Frequency-dependent conductivity in ionic glasses: A possible model

The frequency-dependent (ac) conductivity behaviour of ionically conducting glasses is discussed. A critique is given of the treatment of Almond and West for this behaviour. Two models are proposed instead to account for the observed ac loss data. One involves parallel relaxation processes and is a modification of a model originally developed for the case of large-polaron transport in amorphous semiconductors. The other is based on series relaxation, and is based on a model originally proposed by Glarum for the case of dielectric relaxation in molecular liquids. In the development given here, this model is modified so as to describe ionic transport, and a new microscopic transport mechanism is proposed—“diffusion-controlled relaxation”—which satisfies the requirements of the Glarum theory and which naturally incorporates cooperative ionic motions. For this reason, it is intuitively more appealing.     

Ionic effects on overstretching transition of B-DNA

A three-dimensional model is proposed herein to study the ionic effects of a NaCl solution on the overstretching transition of long B-DNA molecules. In this model, the bending deformations of DNA backbones, cooperativity of base-stacking interactions, electrostatic interactions, and spatial effects of the DNA double-helix structure are taken into account. The energy of electrostatic interactions is given as a function of the ionic strength and of the folding angle based on the Poisson-Boltzmann equation. An expression is derived, which shows that the overstretching force is linear with the natural logarithm of the ionic strength. The analytical results of this model are in good agreement with recently reported experimental results.     

Thermoelectric power of mixed electronic-ionic conductors I. Basic equations

The present work considers thermopower of oxide materials within n-p transition regime. Specifically, basic equations describing the effect of thermocell reactions on both ionic and electronic component of thermoelectric power are derived. The proposed formalism considers the impact of gas/solid reactions on the relationship between thermopower and electrochemical potential within a system involving a metal oxide of nonstoichiometric composition and a metal (such as Pt) that is applied as a measuring electrode. The derived theoretical model allows the determination of the thermopower components corresponding to different charge carriers, including ions, electrons and electron holes, for metal oxides. The proposed model may be used for derivation of defect chemistry models based on thermopower data that are free of the ionic component.     

Modified equation of state for pure and hard sphere mixtures in mean ionic activity coefficient calculations by mean spherical approximation model

A hard sphere equation of state (EOS) based on tetrakaidecahedron cell geometry (instead of spherical shape) and highly optimized molecular dynamic simulation data is proposed. The EOS is extended to hard sphere mixture and its performance for compressibility factor calculation at different diameter size of hard sphere mixtures by using various mixing rule is compared with Monte Carlo simulation data. The results indicated that for all mixing rules, the proposed EOS has minimum error comparing with computer simulation data. Also the residual prosperities are derived by using the proposed EOS. The residual properties are used in mean spherical approximation model (MSA) to evaluate the mean ionic activity coefficient of aqueous electrolyte solutions. The results are compared with those obtained by similar hard sphere equations of state and it is shown that the proposed EOS has a better performance in predicting the mean ionic activity coefficient.     

Phonon dispersion relations of tetrahedrally-bonded crystals

The lattice dynamics of the tetrahedrally-bonded compounds is formulated from first principle by using a model of binding force proposed by us, which mainly includes covalent interactions in the perturbational treatment based on the pseudopotential formalism and also partly on ionic interactions. Numerical calculations are performed for GaP and ZnS. In spite of our introducing no adjustable parameter except the optical phonon-frequency splitting in the long-wavelength limit, the resulting phonon dispersion curves are qualitatively in good agreement with the observed data. We show that ionic contributions are important for the optical phonon frequencies of these compounds.     

Lattice dynamics of thorium

In the present work, a local model pseudopotential has been proposed to study the lattice dynamics of thorium. The model potential depends on the core and ionic radii, and accounts for the s-d-f hybridization effects in a phenomenological way. When this form of potential is applied to derive the phonon dispersion curves of Th, sufficiently good agreement is found between the computed and experimental results.     

