Showing papers on "Ionic bonding published in 2004"

Investigation of multiplet splitting of Fe 2p XPS spectra and bonding in iron compounds

Abstract: Ferrous (Fe2+) and ferric (Fe3+) compounds were investigated by XPS to determine the usefulness of calculated multiplet peaks to fit high-resolution iron 2p3/2 spectra from high-spin compounds. The multiplets were found to fit most spectra well, particularly when contributions attributed to surface peaks and shake-up satellites were included. This information was useful for fitting of the complex Fe 2p3/2 spectra for Fe3O4 where both Fe2+ and Fe3+ species are present. It was found that as the ionic bond character of the iron —ligand bond increased, the binding energy associated with either the ferrous or ferric 2p3/2 photoelectron peak also increased. This was determined to be due to the decrease in shielding of the iron cation by the more increasingly electronegative ligands. It was also observed that the difference in energy between a high-spin iron 2p3/2 peak and its corresponding shake-up satellite peak increased as the electronegativity of the ligand increased. The extrinsic loss spectra for ion oxides are also reported; these are as characteristic of each species as are the photoelectron peaks. Copyright © 2004 John Wiley & Sons, Ltd.

Ionic liquids and eutectic mixtures as solvent and template in synthesis of zeolite analogues

Abstract: The challenges associated with synthesizing porous materials1 mean that new classes of zeolites (zeotypes)—such as aluminosilicate zeolites2,3 and zeolite analogues4—together with new methods of preparing known zeotypes5, continue to be of great importance. Normally these materials are prepared hydrothermally with water as the solvent in a sealed autoclave under autogenous pressure6. The reaction mixture usually includes an organic template or ‘structure-directing agent’ that guides the synthesis pathway towards particular structures. Here we report the preparation of aluminophosphate zeolite analogues by using ionic liquids7 and eutectic mixtures8. An imidazolium-based ionic liquid acts as both solvent and template, leading to four zeotype frameworks under different experimental conditions. The structural characteristics of the materials can be traced back to the solvent chemistry used. Because of the vanishingly low vapour pressure of ionic liquids, synthesis takes place at ambient pressure, eliminating safety concerns associated with high hydrothermal pressures. The ionic liquid can also be recycled for further use. A choline chloride/urea eutectic mixture8 is also used in the preparation of a new zeotype framework.

Li-Storage via Heterogeneous Reaction in Selected Binary Metal Fluorides and Oxides

Abstract: A novel room-temperature molten salt (RTMS) electrolyte based on lithium bis(trifluoromethane sulfone) imide [LiN(SO2CF3)(2), LiTFSI] and acetamide has been prepared and investigated by ac impedance, nuclear magnetic resonance (NMR), Fourier transform infrared, and Raman spectroscopy. The ionic conductivity of the (acetamide)(n) . LiTFSI complex for n = 6 reaches 1.20 x 10(-3) S/cm at 25 degreesC and rises to 5.73 x 10(-3) S/cm at 60 degreesC. The behavior of ion transport obeys well the empirical Vogel-Tamman-Fulcher type relationship for the RTMS electrolyte. Strong cation-solvent interaction has been clarified by the variation of Li-7 NMR chemical shifts and the spectral evolution of the C = O group in the (acetamide)(n) . LiTFSI complex system. The effect of salt concentration and temperature upon the ionic association has also been investigated. It is found that ionic association increases with the increases of salt concentration and temperature in the RTMS electrolyte. (C) 2004 The Electrochemical Society.

Electronic and ionic transport properties and other physical aspects of perovskites

Abstract: The perovskites and perovskite-related structures exhibit several features of technical as well as fundamental interest. Technically useful properties include oxide-ion conduction with/without electronic conduction, oxidation catalysis, ferroic displacements in classic and relaxor ferroelectrics, half-metallic ferromagnetism and high-temperature superconductivity. Of more fundamental interest is the ability to tune, by chemical substitution on the large-cation subarray, transition-metal oxides through the crossover on the transition-metal array from localized dn configurations to itinerant d-electron behaviour without/with changing the valence state of that array. The localized-electron configurations may exhibit cooperative Jahn–Teller distortions that introduce anisotropic exchange interactions. At crossover, bond-length fluctuations may segregate into an ordered array of alternating covalent and ionic bonding in a single-valent perovskite; multicentre polarons or correlation bags may replace small polarons in a mixed-valent system. Bond-length fluctuations at crossover give vibronic conduction and suppression of the phonon contribution to the thermal conductivity; the fluctuations may order, to give high-temperature superconductivity, or transform to quantum–critical-point behaviour at lowest temperatures. Crossover of σ-bonding electrons in the presence of localized spins associated with π-bonding electrons gives rise to the colossal magnetoresistance phenomenon above a ferromagnetic Curie temperature.

