Showing papers on "Ionic bonding published in 2006"

How Ionic Are Room-Temperature Ionic Liquids? An Indicator of the Physicochemical Properties

Abstract: Room-temperature ionic liquids (RTILs) are liquids consisting entirely of ions, and their important properties, e.g., negligible vapor pressure, are considered to result from the ionic nature. However, we do not know how ionic the RTILs are. The ionic nature of the RTILs is defined in this study as the molar conductivity ratio (Λimp/ΛNMR), calculated from the molar conductivity measured by the electrochemical impedance method (Λimp) and that estimated by use of pulse-field-gradient spin−echo NMR ionic self-diffusion coefficients and the Nernst−Einstein relation (ΛNMR). This ratio is compared with solvatochromic polarity scales: anionic donor ability (Lewis basicity), ET(30), hydrogen bond donor acidity (α), and dipolarity/polarizability (π*), as well as NMR chemical shifts. The Λimp/ΛNMR well illustrates the degree of cation−anion aggregation in the RTILs at equilibrium, which can be explained by the effects of anionic donor and cationic acceptor abilities for the RTILs having different anionic and catio...

Semiempirical van der Waals correction to the density functional description of solids and molecular structures

Abstract: The influence of a simple semiempirical van der Waals (vdW) correction on the description of dispersive, covalent, and ionic bonds within density functional theory is studied. The correction is based on the asymptotic London form of dispersive forces and a damping function for each pair of atoms. It thus depends solely on the properties of the two atoms irrespective of their environment and is numerically very efficient. The correction is tested in comparison with results obtained using the generalized gradient approximation or the local density approximation for exchange and correlation. The results are also compared with reference values from experiment or quantum chemistry methods. In order to probe the universality and transferability of the semiempirical vdW correction, a range of solids and molecular systems with covalent, heteropolar, and vdW bonds are studied.

Enhancement of the High-Rate Capability of Solid-State Lithium Batteries by Nanoscale Interfacial Modification

Abstract: Rechargeable lithium batteries are widely used in portable equipment today. However, there have always been safety issues arising from their combustible organic electrolytes. These issues are becoming more serious with the increasing size of batteries for use in electric vehicle (EV) or load-leveling applications. Nonflammable solid electrolytes would be the ultimate solution to the safety issue. Despite their high safety, the energy densities and power densities of solid systems have been too low for their practical use. We have succeeded in increasing their energy densities to levels comparable to those of liquid ones. However, power densities, or high-rate capabilities, remain poor. In this communication, we report that a buffer film with a thickness of only several nanometers interposed between the electrode and electrolyte materials improves the high-rate capability of solid-state lithium batteries. Low ionic conductivities of solid electrolytes have been the reason for the poor high-rate capability of solid-state lithium batteries. Although the conductivities of recently discovered solid electrolytes (> 10 S cm) are slightly lower than those observed for liquid electrolytes, Li ionic conduction in the solid electrolytes has become as fast as that of liquid electrolytes, by taking into account the fact that the transport number of ions in inorganic electrolytes is unity. However, the high-rate capability of solid systems, including solid electrolytes, remains inferior. This fact strongly suggests that the rate-controlling step is not in the bulk of the solid electrolytes, but at the interface between the electrode and the electrolyte materials. Ionic conduction at interfaces between different kinds of ionic conductors, or heterojunctions, is characterized by the term “nanoionics”; a frontier study was done for a LiI– Al2O3 composite, [7] and a sophisticated example was presented by Sata et al. In the latter, two kinds of F ion conductors, BaF2 and CaF2, were brought into contact with each other. Part of the F ions then transferred from one side to the other to reach an equilibrium, which produced vacancies in the former and interstitial ions in the latter, both of which contributed to ionic conduction at the interface and enhanced the ionic conduction. Similar nanoionic phenomena should take place at the interface between the electrode and the electrolyte materials, forming a space-charge layer. Because the compositions and structures of solid electrolytes have been well tailored to achieve high ionic conductivities, the ionic conductivity of the space-charge layer, where the compositions deviate from the optima, should be lower than that of the bulk, increasing the interfacial resistance. For instance, the Li4GeS4–Li3PS4 (thio-LISICON) system used in the present study has a high ionic conductivity of the order of 10 S cm at its optimum composition. However, variation in the composition reduces it to 10 S cm. The detrimental increase in the interfacial resistance would be prominent in bulk-type or non-thin-film solid-state lithium batteries. Sulfide electrolytes should be used in such batteries, because the ionic conductivities of oxide solid electrolytes, for example, are so low that they are available only in thin-film systems. On the other hand, the cathode should be an oxide, such as LiCoO2, because of its high electrode potential. [10,11]

