soluble aggregates. A scheme for the thermal coagulation of egg albumen is proposed.

Numerical calculations relevant to the initial expansion of the polar wind

Abstract: The results of a numerical simulation of the expansion of an H(+)-0(+) electron plasma into a vacuum are reported. Variations were introduced in the ionic density ratios, the scale lengths for the density gradient at the plasma-vacuum interface, and the initial electron-ion temperature ratio. Conditions of a gravitationally bound O(+) plasma were also considered. It was found that if H(+) is a major ion, the H(+) density profile in the expansion region is always concave, with a phase-space divided into three regions, i.e., an undisturbed plasma, a rarefaction region, and an expansion region in which ions are accelerated. When the H(+) is a minor ion, the O(+) ions accelerate the H(+) ions, the H(+) phase-space is divided into five regions, and the increased electron temperatures produce enhanced electrostatic potentials and H(+) drifts in the expansion region.

Electrical conductivity of synthetic and natural quartz crystals

Abstract: Electrical conductivity of α quartz is ionic, the current being carried by alkali ions (M+) moving in channels parallel to the c axis. A study was made of conductivity σ of one natural and three synthetic crystals in order to better understand these ionic defects. ac measurements were made over a wide range of frequencies, and complex‐impedance analysis was used to separate out the bulk conductivity from electrode effects. The activation energy E for the synthetic crystals was always close to 1.36 eV and the pre‐exponential A was in the range of 106 K/Ω cm. A decrease in σ resulted from electrodiffusion (sweeping) in hydrogen. For the as‐received natural crystal, the value of E was 0.82 eV, but this E value increased appreciably after hydrogen sweeping or after irradiation plus high‐temperature anneal. The principal observations could not be interpreted in terms of the conventional theory in which the Al3+‐M+ pair and its dissociation products are the only defects involved. A modified theory, in which additional unassociated Al is present as compensation either for H+ or for oxygen vacancies, allows a detailed interpretation of the results. Supporting data are also obtained from infrared measurements.

Novel pi systems possessing cyclopropenylidene moiety

Abstract: As one of the most attractive Tr systems cyclopropenylidene metal complexes are prepared and their structure, especially carbene carbon—metal bonding is examined. In the lithium complex, this bonding is substantially ionic, on the other hand, in the transition metal complexes the carbene carbon—metal bond is shown to have double bond character by the ii back donation. The G donating and v accepting powers of cyclopropenylidenes change dramatically with presence or absence of amino substituent. Novel triafulvenes, triafulvalenes and related v system are taken up. The simplest triafulvenes and triafulvenes possessing conjugate Tr systems are synthesized. Triafulvalenes (3—5, 3—7, 3—9 systems) are also prepared. The structural features of these triafulvenes and triafulvalenes are discussed based on the spectral data. The parent compound of "cyclic dicalicene" which have peripheral l6v and central 8v structures is synthesized. The X ray analysis indicates that this novel system has planar (D2h) structure. Its v electron configuration is determined by i-H— and 1-3C—NMR spectra and MINDO/3 investigation.

Ionic dynamics in the LiCl–KCl system at liquid state

Abstract: The mixing effects in the fused system LiCl–KCl are studied by molecular dynamics simulation. The predominant contribution of the Coulombic interaction is clearly detected, its nonlinear variation gives rise to the main part of the excess internal energy. This change reflects the structural ionic arrangement; at high potassium concentration a significant decrease of the Li+–Cl− distance is observed. The anion–cation radius ratio has a marked effect: the lithium ions remain fourfold coordinated by chloride ions throughout the concentration range while the potassium ions are about sixfold coordinated. It is shown that the ion motion proceeds from two main mechanisms: an oscillatory displacement in the force field of the surrounding particles which tends to restore the electroneutrality and a collective mode which concerns the ion and its ionic atmosphere. This approach is described by the generalized Langevin equation and may open a better understanding of the transport processes at liquid state. It provides a link between the structure and the dynamical properties, the strong Li+–CL− interactions leading to a significant decrease of the Li+ mobility when the potassium concentration increases.

Recent developments in electron and photon stimulated desorption

Abstract: The realization by Knotek and Feibelman that core ionization can be an important primary mechanism in electron stimulated ionic desorption (ESID) from oxides, and the availability of synchrotron radiation as excitation source for detailed photon stimulated ionic desorption (PSID) studies, have stimulated a number of interesting experiments in the last two years. It is shown that—besides direct valence excitation—adsorbate core ionization is a new channel for ESID and PSID from covalent adsorbates on metals also, while metal core ionization rarely or never causes desorption in such systems. However, detailed threshold studies and comparison to Auger yields show that this cannot be simply explained by treating the core ionization/Auger decay sequence as a composite Franck–Condon transition to multiply valence ionized states. Rather, strong influences of many‐body interactions (e.g., shake ups) exist. This is not the case for ionic surface species; in these cases PSID can be used as species‐specific EXAFS probe. The investigation of desorption of neutrals, which constitute the overwhelming majority of products at low energies, shows that neutrals can be formed by reneutralization of ions. Additional channels exist below the absolute ionic thresholds; core ionization is not an important primary channel for neutrals production. The consequences of these findings are discussed in context with existing models.

