Effective Interionic Potential Research Articles

Temperature-dependent structure of molten eutectic Cu37Sb63

Quoted measurements of viscosity and our results of electrical resistivity experiments indicate two anomalous changes of physical properties take place in molten eutectic Cu37Sb63. Effective pair inter-ionic potentials evaluated using the hyper-netted chain theory further confirms the presence of two discontinuous structural transitions within the ranges from 1273K to 1073K and from 1073K to 903K, respectively. Moreover, the result of electrical resistivity experiments suggests the two structural transitions for molten Cu37Sb63 are reversible.

Pair-effective interionic interaction and metallization of helium

The pair-effective interionic interaction in metallic helium has been calculated at various densities. The terms of the second and third order of perturbation theory have been taken into account in the potential of the pair interaction. The metallic state of helium has been analyzed for both singly and doubly ionized helium atoms. In the second state, which corresponds to higher densities, the potential of the electron-ion interaction is of Coulomb type. The pair-effective interionic potential at densities of several grams per cubic centimeter is characteristic of metals with potential wells of several thousand Kelvins, which makes it possible to assume the existence of a stable liquid metallic phase for helium at these densities and temperatures. In the first case, the simulated potential is applied, the single parameter of which is determined based on the assumption that the transition of helium into a metallic state occurs at the same density of electric current as in the case of hydrogen metallization. In this metallic phase pair, the effective interionic potential is also characteristic of metals; however, the potential well for helium is significantly smaller than for hydrogen and decreases faster with an increase in the electron density. The opposite is true for the semiconductor phase, i.e., the potential well is significantly larger for helium than for hydrogen and increases faster with a decrease in electron density. In this case, significantly higher temperatures are required for the thermal activation of metallic conductance.

Effective pair interionic interactions in supercompressed helium

The effective pair interionic interaction has been calculated for liquid helium in the metallic state. The potential was found at the second order of the perturbation theory in the electron-ion interaction. Because the depth of the potential well depends heavily on the method of approximation of the exchange interaction and electron gas correlations, the calculations were performed for several approximations. The authors propose an original form of the approximation. The effective pair interionic potentials of metallic hydrogen and helium were compared. An assumption about the possibility of the existance of metallic helium after the removal of external pressure was made.

Pressure induced B3-B1 structural phase transition and elastic properties of monopnictides InX (X = N, P, As)

An effective interionic interaction potential is developed to discuss the pressure induced structural phase transformation and mechanical properties of InX (X = N, P, As) semiconducting compounds. The effective interionic potential consists of the long-range Coulomb and three-body interactions and the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbour ions and the van der Waals interaction. The present calculations have revealed reasonably good agreement with the available experimental data on the phase transition pressures (Pt = 11.5, 10, 7.5 GPa) and the elastic properties of InX (X = N, P, As). The equation of state curves (plotted between V (P)/V(0) and pressure) for both the structures zincblende (B3) and rocksalt (B1) structures obtained by us are in fairly good agreement with the experimental results. The calculated values of the volume collapses [ΔV(P)/V(0)] are also closer to their observed data.

Encoding electronic structure information in potentials for multi-scale simulations: SiO 2

Potentials generally used in molecular dynamics (MD) simulation of SiO 2 properties customarily are calibrated to a combination of computed molecular electronic structure data and experimental crystalline data. The present study tests parametrization to data from high-level, first-principles electronic structure calculations alone. The issue is crucial to the success of multi-scale simulations. They require a consistent embedding of the so-called quantum mechanical region (the region in which the forces come from gradients of quantum mechanical total energies) in a classical inter-ionic potential region. The evident challenge is generation of a quantum mechanically consistent parametrization. A simple probe of the issue is to see how parametrization solely from first-principles data influences the simulation outcomes. We parametrized a widely used form of effective inter-ionic potential for SiO 2 and did MD simulations of tensile failure in a 72 formula unit SiO 2 nanorod. Separate parametrizations were done to high quality calculated data for H 4SiO 4 and H 6Si 2O 7 clusters and for α-quartz. The differing parametrizations yield quantitative differences in the prediction of the yield strength and even semi-qualitative differences in the system behavior in that region. Some superficially similar parametrizations do not even provide a stable T = 0 K configuration. These differences highlight the crucial distinction between potential parametrization aimed at replacing realistic quantum mechanical forces entirely in an MD calculation versus a parametrization aimed at embedding an explicitly QM region.