Control of Electron Localization in Molecular Dissociation

Abstract: We demonstrated how the subcycle evolution of the electric field of light can be used to control the motion of bound electrons. Results are presented for the dissociative ionization of deuterium molecules (D2 ⇒ D+ + D), where asymmetric ejection of the ionic fragment reveals that light-driven intramolecular electronic motion before dissociation localizes the electron on one of the two D+ ions in a controlled way. The results extend subfemtosecond electron control to molecules and provide evidence of its usefulness in controlling reaction dynamics.

Proton conduction in rare-earth ortho-niobates and ortho-tantalates

Abstract: Some oxides contain sufficient equilibrium concentrations of protons in wet atmospheres to show useful proton conduction at elevated temperatures1. As an example, Y-doped BaCeO3 has shown promising performance as a thin-film electrolyte in fuel cells at intermediate temperatures (400–600 ∘C)2. In contrast to proton-conducting polymers (for example, Nafion(R)) and acid salts (for example, CsHSO4), such oxidic ceramics are stable at sufficiently elevated temperatures that electrode kinetics are fast and insensitive to poisoning, but they tend to be basic (Ba-based or Sr-based) compounds with poor chemical and mechanical stability3. In search of more stable proton-conducting materials, we have investigated several acceptor-doped rare-earth ortho-niobates and ortho-tantalates, RE1−xAxMO4 (M=Nb,Ta). We show that this class of materials shows mixed protonic, native ionic and electronic conduction depending on conditions. Both the low-temperature monoclinic and high-temperature tetragonal polymorphs show proton conduction. The proton conductivity is dominant in wet atmospheres below roughly 800∘C and the highest proton conductivity of approximately 10−3Scm−1 was found for Ca-doped LaNbO4. These transport characteristics can be used in sensors and fuel cells provided that the electrolyte film thickness is in the micrometre range.

Elements of Molecular and Biomolecular Electrochemistry: An Electrochemical Approach to Electron Transfer Chemistry

Abstract: Preface. Chapter 1. Single Electron Transfer at an Electrode. 1.1 Introduction. 1.2 Cyclic Voltammetry of Fast Electron Transfers. 1.3 Technical Aspects. 1.4 Electron Transfer Kinetics. 1.5 Successive One-Electron Transfers vs. Two-Electron Transfers. References and Notes. Chapter 2. Coupling of Electrode Electron Transfers with Homogeneous Chemical Reactions. 2.1 Introduction. 2.2 Establishing the Mechanisms and Measuring the Rate Constants for Homogeneous Reactions by Means of Cyclic Voltammetry and Potential Step Chronoamperometry. 2.3 Application of Redox Catalysis to the Kinetic Characterization of Fast Follow-up Reactions. 2.4 Product Distribution in Preparative Electrolysis. 2.5 Chemical Classification and Examples. 2.6 Redox Properties of Transient Radicals. 2.7 Electrochemistry as a Trigger for Radical Chemistry or Ionic Chemistry. References and Notes. Chapter 3. Electron Transfer, Bond Breaking, and Bond Formation. 3.1 Introduction. 3.2 Dissociative Electron Transfer. 3.3 Interactions Between Fragments in the Product Cluster. 3.4 Stepwise vs. Concerted Mechanisms. 3.5 Cleavage of Ion Radicals. Reactions of Radicals with Nucleophiles. 3.6 Role of Solvent in Ion-radical Cleavage and in Stepwise vs. Concerted Competitions. 3.7 Dichotomy and Connections between SN2 Reactions and Dissociative Electron Transfers. References and Notes. Chapter 4. Molecular catalysis of Electrochemical Reactions. 4.1 Introduction. 4.2 Homogeneous Molecular Catalysis. 4.3 Supported Molecular catalysis (Immobilized catalysts). References and Notes. Chapter 5. Enzymatic Catalysis of Electrochemical Reactions. 5.1 Introduction. 5.2 Homogeneous Enzymatic Catalysis. 5.3 Immobilized Enzymes in Monomolecular Layers. 5.4 Spatially Ordered Multimonomolecular Layered Enzyme coatings. References and Notes. Chapter 6. Appendixes. 6.1 Single Electron Transfer at an Electrode. 6.2 Coupling of Homogeneous Chemical Reactions with Electron transfer. 6.3 Electron Transfer, Bond Breaking, and Bond Formation. 6.4 Analysis of Supported Molecular catalysis by Rotating Disk Electrode Voltammetry. 6.5 Enzymatic catalysis Responses. References and Notes. Glossary of Symbols. Index.