用半经验CNDO/INDO方法计算离子晶体中的电子态

本文报道在分子簇(MC)模型的框架内,用半经验CNDO/INDO方法对离子晶体及其点缺陷的电子态进行计算。引用计算分子结构的CNDO/INDO程序加以改造,使之适合于离子晶体及其点缺陷的计算。在此基础上,计算了LiF,NaF,LiCl,NaCl四种晶体的价带宽、禁带宽、有效离子电荷等参数和F色心基态能级的位置。所得结果与以前报道的各种计算和实验结果进行了比较。 关键词：     

Ionic pseudopotential for semiconductors without cut-off parameter for core-repulsion with application to Si

An ionic pseudopotential for semiconductors is proposed, which consists of a set of continuous exponential functions. Introduced damping and amplitude parameters into the pseudopotential are to be treated as adjustable. The most important features of the proposed pseudopotential is that (1) it has no sharp cut-off parameter for the core-repulsion and (2) it is continuous and has continuous derivatives to arbitrary order. The proposed pseudopotential is applied to Si and the adjustable parameters are determined so as to be consistent with the Si crystal empirical pseudopotential of high quality by taking a valence electron dielectric screening effect into account. The effectiveness of the proposed ionic pseudopotential is discussed by (1) comparing the calculated ionic energy levels of Si with experiments, (2) checking the consistency between the ionic and crystal pseudopotentials for Si, and so on.     

Modified Kolbensvedt model for the electron impact K-shell ionization cross-sections of atoms and ions

A modification of the Kolbensvedt model, MKLV, in terms of ionic, correlation (between the incident and target electrons) and relativistic corrections, is proposed to calculate the K-shell electron impact ionization cross-sections of neutral and ionic targets. The modified model, with a single set of parameters, is found to reproduce satisfactorily the experimental data for the neutral H, Al, Ar, Mn, Ge, Mo, Ag, Sn, Au, Bi and U and ionic Li⁺, B⁴⁺ and O⁷⁺ targets better than the existing empirical models from threshold energies to as high as 10⁴ MeV. For the Ag target, the calculations from MKLV follow closely the results of Scofield, and for Au, those of Scofield, and Ndefru and Malik, both done in the relativistic Born approximation with the M?ller interaction, at energies higher than the peak region.     

Theoretical Study of Staging Structure in the Neutral-Ionic Transition of Charge-Transfer Complexes

Accompanying the neutral-ionic transition, the co-existence of the neutral and ionic states has been observed in mixed-stack charge-transfer (CT) complexes, (3,3',5,5')-tetramethylbenzidine-(7,7,8,8)tetracyanoquinodimethane (TMB-TCNQ), dimethyltetrathiafulvalene- p -chloranil (DMTTF-CA), and tetrathiafulvalene- p -chloranil (TTF-CA). We calculated electrostatic energies in these crystals in order to investigate the stability of the staging structure proposed for the co-existing state, that is, a superstructure consisting of an alternative sequence of the ionic and neutral domains. We took account of intramolecular charge distributions and the extent of molecules. It was found that the staging state is slightly stable than the uniform ionic and neutral states in TMB-TCNQ. The stability is determined by the difference of the intramolecular charge distribution between the ionic and neutral states. This allows the staging state even under the attractive interaction between the stacking chains, in contrast to the original model of the state. It was found that the interaction is attractive in the three materials.     

On the compressed-ion model of cohesion in alkali halides

First-principle local-density calculations of the total energy of ions confined within a sphere were performed The results for positive alkali and negative halogen ions are used to test the validity of the compressed-ion model for the cohesion of alkali halides recently proposed by Narayan and Ramaseshan It was found that the model is not realistic since the calculated ionic compression energies are by far too large to account for the observed equilibrium lattice constants and bulk moduli of alkali halides.     

A Monte-Carlo calculation of the secondary electron emission of normal metals: I. The model

A model is proposed to describe the secondary electron emission (SEE) of pure normal metals. The various types of collisions suffered by an electron travelling in the solid are analyzed. The dielectric theory is used to account for the collisions with the jellium, including surface effects in an approximate way. The different damping mechanisms for plasmons are reviewed and their influence on SEE is emphasized. The elastic and ionizing collisions with the ionic cores are described by a randium model. The relative importance of each of the above processes for the SEE of normal metals is discussed.     