On the Electronic Structure of the Polar Surface of Compound Crystals

Abstract: To cancel the average bulk macroscopic field compensating charge should be induced on the polar surface of compound crystals. In this paper we clarify the mechanism of the charge compensation by performing the DV-X α calculations on the clusters M 13 X 13 representing the polar surface of the rock-salt type crystals. The MgO (111) and TiC (111) surface are chosen as the example of the polar surfaces of the ionic and covalent crystals, respectively. Peculiar electronic structures of the polar surfaces leading to the charge compensation are revealed for the anion and the cation surface of these crystals. Especially metallic character of the cation surface is found, which is related to the enhanced chemical activities of these surfaces.

Towards a general scheme for estimating the heats of formation of organic ions in the gas phase. Part II. The effect of substitution at charge-bearing sites

Abstract: The effects on ionic heats of formation of the multiple substitution of -CH3, -OH, or -OCH3 groups, in place of H atoms, are described. It is shown that for all three substituents, good linear corr...

Structure and lattice vibrations of Mg-Al spinel solid solution

Abstract: X-ray structure refinements have been made for nonstoichiometric (MgO · 3Al2O3) and stoichiometric Mg-Al spinels. Several structure variations with chemical composition have been observed and are discussed in relation to Al substitution in tetrahedral sites. Infrared reflection and Raman spectra of the single crystal of the nonstoichiometric spinel (MgO · 3Al2O3) have been measured and analyzed. The results obtained are compared with those reported for the stoichiometric sample. From the infrared and Raman frequencies reported for the stoichiometric Mg-Al spinel, which are partly complemented with our results, the effective ionic charges of the ions in MgAl2O4 have been estimated on the basis of the rigid ion model.

Ab initio Cl calculation of the effects of Rydberg-valence mixing in the electronic spectrum of the HF molecule

Abstract: Experimental studies of the HF molecular spectrum have heretofore been unable to arrive at a suitably consistent theoretical assignment for the various measured band systems therein. To aid in this pursuit a series of ab initio Cl calculations in an AO basis containing 40 contracted gaussians has been carried out to an accuracy which is close to the full Cl limit as a result of the use of energy extrapolation techniques described elsewhere. In addition to obtaining generally quite good agreement with experimental spectroscopic constants including the dissociation energy, this treatment allows for a careful description of the change in composition of the HF ground state from ionic to covalent character as molecular stretching occurs, as well as a good representation of the upperB 1Σ+ state with which it undergoes a strongly avoided crossing. The repulsive branch of the latter potential energy curve is shown to intersect the Rydberg1Σ+ manifold at relatively short bond distances, leading to a series of mixed valence-Rydberg states which are ultimately responsible for the large deviations from normal Rydberg series which are observed experimentally. Similar crossings of σ→π* and σ→σ* valence states with Rydberg species of3,1π and/or3 Σ + symmetry are calculated to result in heavy mixing over only a relatively short range of bond distances, and thus do not produce the same magnitude of perturbations as do the1Σ+ states. Finally the calculated potential curve for the parentX 2π ionic state for such Rydberg species also proves to be quite consistent with known structural data, giving independent evidence for the overall high level of accuracy of the theoretical treatment employed in the present work.

Addition-compound formation between anionic and zwitter-ionic surfactants in water

Abstract: The Krafft points of binary mixtures of anionic and zwitterionic surfactants have been measured and found, for the first time, to show a maximum in Krafft point vs. composition curves can be attributed to the addition-compound (or intermolecular-compound) formation between anionic and zwitterionic surfactants in the solid phases. This addition-compound formation has been supported by infrared absorption spectra. The composition of the Krafft-point maximum which corresponds to that of the addition compound varies from 1/1 to 1/9 for mole ratios of anionic/zwitterionic surfactants, depending upon the combination of each component. Most of the micellar solution phases of these surfactant mixtures are viscoelastic in the composition ranges close to those of the addition compounds. The above result and the thermodynamic treatment for the Krafft point vs. composition curves strongly suggest that the addition compound (or intermolecular complexes) between anionic and zwitterionic surfactants are formed also in the micellar solution phases. 26 references.

Chemically bonded multifunctional stationary phases for high-performance liquid chromatography

Abstract: A new class of stationary phases for high-performance liquid chromatography (HPLC) are described which simulate in their retention chracteristics ion-pair separations. The phases consist of mixtures of chemically dissimilar ligands chemically bonded to silica supports. These phases are largely reversed-phase in nature, but also contain significant ion-exchange properties, at levels similar to those demonstrated to occur in ionpairing. By bonding both ionic and hydrophobic groups in the correct proportions, mixed retention mechanisms are created, resulting in unique selectivities, while retaining the excellent stabilities and efficiencies characteristic of bonded phases. The ratio of hydrophobic to ionic character can be controlled during the synthesis, and is used as a tool to vary the stationary phase, rather than only the mobile phase, to effect the separation desired. The synthesis and behavior of both anionic and cationic/reversed-phase materials are described, and are applied to the simultaneous separation of nucleosides and nucleotides, and to the separation of the catecholamines.