Protic Ionic Liquids: Solvents with Tunable Phase Behavior and Physicochemical Properties

Abstract: The phase behavior, including glass, devitrification, solid crystal melting, and liquid boiling transitions, and physicochemical properties, including density, refractive index, viscosity, conductivity, and air−liquid surface tension, of a series of 25 protic ionic liquids and protic fused salts are presented along with structure−property comparisons. The protic fused salts were mostly liquid at room temperature, and many exhibited a glass transition occurring at low temperatures between −114 and −44 °C, and high fragility, with many having low viscosities, down to as low as 17 mPa·s at 25 °C, and ionic conductivities up to 43.8 S/cm at 25 °C. These protic solvents are easily prepared through the stoichiometric combination of a primary amine and Bronsted acid. They have poor ionic behavior when compared to the far more studied aprotic ionic liquids. However, some of the other physicochemical properties possessed by these solvents are highly promising and it is anticipated that these, or analogous protic s...

One-Dimensional Ion-Conductive Polymer Films: Alignment and Fixation of Ionic Channels Formed by Self-Organization of Polymerizable Columnar Liquid Crystals

Abstract: We have prepared two types of one-dimensional ion-conductive polymer films containing ion nanochannels that are both perpendicular and parallel to the film surface. These films have been obtained by photopolymerization of aligned columnar liquid crystals of a fan-shaped imidazolium salt having acrylate groups at the periphery. In the columnar structure, the ionic part self-assembles into the inner part of the column. The column is oriented macroscopically in two directions by different methods: orientation perpendicular to the modified surfaces of glass and indium tin oxide with 3-(aminopropyl)triethoxysilane and orientation parallel to a glass surface by mechanical shearing. Ionic conductivities have been measured for the films with columnar orientation vertical and parallel to the surface. Anisotropic ionic conductivities are observed for the oriented films fixed by photopolymerization. The ionic conductivities parallel to the columnar axis are higher than those perpendicular to the columnar axis because the lipophilic part functions as an ion-insulating part. The film with the columns oriented vertically to the surface shows an anisotropy of ionic conductivities higher than that of the film with the columns aligned parallel to the surface.

Physico-chemical processes in imidazolium ionic liquids.

Abstract: Among the various properties exhibited by ionic liquids (ILs)—especially those based on the imidazolium cation—their inherent ionic patterns, very low vapour pressure and pronounced self-organization in the solid, liquid and even in the gas phase are particularly interesting since this allows the use of these fluids as alternative and complementary media to classical organic solvents and water in many applications. Hence, reaction paths that involve charge-separated intermediates or transition states are accelerated—by lowering the activation barrier—in the presence of ILs when compared with those performed in classical organic solvents. It is also possible, for example, to observe, by electrochemical methods, transient species (ionic and radical) that are otherwise undetectible in water or in molecular organic solvents and to investigate the interactions and behaviour of molecular, biological and macromolecular species in solution using physical and chemical methods which require special conditions such as high-vacuum, and which have been traditionally limited to solid state chemistry.