Bacterial adhesion and transport in porous media: role of the secondary energy minimum.

Abstract: The adhesion of a well-characterized Escherichia coli bacterial strain to quartz sediment grains in the presence of repulsive electrostatic interactions is systematically examined. An increase in the ionic strength of the pore fluid results in an increase in bacterial attachment, despite DLVO calculations indicating a sizable electrostatic energy barrier to deposition. Bacterial deposition is likely occurring in the secondary energy minimum, which DLVO calculations indicate increases in depth with ionic strength. A decrease in the ionic strength of the pore fluid--thereby eliminating the secondary energy minimum--resulted in release of the majority of previously deposited bacteria, suggesting that these cells were deposited reversibly in the secondary minimum. Additionally, bacterial attachment to a quartz surface in a radial stagnation point flow system was absent at ionic strengths less than 0.01 M and resulted in attachment efficiencies over an order of magnitude lower than in the packed-bed column experiments at higher ionic strengths. Because of the hydrodynamics in the radial stagnation point flow system, this observation supports our conclusion that the majority of bacterial deposition in the packed bed occurs in a secondary energy minimum.

Ion Transport in Nanofluidic Channels

Abstract: Theoretical modeling of ionic distribution and transport in silica nanotubes, 30 nm in diameter and 5 μm long, suggest that when the diameter is smaller than the Debye length, a unipolar solution of counterions is created within the nanotube and the colons are electrostatically repelled By locally modifying the surface charge density through a gate electrode, the ion concentration can be depleted under the gate and the ionic current can be significantly suppressed It is proposed that this could form the basis of a unipolar ionic field-effect transistor

One-dimensional ion transport in self-organized columnar ionic liquids.

Abstract: New fan-shaped ionic liquids forming columnar liquid crystalline phases have been prepared to obtain one-dimensional ion-transporting materials. The ionic liquids consist of two incompatible parts: an imidazolium-based ionic part as an ion-conducting part and tris(alkyloxy)phenyl parts as insulating parts. Two compounds having octyl and dodecyl chains have been synthesized. Self-assembly of these materials leads to the formation of thermotropic hexagonal columnar liquid crystalline states at room temperature. Anisotropic one-dimensional ionic conductivities have been successfully measured by the cells having comb-shaped gold electrodes. The self-organized columns have been aligned macroscopically in two directions by shearing perpendicular and parallel to the electrodes. The ionic conductivities parallel to the column axis are higher than those perpendicular to the axis. The incorporation of lithium salts in these columnar materials leads to the enhancement of the ionic conductivities and their anisotrop...

Controlling the charge state of individual gold adatoms.

Abstract: The nature and control of individual metal atoms on insulators are of great importance in emerging atomic-scale technologies. Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states, which are stabilized by the large ionic polarizability of the film. The charge state and associated physical and chemical properties such as diffusion can be controlled by adding or removing a single electron to or from the adatom with a scanning tunneling microscope tip. The simple physical mechanism behind the charge bistability in this case suggests that this is a common phenomenon for adsorbates on polar insulating films.

