Interionic Potentials Research Articles

Temperature-dependent potential for the molecular dynamics of the superionic conductor β-PbF2

Molecular dynamic (MD) calculations were performed to investigate the thermodynamic and structural properties of lead fluoride (PbF2) by using a proposed inter-ionic temperature-dependent potential. This potential allows calculating with high precision the linear thermal expansivity and the lattice parameter as a temperature function. In addition, the potential can be represented as a sum of two contributions, a temperature-independent potential added to another temperature-dependent potential, considered last as a correction justified by the one-dimensional Newtonian quantum equation. Two fitting regions were considered, the first region from 300 to 700 K and the other one from 700 to 900 K. These regions arise naturally due to the smooth and continuous transition that PbF2 undergoes until it reaches the superionic state and, allows us to model with high precision the anomaly in the dependence of the lattice parameter with the temperature of this material, a feature that until now under the molecular dynamic method has not been studied. These results are all in good agreement with the experimental measurements.

Thermodynamic or density scaling of the electrical conductivity of molten salts.

Thermodynamic or density scaling of high-pressure conductivities and molar conductivities of the high-temperature molten salts NaOH, and the alkali chlorides, bromides, and nitrates, from Na to Cs, taken from the literature, is found to be consistent with the simulations of Knudsen, Niss, and Bailey (KNB). They used a simple model fluid of point particles interacting through an interionic potential with a repulsive inverse power law part varying as r-9 and an attractive Coulombic part. This yields values between the limits 0.33-3 for the scaling parameter, γ. The Coulombic potential reduces the scaling parameter to values much lower than are normally found for molecular liquids, and KNB used this to explain the low values typically found for ionic liquids. Here, it is shown that the high-temperature molten salts examined behave similarly.

Ionic screening in bulk and under confinement.

Recent experiments have shown that the repulsive force between atomically flat, like-charged surfaces confining room-temperature ionic liquids or concentrated electrolytes exhibits an anomalously large decay length. In our previous publication [J. Zeman, S. Kondrat, and C. Holm, Chem. Commun. 56, 15635 (2020)], we showed by means of extremely large-scale molecular dynamics simulations that this so-called underscreening effect might not be a feature of bulk electrolytes. Herein, we corroborate these findings by providing additional results with more detailed analyses and expand our investigations to ionic liquids under confinement. Unlike in bulk systems, where screening lengths are computed from the decay of interionic potentials of mean force, we extract such data in confined systems from cumulative charge distributions. At high concentrations, our simulations show increasing screening lengths with increasing electrolyte concentration, consistent with classical liquid state theories. However, our analyses demonstrate that-also for confined systems-there is no anomalously large screening length. As expected, the screening lengths determined for ionic liquids under confinement are in good quantitative agreement with the screening lengths of the same ionic systems in bulk. In addition, we show that some theoretical models used in the literature to relate the measured screening lengths to other observables are inapplicable to highly concentrated electrolytes.

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.

Thermophysical properties of magnesium arsenide with atomistic simulation methods

A classical interionic potential for the Buckingham potential model was developed for magnesium arsenide (Mg3As2). Potential parameters were fitted to available experimental and first-principles data in the literature. They were implemented in classical molecular dynamics programs, and static properties such as lattice constant, bulk modulus, shear modulus, Young's modulus, elastic constants, Poisson's ratio and anisotropy factor were obtained at 0 K. Lattice constants, linear thermal expansion coefficients, heat capacities, self-diffusion coefficients for each axis, bulk moduli and radial distribution functions were calculated from room temperature up to the melting point. A superionic phase transition was observed. The superionic phase transition and the melting temperature points were found ~1505 K and ~1700 K by two-phase simulation box method, respectively. This method was used for the first time to determine the superionic phase transition temperature. Lattice parameter and bulk modulus were also calculated by the first-principles method. All the calculated data displayed an agreement with the available experimental and first-principles data in the literature. The present parameters of Buckingham potential provide a good description of magnesium arsenide (Mg3As2) crystal.