Cyclic quaternary ammonium ionic liquids with perfluoroalkyltrifluoroborates: synthesis, characterization, and properties.

Abstract: New cyclic quaternary ammonium salts, composed of N-alkyl-(alkyl ether)-N-methylpyrrolidinium, -oxazolidinium, -piperidinium, or -morpholinium cations (alkyl=nC 4 H 9 , alkyl ether=CH 3 OCH 2 , CH 3 OCH 2 CH 2 ) and a perfluoroalkyltrifluoroborate anion ([R F BF 3 ] - , R F =CF 3 , C 2 F 5 , nC 3 F 7 , nC 4 F 9 ), were synthesized and characterized. Most of these salts are liquids at room temperature. The key properties of these salts-phase transitions, thermal stability, density, viscosity, conductivity, and electrochemical windows-were measured and compared to those of their corresponding [BF 4 ] - and [(CF 3 SO 2 ) 2 N] - salts. The structural effect on all the above properties was intensively studied in terms of the identity of the cation and anion, variation of the side chain in the cation (i.e., alkyl versus alkyl ether), and change in the length of the perfluoroalkyl group (R F ) in the [R F BF 3 ] - ion. The reduction of Li + ions and reoxidation of Li metal took place in pure N-butyl-N-methyl-pyrrolidinium pentafluoroethyltrifluor-oborate as the supporting electrolyte. Such comprehensive studies enhance the knowledge necessary to design and optimize ionic liquids for many applications, including electrolytes. Some of these new salts show desirable properties, including low melting points, high thermal stabilities, low viscosities, high conductivities, and wide electrochemical windows, and may thus be potential candidates for use as electrolytes in high-energy storage devices. In addition, many salts are ionic plastic crystals.

Half-metallic ferromagnetism in Cu-doped ZnO: Density functional calculations

Abstract: Half-metallic ferromagnetism in Cu-doped ZnO is predicted by accurate full-potential linearized augmented plane-wave and ${\mathrm{DMol}}^{3}$ calculations based on density functional theory. A net magnetic moment of $1{\ensuremath{\mu}}_{B}$ is found per Cu. At a Cu concentration of 12.5%, total energy calculations show that the ferromagnetic state is $43\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ lower than the antiferromagnetic state and is thus predicted to be the ground state with a ${T}_{c}$ estimated to be about $380\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The magnetic moments are localized within the $\mathrm{Cu}{\mathrm{O}}_{4}$ tetrahedron with ferromagnetic coupling between Cu and O. The electronic states near ${E}_{F}$ are dominated by strong hybridization between O $2p$ and Cu $3d$ which implies that the Cu-O bond is quite covalent instead of purely ionic. We examine the interplay between the carrier density and the ferromagnetism with N codoping and oxygen vacancies where we find no apparent relation between them. Oxygen vacancies tend to destroy the ferromagnetism and therefore should be avoided during sample fabrication. We found no clustering tendency of the Cu atoms. Since there is no magnetic element in this compound, Cu-doped ZnO appears to be an unambiguous dilute magnetic semiconductor where ferromagnetic precipitate problems can be avoided.

One-Step Conversion of Cellobiose to C6-Alcohols Using a Ruthenium Nanocluster Catalyst

Abstract: The one-step conversion of cellulose to C6-alcs. via green and energy efficient approaches has, as far as we are aware, not been reported. Such a process presents a considerable challenge, the two key problems being (1) finding a suitable solvent that dissolves the cellulose, and (2) the development of advanced catalytic chem. for selective cleavage of the C-O-C bonds (glycosidic bonds) connecting glucose residues. The dissoln. of cellulose has been recently realized by using ionic liqs. as green solvents; there is still no efficient method, such as selective hydrogenation, for the precise C-O-C cleavage under mild conditions, however. Cellobiose is a glucose dimer connected by a glycosidic bond and represents the simplest model mol. for cellulose. We disclose in this communication that the one-step conversion of cellobiose to C6-alcs. can be realized by selectively breaking the C-O-C bonds via selective hydrogenation using a water-sol. ruthenium nanocluster catalyst under 40 bar H2 pressure.