Effective Interaction Potentials for Alkali and Alkaline Earth Metal Ions in SPC/E Water and Polarization Model of Hydrated Ions

In the first paper (J. Phys. Chem. B, 2006, 110, 10878), effective ion-ion potentials in SPC/E water were obtained for Me-Me, Me-Cl-, and Cl(-)-Cl- pairs, where Me is Li+, Na+, K+, Mg2+, Ca2+, Sr2+, and Ba2+ cations. In this second part of the study of effective interionic potentials, ion-ion distribution functions obtained from implicit-water Monte Carlo simulations of electrolyte solution with these potentials have been explored. This analysis verifies the range of applicability of the primitive model of electrolyte. It is shown that this approximation can be applied to monovalent electrolyte solutions in a wide range of concentrations, whereas the nature of ion-ion interactions is notably different for 2:1 electrolytes. An improved model of ions is discussed. The model includes approximations of the ion hydration shell polarization and specific short-range ion-ion interaction. It allows approximation of the potential of mean force acting on ions in strong electric fields of highly charged macromolecules and bilayers.

Density Functional Energetics of α-Quartz for Calibration of SiO2 Interatomic Potentials

All-electron, Gaussian basis density functional theory (DFT) total energies for alpha-quartz were generated on a substantial grid of points for eventual use in a first-principles check of calibration of a widely used effective interionic potential for SiO2. Unlike a previous all-electron, Gaussian basis calculation, we do not find a weak double minimum with respect to cell volume. Rather there is a weak double minimum with respect to internal cell parameters that is at the margins of numerical precision and, hence, may only be the signal of a very flat potential surface. The results show typical dependence on the choice of approximate DFT functional and differ from experimental values enough that both aspects may be problematic for parametrization of potentials.

Model development and analysis of temperature-dependent lithium sputtering and sputtered Li+ transport for tokamak plasma-facing applications

We developed and applied models for overall (atom+ion) sputtering and sputtered Li+ transport of liquid lithium tokamak divertor surfaces. The model/analysis has four parts: (1) a temperature-dependent data-calibrated empirical/code (TRIM) model of lithium sputtering by D+ and Li+ as a function of incident particle energy and angle; (2) temperature and energy-dependent molecular dynamics (MolDyn) modeling using an effective interionic pair potential of surface-reflected redeposited Li+; (3) analytical model of reflected lithium charge state; and (4) analytic model of Li+ near-surface emission/redeposition cascade. We predict: (1) strong temperature dependence of sputter yields, (2) reflection coefficients of order 50% (thermal energies) and 10% (hyperthermal energies), (3) reflected lithium charge fractions of 10–30% near 1eV incidence, and (4) enhanced but non-runaway Li emission for the studied surface temperature range between 473 and 653K.

A STATISTICAL MECHANICAL STUDY OF THERMODYNAMIC PROPERTIES OF SOLID SODIUM UNDER PRESSURE BASED ON AN EFFECTIVE INTERIONIC POTENTIAL

Basing on the Schiff effective interionic potential that has an oscillatory character and the correlative method of unsymmetrized self-consistent field (CUSF) that enables one to take into account the strong anharmonicity of the crystal lattice vibrations, we have calculated a complete set of equilibrium thermodynamic properties of solid sodium as functions of pressure and temperature: the lattice parameter, the elastic moduli, the thermal expansion coefficient, the Grüneisen parameter and the isochoric and isobaric heat capacities. Our results are compared with available experimental data. We also discuss the thermodynamic stability of the BCC lattice, the mechanism of its loss and its change under pressure.