Ionic liquids as stable solvents for ionic polymer transducers

Abstract: Nafion™membranes are known to operate as electromechanical actuators and sensors. The transduction in the material is caused by redistribution of the mobile cations in the material, which is made possible because the material is saturated with a solvent. Typically, the solvent used is water, although its use limits the performance of these materials. This is due to the chemical breakdown of the water at relatively low operating voltages and the loss of the water to evaporation when these devices are operated in air, causing a corresponding loss of performance. In the current work, the use of highly stable ionic liquids to replace water is explored. Ionic liquids have the advantage of greater electrochemical stability than water, thus offering the possibility of higher actuation voltages for these materials. Also, ionic liquids are known to be non-volatile and therefore will not evaporate out of the polymer as water will. In this work, the use of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ionic liquid is demonstrated as a viable solvent for Nafion™polymer actuators and sensors. This ionic liquid melts at −9 °C and has an electrochemical stability Window of 4.1 V [Inorg. Chem. 35 (1996) 1168], making it a promising candidate to replace water in ionic polymer transducers. Experimental results indicate that Nafion™transducers solvated with this ionic liquid have improved staility when operated in air as compared to the same materials solvated with water, although the magnitude of the response is decreased as compared to the water samples at high frequencies. The main drawback associated with the use of ionic liquids is a reduction in the speed of the response as compared to water, although the initial results are promising and demonstrate the potential for this approach.

Ideal shear strain of metals and ceramics

Abstract: Using density functional theory we analyze the stress-strain responses of 22 simple metals and ceramics to determine the maximum shear strain a homogeneous crystal can withstand, a property for which we suggest the name shearability. A shearability gap is found between metals and covalent ceramics. Shearability of metals further correlates with the degree of valence charge localization and directional bonding. Depending on the deformation constraints, ionic solids may possess even larger shearability than covalent solids. The Frenkel model of ideal shear strength works well for both metals and ceramics when shearability is used in the scaling.

Physical Chemistry of Ionic Materials: Ions and Electrons in Solids

Abstract: Preface. 1. Introduction. Motivation. The defect concept: Point defects as the main actors. 2. Bonding aspects: From atoms to solid state. Chemical bonding in simple molecules. Many atoms in contact: The solid state as a giant molecule. 3. Phonons. Einstein and Debye models. Complications. 4. Equilibrium thermodynamics of the perfect solid. Preliminary remarks. The formalism of equilibrium thermodynamics. Examples of equilibrium thermodynamics. 5. Equilibrium thermodynamics of the real solid. Preliminary remarks. Equilibrium thermodynamics of point defect formation. Equilibrium thermodynamics of electronic defects. Higher dimensional defects. Point defect reactions. Doping effects. Interactions between defects. Boundary layers and size effects. 6. Kinetics and irreversible thermodynamics. Transport and reaction. Electrical mobility. Phenomenological diffusion coefficients. Concentration profiles. Diffusion kinetics of stoichiometry change. Complications of matter transport. Surface reactions. Catalysis. Solid state reactions. 7. Solid state electrochemistry: Measurement techniques and applications. Preliminary remarks: Current and voltage in the light of defect chemistry. Open circuit cells. Cells under current load. Cells generating current. 8. Bibliography. Index.

Phase Behavior of Ionic Liquid−LiX Mixtures: Pyrrolidinium Cations and TFSI- Anions

Abstract: Phase diagrams are reported for mixtures between bis(trifluoromethanesulfonyl)imide (TFSI-) salts containing Li+ and N-alkyl-N-methylpyrrolidinium (PYR1R+) cations. The latter salts readily form both room temperature ionic liquids and plastic crystalline phases. Mixed salt crystalline phases are found which are likely to influence the performance of such mixed salt systems when utilized as electrolytes for electrochemical devices as well as give insight into ionic liquid−solute interactions.

Catalytic Ionic Hydrogenations

Abstract: Catalytic ionic hydrogenation of ketones occurs by proton transfer to a ketone from a cationic metal dihydride, followed by hydride transfer from a neutral metal hydride. This contrasts with traditional catalysts for ketone hydrogenation that require binding of the ketone to the metal and subsequent insertion of the ketone into a M-H bond. Ionic hydrogenation catalysts based on the inexpensive metals molybdenum and tungsten have been developed based on mechanistic understanding of the individual steps required in the catalytic reaction.