Salt screening and specific ion adsorption determine neutral-lipid membrane interactions

Abstract: The simplest, single-component biological membrane challenges accepted models of macromolecular interactions: lipid lamellar phases swell when immersed in monovalent salt solutions. Moreover, typical of a Hofmeister series, Br salts swell multilayers more than Cl salts, offering an excellent opportunity to investigate long-standing questions of ionic specificity. In accord with earlier measurements of liposome mobilities in electric fields, we find an added electrostatic repulsion of membranes due to anion binding, with a much stronger Br binding compared with Cl. However, contrary to the expectation that electrostatic repulsion should vanish in high salinity, swelling of lipid multilayers is monotonic with increasing salt concentration for both Br and Cl salts. The apparent contradiction is resolved by recognizing that although the electrostatic repulsion is progressively screened by increasing salt concentration, so is the van der Waals (vdW) attraction. Negligible in low salt, weakening of vdW forces becomes significant by the time electrostatic forces vanish. The result is a smooth monotonic swelling curve with no apparent distinction between low and high salt concentration regimes. Furthermore, when compared with theoretical predictions, measured vdW forces decay much too slowly with added salt. However, by accounting for the recently measured salt deficit near lipid bilayers, the expected scaling with Debye screening length is recovered. The combination of ion-specific binding and nonspecific ionic screening of low-frequency fluctuations explains salt effects on lipid membrane interactions and, by extension, explains specific (Hofmeister) effects at macromolecular interfaces between low and high dielectric.

Covalent bonding and the nature of band gaps in some half-Heusler compounds

Abstract: Half-Heusler compounds XYZ, also called semi-Heusler compounds, crystallize in the C1b MgAgAs structure, in the space group . We report a systematic examination of band gaps and the nature (covalent or ionic) of bonding in semiconducting 8- and 18-electron half-Heusler compounds through first-principles density functional calculations. We find that the most appropriate description of these compounds from the viewpoint of electronic structures is one of a YZ zinc blende lattice stuffed by the X ion. Simple valence rules are obeyed for bonding in the 8-electron compound. For example, LiMgN can be written Li+ + (MgN)− and (MgN)−, which is isoelectronic with (SiSi), forms a zinc blende lattice. The 18-electron compounds can similarly be considered as obeying valence rules. A semiconductor such as TiCoSb can be written Ti4+ + (CoSb)4−; the latter unit is isoelectronic and isostructural with zinc-blende GaSb. For both the 8- and the 18-electron compounds, when X is fixed as some electropositive cation, the computed band gap varies approximately as the difference in Pauling electronegativities of Y and Z. What is particularly exciting is that this simple idea of a covalently bonded YZ lattice can also be extended to the very important magnetic half-Heusler phases; we describe these as valence compounds, i.e. possessing a band gap at the Fermi energy albeit only in one spin direction. The local moment in these magnetic compounds resides on the X site.

Electronic Structure, Defect Chemistry, and Transport Properties of SrTi1-xFexO3-y Solid Solutions

Abstract: The electronic structure, defect chemistry, and transport properties of members of the mixed ionic electronic conducting SrTi1-xFexO3-y (STF) solid-solution system are revisited, and an improved defect chemical model is proposed in which Fe is considered to be one of the main constituents that shape the energy-band structure of STF, rather than an impurity dopant with acceptor-like character. As a consequence of the high inherent deficiency in the oxygen sublattice, introduced by the mixed-valence states of the B-site cations Ti4+ and Fe3+, oxygen vacancies and interstitials generated by the anion Frenkel reaction dominate the defect equilibria, leading to predominant ionic conductivity at intermediate partial pressures of oxygen. Increasing Fe content results in both a systematic decrease in band-gap energy, Eg0 = 3.2 − 1.9x + 0.5x2 eV, and reduction enthalpy, ΔHred = 5.8 − 3.4x + 1.7x2 eV. The decrease in band gap is explained on the basis of the systematic broadening of the Fe-derived 3d band lying abo...

Electrical properties and defect chemistry of TiO2 single crystal. I. Electrical conductivity.