Hybrid Plasma/Molecular-Dynamics Approach for Efficient XFEL Radiation Damage Simulations

X-ray free-electron laser pulses initiate a complex series of changes to the electronic and nuclear structure of matter on femtosecond timescales. These damage processes include widespread ionization, the formation of a quasi-plasma state and the ultimate explosion of the sample due to Coulomb forces. The accurate simulation of these dynamical effects is critical in designing feasible XFEL experiments and interpreting the results. Current molecular dynamics simulations are, however, computationally intensive, particularly when they treat unbound electrons as classical point particles. On the other hand, plasma simulations are computationally efficient but do not model atomic motion. Here we present a hybrid approach to XFEL damage simulation that combines molecular dynamics for the nuclear motion and plasma models to describe the evolution of the low-energy electron continuum. The plasma properties of the unbound electron gas are used to define modified inter-ionic potentials for the molecular dynamics, including Debye screening and drag forces. The hybrid approach is significantly faster than damage simulations that treat unbound electrons as classical particles, enabling simulations to be performed on large sample volumes.

A new position in α-RbAg4I5 at room temperature by molecular dynamics simulations

ABSTRACTBy using a four-interaction interionic potential, constant-energy (classical microcanonical ensemble) molecular dynamics simulations, at room temperature, ionic positions in α-RbAg4I5 compound are reported. In this crystal structure (cubic with a = 11.23934(3) Å and P4132 as space group) a new site has been found. The unit cell has 16- ions, and its disordered partial occupancies have been the subject of discussion. Some authors report 72 [ = 24(e) + 24(e) + 8(c) + 4(b) + 12(d) or Ag1, Ag2, Ag3, Ag4, Ag5] sites with equal vacancy probabilities (Bradley and Greene), 60 [ = 24(e) + 24(e) + 8(c) + 4(b) or Ag1, Ag2, Ag3, Ag4 respectively] with different probability (Geller), 56 [ = 24(e) + 24(e) + 8(c) + 4(b) or Ag1, Ag2, Ag3] with different probability and 4 sites having zero probability (Hull et al). We report 96 interstitial sites [ = 24(e) + 24(e) + 8(c) + 4(b) + 12(d) + 24(e) or Ag1, Ag2, Ag3, Ag4, Ag5, Ag6, respectively] with different probabilities. The new site was denoted as Ag6 with 0.77% similar to Ag5 (∼0.66%). Good results (occupancy probabilities) compared to experimental data were found.

High pressure structural phase transition and elastic properties of ZnSexTe1−x semiconducting compounds

This paper discusses the high pressure-dependent crystal structure phase transformation and elastic behavior of ZnSe[Formula: see text]Te[Formula: see text] (x = 0.0, 0.2, 0.55, 0.81, 0.93 and 1.0) by speculating effective interionic interaction potential (EIoIP), that contains long-range type Coulomb force, short-range overlap repulsion of the Hafemeister and Flygare types and the van der Waals (vdW) interaction. The assessed values of structural phase transition pressure and the considerable volume discontinuity associated with the pressure–volume phase diagram support the structural phase transformation from zinc blende (B3) type to rock-salt (B1) type crystal structure. The equation of state curves (relationship between V(p)/V(0) and pressure in the pressure–volume phase diagram) for both the zinc blende (B3) and rock-salt (B1) structures is in reasonably good agreement with that of experimental observations. Moreover, systematic variations of the second-order elastic constants (SOEC) with pressure have been found to follow systematic trends showed by the other systems with B3 type to B1 type pressure-dependent structural phase transition.

Role of neutral and non-hard sphere interaction in differential capacitance of electrical double-layer

Classical density functional theory is combined with the extended primitive model and solvent primitive model to investigate how differential capacitance Cd of electrical double-layer formed inside cylindrical pore is influenced by solvent granularity, bulk concentration, counter-ion diameter, and neutral and non-hard sphere (HS) potential utail_αβ(r) between ion species, between solvent and counter-ion (and co-ion). Several main conclusions are drawn. (i) The Cd−surface charge strength (|σ|) curve generally rises as a result of consideration of the solvent granularity. (ii) An interionic attractive utail_αβ(r) helps in raising the Cd curve whereas a repulsive utail_αβ(r) tends to lower the Cd curve. Moreover, a secondary Cd peak appears when |σ| increases to an appropriately large value, and becomes increasingly evident with the strength of the attractive utail_αβ(r) and eventually disappears as the attractive utail_αβ(r)reduces in attraction or changes into repulsion at all. (iii) The Cd − |σ| curve is raised by increasing the repulsion between the solvent and counter-ion; however, both counter-ion diameter and bulk concentration cause obvious changes of the Cd − |σ∗| curve morphology. Concretely speaking, the Cd peak height and the peak position rise with the counter-ion size decreasing. Moreover, changing the utail_αβ(r) between solvent and counter-ion from attraction to repulsion facilitates a transition from bell-shaped curve to camel-shaped curve. (iv) Effects of all of the factors considered in influencing the Cd curve diminish increasingly with |σ|. All of the above observations can be explained by considering the interionic depletion potential induced by the solvent and its changes with the system parameters.