Representation of the nonideality of electrolyte solutions using the cluster expansion theory

An explicit model for electrolyte solutions has been developed in terms of the cluster expansion theory, with which the osmotic coefficient and activity coefficient in the dilute concentration range can be predicted using the Pauling ionic diameter. In this model, the electrolyte solution is modeled as an ion gas immersed in a dielectric continuum, and the ion is represented as a charged hard sphere. Based on the Debye–Hückel theory, the effects of solvent and mean field of surrounding ions have been considered by virtue of an effective interionic pair potential. The applicability of the model as well as the effective pair potential adopted have been tested on a HBr solution and fourteen 1:1 haloid electrolyte solutions. The results suggest that the cluster expansion theory with incorporated effective pair potential can qualitatively predict the activity and osmotic coefficient of electrolyte solutions.

Polarization effects in liquid binary alloys

The contributions of dispersed and induced polarization forces to the effective interionic potentials of liquid binary alloys are presented in this paper. The latter are obtained within a perturbative scheme up to first order in the polarizability of the ions. The formalism is illustrated by analyzing how the polarization terms affect the structure of an example liquid alloy, namely, NaCs.

Dynamical properties of liquid Al near melting: An orbital-free molecular dynamics study

The static and dynamic structure of liquid Al is studied using the orbital free ab-initio molecular dynamics method. Two thermodynamic states along the coexistence line are considered, namely T = 943 K and 1323 K for which X-ray and neutron scattering data are available. A new kinetic energy functional, which fulfills a number of physically relevant conditions is employed, along with a local first principles pseudopotential. In addition to a comparison with experiment, we also compare our ab-initio results with those obtained from conventional molecular dynamics simulations using effective interionic pair potentials derived from second order pseudopotential perturbation theory.

Transferable Pair Potentials for Liquid Iron, Cobalt and Nickel

Effective inter-ionic pair potentials for liquid iron, cobalt and nickel are derived from second order pseudopotential perturbation theory with the transferable electron-ion potential of Fiolhais and co-workers which was originally developed for the solid state. The liquid structure is obtained by the thermodynamically self consistent liquid state theory, the variational modified hypernetted chain (VMHNC) approximation. It has been found that the calculated structural properties at the investigated thermodynamic states just above the melting point agree well with experiment. In this work we have determined the parameters of liquids Fe, Co and Ni for the universal choice of the evanescent core pseudopotentials of Fiolhais'. We have shown that this pseudopotential is transferable to the liquid state if used this parameterisation.

McMillan–Mayer theory for solvent effects in inhomogeneous systems: Calculation of interaction pressure in aqueous electrical double layers

We demonstrate how to use the McMillan–Mayer theory to include solvent effects in effective solute–solute interactions for inhomogeneous systems, extending a recent derivation [S. Marčelja, Langmuir 16, 6081 (2000)] for symmetric planar double layers to the general case. In the exact treatment, the many-body potential of mean force between the solute molecules can be evaluated for an inhomogeneous reference system in equilibrium with pure bulk solvent. The reference system contains only solvent and a finite number, n, of fixed solute molecules and it has an external potential that in some cases is different from that of the original system. It is discussed how the n-body potential of mean force between the ions for the relevant cases of large n values can be approximated by a sum of effective singlet and pair interactions evaluated in the presence of, on average, all n ions, i.e., at finite concentration. In examples considered in this work we use effective interionic pair potentials evaluated from bulk electrolyte calculations at finite electrolyte concentrations. We calculate the contribution to the double layer interaction pressure arising from the interaction between ions dissolved in aqueous electrolyte. In cases of moderate or high surface charge, calculations show several new effects. At small surface separations one finds attractive and then strongly repulsive contributions. For surface charge density around one negative charge per 70 Å2 the full results for pressures resemble “secondary hydration force” measured in classical experiments in 1980s. When there is a tendency for ions to adsorb at the surfaces there is a marked change in behavior. The force is then oscillatory, reminiscent of results obtained with the surface force apparatus at low electrolyte concentration.

Polarization effects in liquid metals and charge stabilized colloidal dispersions

The induced polarization contributions to the effective interionic potentials are derived for liquid metals, liquid alloys and charge stabilized colloidal dispersions. These contributions have been obtained within a perturbative scheme that includes up to three-body terms. The formalism is illustrated by calculating the static structure factor S(k) of liquid Ga near melting. It is found that the main effects due to the induced polarization in S(k) are first lowering the height of the principal diffraction peak, and second sharpening the subsidiary peak present in liquid Ga.