Abstract: The present work reports the electrical properties of high-purity single-crystal TiO(2) from measurements of the electrical conductivity in the temperature range 1073-1323 K and in gas phases of controlled oxygen activities in the range 10(-13) to 10(5) Pa. The effect of the oxygen activity on the electrical conductivity indicates that oxygen vacancies are the predominant defects in the studied ranges of temperature and oxygen activities. The electronic and ionic lattice charge compensations were revealed at low and high oxygen activities, respectively. The determined semiconducting quantities include: the activation energy of the electrical conductivity (E(sigma) = 125-205 kJ.mol(-1)), the activation energies of the electrical conductivity components associated with electrons (E(n) = 218 kJ.mol(-1)), electron holes (E(p) = 34 kJ.mol(-1)), and ions (E(i) = 227 kJ.mol(-1)), and the enthalpy of motion for electronic defects (DeltaH(m) = 4 kJ/mol). The electrical conductivity data are considered in terms of the components related to electrons, holes, and ions. The obtained data allow the determination of the n-p demarcation line in terms of temperature and oxygen activities. The band gap determined from the electronic component of the electrical conductivity is 3.1 eV.

First-principles description of correlation effects in layered materials.

Abstract: We present a first-principles description of anisotropic materials characterized by having both weak (dispersionlike) and strong covalent bonds, based on the adiabatic-connection fluctuation-dissipation theorem with density functional theory. For hexagonal boron nitride the in-plane and out-of-plane bonding as well as vibrational dynamics are well described both at equilibrium and when the layers are pulled apart. Bonding in covalent and ionic solids is also described. The formalism allows us to ping down the deficiencies of common exchange-correlation functionals and provides insight toward the inclusion of dispersion interactions into the correlation functional.

Sodium carboxymethylcellulose-CTAB interaction: a detailed thermodynamic study of polymer-surfactant interaction with opposite charges.

Abstract: Interaction between polymer and surfactant bearing opposite charges is much more complex from a physicochemical point of view as compared to interaction between ionic surfactant and nonionic polymer. Electrostatic and hydrophobic interactions interplay in the former, whereas the hydrophobic effect is the prevailing factor in the latter. We have studied the interaction between a water-soluble polyanion, sodium salt of carboxymethylcellulose (NaCMC), with a cationic amphiphile, CTAB, in aqueous medium. There were manifold discrepancies with the reported works in NaCMC-alkyltrimethylammonium bromide, which is assumed to be an effect of difference in degree of substitution, which in turn affects the charge density of the polymer chain. We have noticed that the bulk complexation and interfacial interaction driven by electrostatic forces operate side by side. Thereafter, there is a wrapping process by the polyanion to the polymer-induced smaller surfactant aggregates driven by increase in entropy of the solution as a result of expulsion of the counterions from the ionic atmosphere around the surfactant aggregate. Because of the electrostatic interaction, hydrophobicity of the polymer-surfactant complex increases, leading to coacervation, and again solubilization in the hydrophobic core of the self-aggregated structure provided by the added excess CTAB. The tensiometric, conductometric, microcalorimetric, and turbidimetric techniques have been applied to address these problems.

Computation of methodology-independent ionic solvation free energies from molecular simulations. II. The hydration free energy of the sodium cation.

Abstract: The raw ionic solvation free energies computed from atomistic (explicit-solvent) simulations are extremely sensitive to the boundary conditions (finite or periodic system, system shape, and size) and treatment of electrostatic interactions (Coulombic, lattice sum, or cutoff based) used during these simulations. In the present article, it is shown that correction terms can be derived for the effect of (A) an incorrect solvent polarization around the ion due to the use of an approximate (not strictly Coulombic) electrostatic scheme; (B) the finite size or artificial periodicity of the simulated system; (C) an improper summation scheme to evaluate the potential at the ion site and the possible presence of a liquid-vacuum interface in the simulated system. Taking the hydration free energy of the sodium cation as a test case, it is shown that the raw solvation free energies obtained using seven different types of boundary conditions and electrostatic schemes commonly used in explicit-solvent simulations (for a total of 72 simulations differing in the corresponding simulation parameters) can be corrected so as to obtain a consistent value for this quantity.