The Verwey transition: a new perspective

Abstract: This review puts in doubt the classical description of the Verwey (metal–insulator) transition in magnetite on the basis of the wide set of experiments carried out over the last 60 years. We re-analyse here the most relevant experiments used to study the Verwey transition from the point of view of their degree of agreement with the proposed Fe2+–Fe3+ charge ordering model. We will consider three groups of experimental studies, according to their capability of detecting different ionic species and/or a charge periodicity: (1) Experiments which have been interpreted using the charge ordering model as the starting point though they are not able to demonstrate its validity. This is the case for macroscopic properties such as the electrical resistivity, the heat capacity and the magnetic properties. (2) Experiments which can distinguish different types of Fe ions, such as Mossbauer, nuclear magnetic resonance (NMR) and electronic spectroscopies. However, we show that they are not able to associate them with a specific valence (2+ or 3+ in our case) and, in some cases, they observe more than two different kinds of iron atoms. (3) Diffraction (x-ray, neutron and electron) experiments, which are the most conclusive ones for determining a periodic ordering of different entities. These experiments, instead, point to the lack of ionic charge ordering. We will focus, in particular, on the discussion of the results of some recent x-ray resonant scattering experiments carried out on magnetite that directly prove the lack of ionic charge ordering in such mixed valence oxide. Furthermore, we also reconsider some so-called Verwey-type transition metal oxides in terms of the applicability of the Verwey charge ordering model. We show that a complete charge disproportionation (δ) is not experimentally observed in any of these compounds, the maximum δ being less than 0.5 e−. Regarding the theoretical framework, we will outline some relevant implications for the description of the physics of 3d transition metal oxides of this critical re-examination of the experimental facts on magnetite. Electronic localization should then occur involving more than one transition metal atom, so the definition of ionic d states loses its meaning in mixed valence transition metal oxides.

The partial hydrogenation of benzene to cyclohexene by nanoscale ruthenium catalysts in imidazolium ionic liquids.

Abstract: The controlled decomposition of an Ru(0) organometallic precursor dispersed in 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI.PF(6)), tetrafluoroborate (BMI.BF(4)) or trifluoromethane sulfonate (BMI.CF(3)SO(3)) ionic liquids with H(2) represents a simple and efficient method for the generation of Ru(0) nanoparticles. TEM analysis of these nanoparticles shows the formation of superstructures with diameters of approximately 57 nm that contain dispersed Ru(0) nanoparticles with diameters of 2.6+/-0.4 nm. These nanoparticles dispersed in the ionic liquids are efficient multiphase catalysts for the hydrogenation of alkenes and benzene under mild reaction conditions (4 atm, 75 degrees C). The ternary diagram (benzene/cyclohexene/BMI.PF(6)) indicated a maximum of 1 % cyclohexene concentration in BMI.PF(6), which is attained with 4 % benzene in the ionic phase. This solubility difference in the ionic liquid can be used for the extraction of cyclohexene during benzene hydrogenation by Ru catalysts suspended in BMI.PF(6). Selectivities of up to 39 % in cyclohexene can be attained at very low benzene conversion. Although the maximum yield of 2 % in cyclohexene is too low for technical applications, it represents a rare example of partial hydrogenation of benzene by soluble transition-metal nanoparticles.

Improved dye-sensitized solar cells using ionic nanocomposite gel electrolytes

Abstract: In this study, an effect of addition of nanoparticles into a dye-sensitized solar cells (DSCs) ionic liquid electrolyte was explored. Carbon nanotubes, other carbon nanoparticles and titanium dioxide nanoparticles were dispersed individually into a 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm-TFSI) ionic liquid electrolyte by grinding. It was centrifuged to form an ionic nanocomposite gel electrolyte. The dispersion of nanoparticles resulted in a substantial increase in their viscosity. Their electric conductivity increased as well. Notable effects were obtained in photocurrent density and voltage measurements of the DSC assembled with them. Energy conversion efficiency of them was significantly improved and increased compared with a DSC using a bare ionic liquid electrolyte.