Local minimum in pair potentials of polyvalent metals: A limitation of pseudopotential theory

Local minimum appearing in the interionic pair potentials, when derived from local model pseudopotential, for Al (and some other polyvalent metals) remains as a long standing problem of clear understanding of its origin, although some attempts have been made by a few authors. The origin of this feature of local minimum is systematically investigated for the first time in this paper considering both the core size and the conduction electron density as variables. Interionic pair potential is derived from Ashcroft’s empty core model because it depends on these two variables only. Results of this investigation show monovalent metals do not exhibit a local minimum at all but trivalent Al and some other polyvalent metals do exhibit at their normal densities. Here, the combined effect of the core size and the conduction electron density results whether the local minimum will appear or not. More interestingly, for smaller core size, conduction electron density plays major role and for larger core size the core radius plays the major role in determining the depth of the local minimum.

Interionic potential for α-RbAg4I5-system at room temperature by molecular dynamics

ABSTRACTA rigid-ion empirical pairwise potential for RbAgI including Coulomb (long range), Born–Mayer (short range), charge–dipole and dipole–quadrupole (Van der Waals) interactions, has been tested by molecular dynamics simulation with a statistical ensemble where the number of particle, volume, and total energy are constant, non-pressure (P = 0), and as space group were used. Structural (radial, pair, distribution function) and dynamical behaviour (mean square function) of rubidium, iodines, and silvers were evaluated and compared to experimental data reported at room temperature. The potential parameters were taken from different works.

Ab initio based interionic potential for silver iodide

In this paper, a new interionic potential is derived for silver iodide (AgI) via the Chen-Möbius lattice inversion and ab initio calculation. The accuracy of the proposed potential is checked by comparing the molecular dynamics simulation results on the static properties, structural stability, disordered states, mean-squared displacement and phase transition of AgI with experimental data. The simulation results are consistent with experimental data and density functional theory (DFT) calculations, indicating that the proposed interionic potential is valid over a wide range of interionic separations and applicable for describing the major properties of AgI.

Theoretical approach to predict the phase transition of NaH under pressure

This paper is dealing with the high pressure behavior of alkali halide (NaH) and a structural phase transition from NaCl to CsCl phase is predicted under high pressure. We have adopted an effective interionic interaction potential, which includes the long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach. The thermodynamic requirement, for the equality of free energies, at a phase boundary, suggests that the transition pressure of NaH occur at 37 GPa. The transition involves the coordination number increasing from 6 for the NaCl-type structure to 8 for the CsCl-type structure.

Theoretical Analysis of the Structural Phase Transformation and Elastic Properties in the ZrN under High Pressure

We report a phenomenological model based calculation of pressure-induced structural phase transition and elastic properties of ZrN compound. Gibb’s free energy is obtained as a function of pressure by applying an effective interionic interaction potential, which includes the long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach. From the present study, we predict a structural phase transition from NaCl structure (B1) to the CsCl structure (B2). The variations of elastic constants with pressure follow a systematic trend identical to that observed in others compounds of NaCl type structure family.

Study of Structural, Elastic Properties and their Pressure Dependence in IrN Compound

A theoretical study of the elastic behavior in IrN compound using effective interionic interaction potential is carried out. The estimated values of phase transition pressure and the vast volume discontinuity in pressure-volume (PV) phase diagram indicate the structural phase transition from zinc blende (B3) to CsCl structure (B2). C11, C12 and C44 increase nearly linearly with pressure. At phase transition pressure IrN has shown a discontinuity in second order elastic constants, which is in accordance with the first-order character of the phase transition.

High-Pressure Structural Phase Transition in Mg<sub>x</sub>Cd<sub>1-x</sub>O Compounds

The high-pressure technique is useful to understand physical properties because the technique can directly control bond length and phase transition. As a general trend, the pressure-induced phase transition causes an increase of coordination number with a drastic change of their physical properties. Here, we attempt to explore the pressure-induced phase transitions from the sixfold-coordinated NaCl structure (B1) to the eightfold-coordinated CsCl structure (B2) in MgxCd1−xO by applying an effective interionic interaction potential, which includes the long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach. Assuming that both the ions are polarizable, the Slater-Kirkwood variational method is employed to estimate the vdW coefficients for parent compounds. The estimated values of the phase transition pressure (Pt) increase with Mg concentration. The vast volume discontinuity in pressure volume phase diagram identifies the structural phase transition from B1 to B2 structure. The results obtain from the present calculations requires the complete understanding of many physical interactions that are essential to ternary oxides, containing elements with size and chemical mismatch, will lead to a consistent explanation of the documented structural properties.