Solid-state acid-base interactions in complexes of heterocyclic bases with dicarboxylic acids: crystallography, hydrogen bond analysis, and 15N NMR spectroscopy.

Abstract: A cancer candidate, compound 1, is a weak base with two heterocyclic basic nitrogens and five hydrogen-bonding functional groups, and is sparingly soluble in water rendering it unsuitable for pharmaceutical development. The crystalline acid−base pairs of 1, collectively termed solid acid−base complexes, provide significant increases in the solubility and bioavailability compared to the free base, 1. Three dicarboxylic acid−base complexes, sesquisuccinate 2, dimalonate 3, and dimaleate 4, show the most favorable physicochemical profiles and are studied in greater detail. The structural analyses of the three complexes using crystal structure and solid-state NMR reveal that the proton-transfer behavior in these organic acid−base complexes vary successively correlating with ΔpKa. As a result, 2 is a neutral complex, 3 is a mixed ionic and zwitterionic complex and 4 is an ionic salt. The addition of the acidic components leads to maximized hydrogen bond interactions forming extended three-dimensional networks....

Ionic conduction in the solid state

Abstract: Solid state ionic conductors are important from an industrial viewpoint. A variety of such conductors have been found. In order to understand the reasons for high ionic conductivity in these solids, there have been a number of experimental, theoretical and computational studies in the literature. We provide here a survey of these investigations with focus on what is known and elaborate on issues that still remain unresolved. Conductivity depends on a number of factors such as presence of interstitial sites, ion size, temperature, crystal structure etc. We discuss the recent results from atomistic computer simulations on the dependence of conductivity in NASICONs as a function of composition, temperature, phase change and cation among others. A new potential for modelling of NASICON structure that has been proposed is also discussed.

Additivity in the optical kerr effect spectra of binary ionic liquid mixtures: implications for nanostructural organization.

Abstract: Low-frequency spectra of binary room-temperature ionic liquid (RTIL) mixtures of 1-pentyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and 1-pentyl-3-methylimidazolium bromide in the 0−250 cm-1 region were studied as a function of mole fraction at 295 K. The spectra were obtained by use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The spectra of these binary mixtures are well described by the weighted sums of the spectra for the neat RTILs. This surprising result implies that the intermolecular modes giving rise to the spectra of the neat liquids must also produce the spectra of the mixtures. Additivity of the OKE spectra can be explained by a model in which locally ordered domains are assumed to exist in the neat liquid with the structures of these locally ordered domains preserved upon mixing. Recently published molecular dynamics simulations show that RTILs are nanostructurally organized with ionic networks and nonpolar regions. If ionic networks also exi...

Computation of methodology-independent ionic solvation free energies from molecular simulations. I. The electrostatic potential in molecular liquids.

Abstract: The computation of ionic solvation free energies from atomistic simulations is a surprisingly difficult problem that has found no satisfactory solution for more than 15 years The reason is that the charging free energies evaluated from such simulations are affected by very large errors One of these is related to the choice of a specific convention for summing up the contributions of solvent charges to the electrostatic potential in the ionic cavity, namely, on the basis of point charges within entire solvent molecules (M scheme) or on the basis of individual point charges (P scheme) The use of an inappropriate convention may lead to a charge-independent offset in the calculated potential, which depends on the details of the summation scheme, on the quadrupole-moment trace of the solvent molecule, and on the approximate form used to represent electrostatic interactions in the system However, whether the M or P scheme (if any) represents the appropriate convention is still a matter of on-going debate The goal of the present article is to settle this long-standing controversy by carefully analyzing (both analytically and numerically) the properties of the electrostatic potential in molecular liquids (and inside cavities within them) Restricting the discussion to real liquids of "spherical" solvent molecules (represented by a classical solvent model with a single van der Waals interaction site), it is concluded that (i) for Coulombic (or straight-cutoff truncated) electrostatic interactions, the M scheme is the appropriate way of calculating the electrostatic potential; (ii) for non-Coulombic interactions deriving from a continuously differentiable function, both M and P schemes generally deliver an incorrect result (for which an analytical correction must be applied); and (iii) finite-temperature effects, including intermolecular orientation correlations and a preferential orientational structure in the neighborhood of a liquid-vacuum interface, must be taken into account Applications of these results to the computation methodology-independent ionic solvation free energies from molecular simulations will be the scope of a forthcoming article