A model for the heat of transport in ionic conductors

The ion flow caused by a temperature gradient originates the ionic thermopower which is quantified by the heat of transport. Experimentally, it is known that in superionic conductors, the heat of transport Q is nearly equal to the activation energy for ion transport E_a. In the present paper, a model for the heat of transport in ionic conductors has been proposed based on a lattice dynamical theory of diffusion. We have shown that the relationship between Q and E_a is determined by the participation degree of different phonon modes, in particular the short wavelength phonons to the atomic jump processes. The implication of this finding to the transport properties of superionic conductors has been discussed, and it is suggested that the degree of the collective motion in ionic conductors increases with the increase in Q/E_a. The model predicts that good ionic conductors will show large value of Q/E_a. The importance of the acoustic phonons in the ion transport processes has been also pointed out.     

Temperature dependence of phonon linewidth in β-AgI

The temperature dependence of the linewidth and frequency of a TO one-phonon mode of β-AgI is measured using far infrared and Raman spectroscopy. Near the β-α transition an anomalous damping is observed. A very simple model is proposed to explain this damping by interactions between phonons and ionic carriers.     

Statistical interpretation of the kinetic equation in the adsorption problem

A statistical interpretation for the adsorption phenomenon is presented in the framework of the Maxwell-Boltzmann statistics. A model for an adsorbing surface is proposed and the connection between the phenomenological parameters, entering in the kinetic equation at the boundary surfaces, with the microscopic model is derived.Received: 14 June 2004, Published online: 14 September 2004PACS: 66.10.-x Diffusion and ionic conduction in liquids - 68.43.-h Chemisorption/physisorption: adsorbates on surfaces - 68.43.Mn Adsorption/desorption kinetics     

Local ionic motion in the superionic conductor Li3N

Anisotropic dielectric behavior of the superionic conductor lithium nitride (Li₃N) is reported for low temperature where the dc ionic conductivity can be neglected (T<17oK). For E ⊥ c the static dielectric constant followed a Langevin-Debye (ε ∝ 1/T) type law from 170 to 80K which changed to a temperature independent value below 40K. Debye relaxations were measured between 10 and 50K in the frequency range 10Hz to 1MHz. The observed properties are discussed in terms of a locally restricted ionic motion in shallow potentials. Different types of potentials are considered and an overdamped oscillator model is proposed.     

Parabolic oxidation of metals in homogeneous electric fields

A previously proposed thin film parabolic growth law (Fromhold, 1963) is extended to include film growth due to any number of diffusing defect species of arbitrary valence, and an analysis is made of the effects of applying external electrostatic potentials during oxidation. The total electrical conductivity and the partial conductivities are markedly position-dependent in the protective film, varying by orders of magnitude from one interface to the other. The built in electrostatic potential across the film is independent of thickness of the film and is a function of the partial conductivities of the diffusing ionic and electronic defect species. Effects of electrical shorting of the oxide film by external circuitry are analyzed. Depending on polarity, a constant applied potential can increase or decrease the rate constant but does not alter the kinetics from the parabolic form, in accordance with published experimental data. The net electrostatic potential required to stop metal oxidation is derived for the model in question. For growth by a single ionic species, the stopping potential is that electrostatic potential which gives an equal electrochemical potential at the metal-oxide and the oxideoxygen interfaces. For growth by multiple ionic species, the stopping potential is a function of the ionic partial conductivities.     

A self-consistent model for the electronic component of a thin current sheet in the Earth’s magnetosphere

A self-consistent model for the electronic component of a thin current sheet (TCS) of the Earth’s magnetosphere based on the concept of the tension of lines of force is proposed and studied analytically and numerically. It has been shown that the model has a certain range of applicability, beyond which the results may be incorrect; within the range of correctness the model allows for a quite significant increase in the rate of convergence of the iterative algorithm for solving the total self-consistent model of the TCS, which also includes the Boltzmann equations for the ionic component.     

Mixed (ionic and hole) conductivity of Tl<Subscript>3</Subscript>VS<Subscript>4</Subscript> crystals

A model of mixed (ionic and hole) conductivity in Tl₃VS₄ crystals at close-to-room temperatures is proposed. The significant fraction of the ionic conductivity component (～70% of the total conductivity) is explained by the nonstoichiometric electrically active thallium vacancies, whose acceptor levels provide p-type conductivity. The characteristic time dependence of the voltage developing across a sample due to its polarization and depolarization is described using the diffusion theory of mixed conductivity previously developed by Yokota. The charge transport phenomena in Tl₃VS₄ are studied experimentally, and the data are processed according to the theoretical model.