Comment on “Pressure Induced Phase Transition, Elastic and Thermal Properties of Rare Earth Tellurides”

In the recent article ‘Pressure Induced Phase Transition, Elastic and Thermal Properties of Rare Earth Tellurides’ published by Rukmangad et al. (Trans Indian Inst Met 67(2):185–192, 2014), the high pressure phase transition, elastic and thermal properties of rare earth tellurides (RETe; RE = La, Ce, Pr, Sm and Eu) have been investigated by using a suitable inter-ionic potential with modified ionic charge (Zme) to include Coulomb screening effect. After studying their article, we have found some discrepancies in predicted crystal density, longitudinal, transverse and average elastic wave velocities, and Debye temperature data. The crystal density has been found multiplied by 2. Also the longitudinal, transverse and average elastic wave velocities and Debye temperature are different from our reexamined values. Although these discrepancies have no influence on the authors’ conclusion.

The Research on the Bionic Friction Layers of TiAl-10wt.%V2O5 Nanowires at the Applied Loads of 6-24 N

The objective of searching the optimum applied load was to minimize friction and decrease wear in mechanical moving components. The friction and wear behaviors of TiAl-10wt.%V2O5 nanowires (TiAl-10wt.%V2O5) were evaluated sliding against the fixed Al2O3 balls with the Vickers hardness of 18.07 GPa at the applied loads of 6-24 N. The testing results showed that the smaller friction coefficient and wear rate of TiAl-10wt.%V2O5 were obtained at 18 N, if compared to those at 6, 12 and 24 N. The smaller friction coefficient and wear rate were mainly attributed to the forming of cambiform structure of large crystalline grain, the higher totals of oxide interionic potential and the higher thickness of friction layer at 18 N, if compared to those at the other applied loads. It was obvious that the applied load of 18 N was reasonable for obtaining the lower sliding friction coefficient and the less wear rate of TiAl-10wt.%V2O5.

The influence of the mean charge state on the Coulomb heating of fast diclusters through the Si〈1 1 1〉 direction

In the present work, we report a theoretical and experimental study of the Coulomb heating of H2+ and C2+ in Si〈111〉 channel, covering an energy range from 200keV/ion to 2200keV/ion. The experimental values for Coulomb heating were obtained by combining the Rutherford backscattering spectrometry (RBS) and the particle induced X-ray emission (PIXE) techniques under channeling conditions. Theoretical values were obtained by performing classical trajectory Monte-Carlo (CTMC) simulations of the ion paths inside the 〈111〉 Si channel, using Dirac–Hartree–Fock–Slater (DHFS) results for the interionic potential. As seen for the 〈110〉 case, it is shown that the use of a DHFS potential based on the ion mean charge states in amorphous targets leads to a disagreement between the Coulomb heating values and the expected potential energies stored in the dicluster prior to the Coulomb explosion. Therefore, a numerical procedure was used in order to calculate the mean charge state values for ions traveling in Si〈111〉. The use of the resulting charge states led to a linear relationship between the Coulomb heating values and the stored potential energy per ion of the diclusters. Finally, the Coulomb heating/stored potential energy ratio amounts to about 2/3, as expected from an isotropic Coulomb explosion.

Pressure induced stiffening, thermal softening of bulk modulus and brittle nature of mercury chalcogenides

The pressure and temperature dependent elastic properties of mercury chalcogenides ( Hg X; X = S , Se and Te ) with pressure induced structural transition from ZnS -type (B3) to NaCl -type (B1) structure have been analyzed within the framework of a model interionic interaction potential with long-range Coulomb and charge transfer interactions, short-range overlap repulsion and van der Waals (vdW) interactions as well as zero point energy effects. Emphasis is on the evaluation of the Bulk modulus with pressure and temperature dependency to yield the Poisson's ratio ν, the Pugh ratio ϕ, anisotropy parameter, Shear and Young's modulus, Lamé's constant, Klein man parameter, elastic wave velocity and Debye temperature. The Poisson's ratio behavior infers that Hg X are brittle in nature. To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of elastic and thermodynamical properties explicitly the ductile (brittle) nature of Hg X and still awaits experimental confirmations.