Structural stability and pressure-induced phase transitions in MgH 2

Abstract: The structural stability of ${\mathrm{MgH}}_{2}$ has been studied up to $16\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ using a high-pressure synchrotron x-ray diffraction technique. Several pressure-induced phase transitions have been identified in this pressure range. Owing to the close structural similarity between the $\ensuremath{\alpha}$ and $\ensuremath{\gamma}$ modifications the high-pressure $\ensuremath{\gamma}$ form can be stabilized as a metastable phase after pressure release. The experimentally observed structural transition sequence and the volume changes at the transition points as well as bulk modulii are found to be in good agreement with theoretically calculated data. The bonding nature of ${\mathrm{MgH}}_{2}$ is analyzed with the help of charge-density, charge-transfer, electron-localization-function, and Mulliken-population analyses which clearly show that all polymorphs of ${\mathrm{MgH}}_{2}$ are to be classified as ionic materials with Mg and H in nearly 2+ and $1\ensuremath{-}$ states, respectively.

Impact of porous electrode properties on the electrochemical transfer coefficient.

Abstract: The rate of an activation-controlled electrochemical reaction is determined by two key parameters, the exchange current density, io, and the transfer coefficient, α, which is inversely related to the Tafel slope. Assuming that the symmetry factor, β, is 0.5, the minimum α value should be 0.5 for all standard reaction mechanisms, with α values larger than this indicating a better electrocatalytic mechanism. The primary goal of this paper is to better understand why α values of <0.5 are often observed experimentally, with specific examples given for the oxygen reduction reaction. These low α values cannot be explained by adsorption behavior, but they can result when reactions occur within a porous electrode structure. Consistent with past literature related to Tafel slope predictions, we show that long and narrow pores, a low ionic or electronic conductivity of the electrode layer, and a high io value can cause α to be <0.5, most typically 0.25. However, α values between 0.25 and 0.5 are also encountered in...

Molecular Simulation Study of Some Thermophysical and Transport Properties of Triazolium-Based Ionic Liquids

Abstract: Results of a molecular dynamics study of several triazolium-based ionic liquids are reported. Triazolium cations include 1,2,4-triazolium, 1,2,3-triazolium, 4-amino-1,2,4-triazolium, and 1-methyl-4-amino-1,2,4-triazolium. Each cation was paired with a nitrate or perchlorate anion. These materials are part of a class of ionic compounds that have been synthesized recently but for which little physical property data are available. Properties of the more common ionic liquid, 1-n-butyl-3-methylimidazolium nitrate, are also computed and compared with the properties of the triazolium-based compounds. A molecular mechanics force field was developed for these materials using a mix of ab initio calculations and parameter fitting using the molecular compound 1H-1,2,4-triazole as a basis for the triazolium cations. Liquid-phase properties that were computed include heat capacities, cohesive energy densities, gravimetric densities/molar volumes as a function of temperature and pressure, self-diffusivities, rotational time constants, and various pair correlation functions. In the solid phase, heat capacities and lattice parameters were computed. Of all of these properties, only lattice parameters have been measured experimentally (and only for four of the triazolium compounds). The agreement with the experimental crystal structures was good. When compared with that of the imidazolium-based ionic liquid, the triazolium-based materials have much smaller molar volumes, higher cohesive energy densities, and larger specific heat capacities. They also tend to be less compressible, have a higher gravimetric density, and have faster rotational dynamics but similar translational dynamics.

Basics of oxygen and SnO2 interaction; work function change and conductivity measurements

Abstract: The interaction of oxygen and nanocrystalline SnO 2 has been investigated by means of simultaneous work function change and resistance measurements. Various oxygen concentrations between 50 ppm and 6000 ppm have been dosed at two different temperatures. At 400 °C, only ionic species have been identified. However, at 200 °C dipolar species play the major role reflected by a massive decrease in electron affinity.