Mechanisms of Ion and Water Transport in Perfluorosulfonated Ionomer Membranes for Fuel Cells

Abstract: To clarify transport mechanisms of ions and water molecules in perfluorosulfonated ionomer membranes, various membranes, such as one Nafion, two Aciplex, and four Flemion types, having different equivalent weight values (EW) were examined. H-, Li-, and Na-form samples were prepared for each membrane by immersion in 0.03 M HCl, LiCl, and NaCl aqueous solutions, and their properties in the fully hydrated state were investigated systematically. The water content of the membranes showed the tendency that the size and/or the number of ionic cluster region increases with decreasing EW value and the Li-form membranes have the most largely expanded ionic cluster regions. The ionic conductivity of the H-form membranes was considerably higher than that of the Li- and Na-form membranes. It was suggested that the proton in the membranes transports by the hopping mechanism and the Li+ and Na+ ions by the vehicle mechanism. In addition, the ionic conductivity of all membranes increased with increasing water content wit...

Atomic Charges for Classical Simulations of Polar Systems

Abstract: Structure and reactivity often are dependent on the polarity of chemical bonds. This relationship is reflected by atomic charges in classical (semiempirical) atomistic simulations; however, disagreement between atomic charges from accurate experimental investigations, ab initio methods, and semiempirical methods has not been resolved. Our aim is to improve the basic understanding of the polarity of compounds with a view to make force-field parametrizations more consistent and physically realistic. The concept is based on the relationship between the atomization energies of the elements and the possible strength of covalent bonding and the relationship between the ionization energies/electron affinities of the elements and the possible strength of ionic bonding. Both quantities, energetically, are of the same order of magnitude and influence atomic charges in a compound, which we illustrate by trends across the periodic table. The relationship between the pure elements and a given compound is shown in an e...

Structure, Reactivity, and Density Functional Theory Analysis of the Six-Electron Reductant, [(C5Me5)2U]2(μ-η6:η6-C6H6), Synthesized via a New Mode of (C5Me5)3M Reactivity

Abstract: The sterically crowded (C5Me5)3U complex reacts with KC8 or K/(18-crown-6) in benzene to form [(C5Me5)2U]2(-6:6-C6H6), 1, and KC5Me5. These reactions suggested that (C5Me5)3U could be susceptible to (C5Me5)1- substitution by benzene anions via ionic salt metathesis. To test this idea in the synthesis of a more conventional product, (C5Me5)3U was treated with KN(SiMe3)2 to form (C5Me5)2U[N(SiMe3)2] and KC5Me5. 1 has long U-C(C5Me5) bond distances comparable to (C5Me5)3U, and it too is susceptible to (C5Me5)1- substitution via ionic metathesis: 1 reacts with KN(SiMe3)2 to make its amide-substituted analogue {[(Me3Si)2N](C5Me5)U}2(-6:6-C6H6), 2. Complexes 1 and 2 have nonplanar C6H6-derived ligands sandwiched between the two uranium ions. 1 and 2 were examined by reactivity studies, electronic absorption spectroscopy, and density functional theory calculations. [(C5Me5)2U]2(-6:6-C6H6) functions as a six-electron reductant in its reaction with 3 equiv of cyclooctatetraene to form [(C5Me5)(C8H8)U]2(-3:3-C8H8), (C5Me5)2, and benzene. This multielectron transformation can be formally attributed to three different sources: two electrons from two U(III) centers, two electrons from sterically induced reduction by two (C5Me5)1- ligands, and two electrons from a bridging (C6H6)2- moiety.

Surface Ordering of Amphiphilic Ionic Liquids

Abstract: Neutron reflectometry measurements suggest that the interfacial structure normal to the free surface of two short-chain amphiphilic 1-alkyl-3-methylimidazolium-based ionic liquids is inhomogeneous, with the inhomogeneity extending ∼40 A. For the two liquids studied (1-butyl-3-methylimidazolium tetrafluoroborate and 1-octyl-3-methylimidazolium hexafluorophosphate), the data are consistent with a stratified surface model with distinct regions of alkyl chains and ionic headgroups.

Size dependent ion hydration, its asymmetry, and convergence to macroscopic behavior.