Calculations of structure and shear viscosity for less simple liquid metals

The results of calculations for static structure factors, S( q), and shear viscosities for less simple liquid metals, Zn, Cd, In, Hg and Pb, are presented. These have been studied by using effective interionic pair potentials which are derived from a local approach that successfully combines the sp-band contribution with the contribution of sd mixing. Specifically the sp-band contribution is described by an empty core model and the d-band contribution is deduced from the scattering phase shift by applying an inverse scattering method. The liquid structure is investigated by using two liquid state theories. These are the thermodynamically self consistent variational modified hypernetted chain integral equation theory and the linearised version of the Weeks–Chandler–Andersen thermodynamic perturbation theory of liquids. The viscosity is calculated on the assumption that this property of a liquid metal resembles that of an appropriate hard sphere liquid. Theoretical results for both properties are found to be in good agreement with available experimental data.

A model for the Verwey transition based on an effective interionic potential

A model for the Verwey transition has been developed and tested against experimental data. The model describes the electron-ordering process in terms of an average interionic Coulomb potential and a simplified but realistic energy level scheme. The strength of the interionic interaction is captured in a set of parameters related to the Madelung energy of a single ion. These constants can be evaluated easily through a computational procedure. A mean field analysis, in which the experimentally determined value of the energy gap between the octahedral 2 + and 3 +-levels serves as an input parameter, reproduces the observed value of the Verwey transition of 125 K quite well, thereby indicating dominant long range interactions. Madelung calculations confirm that these long range interactions are most likely Coulomb-type interactions. Finally, the strong effect of cation substitutions on the Verwey transition is discussed and explained quantitatively. Interionic Coulomb interactions and balancing of the local charge distribution, as implied by the Anderson-condition, are the basis of this discussion.

The role of volume effects in the Verwey transition inmagnetite

Fe2+/Fe3+ ordering in magnetite is described in terms of a simple mean-field approach based on an effective interionic Coulomb potential. It is shown that first-order electronic order-disorder transformations, as reported in the literature, can only be reproduced when the dependence of the interionic potential on the unit-cell dimensions is taken into account. In this case the first-order transitions can then be viewed as the result of an interplay between the lattice-deformation energy and the free-energy contribution related to the electronic ordering of the octahedral Fe lattice. Furthermore, the effect of the lattice deformation by hydrostatic pressure on the Verwey transition can be successfully explained to some extent within the context of the same framework. In comparison to experimental data available from the literature, the mean-field approach developed in this paper yields very acceptable results with respect to both qualitative and quantitative aspects, thus opening an interesting new viewpoint on the mechanism of the Verwey transition.

Transferable pair potentials for the description of liquid alkali metals

The structural and thermodynamic properties of liquid alkali metals near the melting point are presented. For this purpose, we use the thermodynamically self-consistent integral equation of the liquid state theory, called the hybridized mean-spherical approximation (HMSA). The effective interionic potential is derived via the second-order pseudopotential-perturbation method, in which the very recent pseudopotential of Fiolhais et al. [Phys. Rev. B 51, 14001 (1995)], designed for the solid state, is combined with our semianalytical local-field function [J. L. Bretonnet and M. Boulahbak, Phys. Rev. B 53, 6859 (1996)]. We show that this pseudopotential is transferable to the disordered environment characteristic of the liquid state and, if used with our local-field function, predicts the structural and thermodynamic properties of the liquid alkali metals with a reasonable accuracy.

Thermal and Elastic Properties of Solid Sodium Based on an Effective Interionic Potential

Basing on the Schiff effective interionic potential which has oscillatory character we have calculated a complete set of equilibrium thermodynamic properties of solid sodium at normal pressure above the martensitic transition temperature. To take into account the anharmonicity which is strong at high temperatures we use the correlative method of unsymmetrized self-consistent field elaborated previously. Our results are in agreement with measured values and with available data of computer simulations. We discuss the thermodynamic stability of the b.c.c. lattice of sodium and the mechanism of its loss. The metastability of a hypothetical f.c.c. lattice with Schiff potential is discussed.