Abstract: The packing and orientation of water molecules in the vicinity of solutes strongly influence the solute hydration thermodynamics in aqueous solutions. Here we study the charge density dependent hydration of a broad range of spherical monovalent ionic solutes (with solute diameters from approximately 0.4 nm to 1.7 nm) through molecular dynamics simulations in the simple point charge model of water. Consistent with previous experimental and theoretical studies, we observe a distinct asymmetry in the structure and thermodynamics of hydration of ions. In particular, the free energy of hydration of negative ions is more favorable than that of positive ions of the same size. This asymmetry persists over the entire range of solute sizes and cannot be captured by a continuum description of the solvent. The favorable hydration of negative ions arises primarily from the asymmetric charge distribution in the water molecule itself, and is reflected in (i) a small positive electrostatic potential at the center of a neutral solute, and (ii) clear structural (packing and orientation) differences in the hydration shell of positive and negative ions. While the asymmetry arising from the positive potential can be quantified in a straightforward manner, that arising from the structural differences in the fully charged states is difficult to quantify. The structural differences are highest for the small ions and diminish with increasing ion size, converging to hydrophobiclike hydration structure for the largest ions studied here. We discuss semiempirical measures following Latimer, Pitzer, and Slansky [J. Chem. Phys. 7, 108 (1939)] that account for these structural differences through a shift in the ion radius. We find that these two contributions account completely for the asymmetry of hydration of positive and negative ions over the entire range of ion sizes studied here. We also present preliminary calculations of the dependence of ion hydration asymmetry on the choice of water model that demonstrate its sensitivity to the details of ion-water interactions.

First Principles Analysis of the Stability and Diffusion of Oxygen Vacancies in Metal Oxides

Abstract: Oxygen vacancies in metal oxides are known to determine their chemistry and physics. The properties of neutral oxygen vacancies in metal oxides of increasing complexity (MgO, CaO, � -Al2O3, and ZnO) have been studied using density functional theory. Vacancy formation energies, vacancy-vacancy interaction, and the barriers for vacancy migration are determined and rationalized in terms of the ionicity, the Madelung potential, and lattice relaxation. It is found that the Madelung potential controls the oxygen vacancy properties of highly ionic oxides whereas a more complex picture arises for covalent ZnO.

The polarization of mixed conducting SOFC cathodes: Effects of surface reaction coefficient, ionic conductivity and geometry

Abstract: Multi-dimensional finite element simulations of current distributions in mixed ionic and electronic conducting cathodes (MIEC) are presented for the case that the cathodic oxygen incorporation into an electrolyte takes place through the bulk of the electrode. The effects of the ionic conductivity and the surface reaction coefficient on the overall process are analyzed. Depending on these material parameters different parts of the cathode are involved in the oxide ion transport to the electrolyte (from a very small region close to the three phase boundary for a fast surface reaction up to the entire cathode for a very slow surface reaction). The calculations also reveal which combinations of ionic conductivity and surface reaction coefficient are appropriate to achieve acceptable polarization resistances. The influence of the particle size is discussed and interpolation formulae are given to estimate the cathodic polarization in porous MIECs.

Magnetoplastic effects in solids

Abstract: This paper is an overview of the studies into the effect of weak magnetic fields on the structure and mechanical properties of nonmagnetic solids of various nature (ionic, covalent, molecular, and metallic crystals, polymers, etc.). The various effects and aftereffects initiated by static, pulsed, and microwave magnetic fields that have been discovered over the past 15 years are classified and critically analyzed. The thermodynamic and kinetic aspects of the magnetic-field sensitivity of real solids with structural defects containing paramagnetic centers (electrons, holes, radicals, excitons, etc.) are discussed. Possible mechanisms for the effect of a weak magnetic field on the defect structure of crystals are considered. Special attention is given to the most developed chemical-physical theory of spin-dependent reactions between mobile particles and unpaired electrons. Interpretation of magnetoplastic effects is proposed in terms of the spin, electron, molecular, and dislocation dynamics of the complex multistage processes initiated by a magnetic field in a system of metastable structural defects.

Bonding, energies, and band offsets of Si-ZrO2 and HfO2 gate oxide interfaces.

Abstract: New oxides with high dielectric constant are required for gate oxides. ZrO2 is a typical example with ionic bonding. We give the rules for bonding at interfaces between Si and ionic oxides, to satisfy valence requirements and give an insulating interface. Total energies and band offsets are calculated for various (100)Si:ZrO(2) and HfO2 interface structures. The oxygen-terminated interface is found to be favored for devices, because it has no gap states and has a band offset which is rather independent of interfacial bonding.