Ionic Potential Model Research Articles

Evolutionary Optimization of a Charge Transfer Ionic Potential Model for Ta/Ta-Oxide Heterointerfaces

Tantalum, tantalum oxide and their hetero-interfaces are of tremendous technological interest in several applications spanning electronics, thermal management, catalysis and biochemistry. For example, local oxygen stoichiometry variation in TaOx memristors comprising of metallic (Ta) and insulating oxide (Ta2O5) have been shown to result in fast switching on the sub-nanosecond timescale over a billion cycles, relevant to neuromorphic computation. Despite its broad importance, an atomistic scale understanding of oxygen stoichiometry variation across Ta/TaOx hetero-interfaces, such as during early stages of oxidation and oxide growth, is not well understood. This is mainly due to the lack of a variable charge interatomic potential model for tantalum oxides that can accurately describe the ionic interactions in the metallic (Ta) and oxide (TaOx) environment as well as at their interfaces. To address this challenge, we introduce a charge transfer ionic potential (CTIP) model for Ta/Ta-oxide system by training against lattice parameters, cohesive energies, equations of state, and elastic properties of various experimentally observed Ta2O5 polymorphs. The best set of CTIP parameters are determined by employing a single-objective global optimization scheme driven by genetic algorithms followed by local Simplex optimization. Our newly developed CTIP potential accurately predicts structure, thermodynamics, energetic ordering of polymorphs, as well as elastic and surface properties of both Ta and Ta2O5, in excellent agreement with DFT calculations and experiments. We employ our newly parameterized CTIP potential to investigate the early stages of oxidation of Ta at different temperatures and atomic/molecular nature of the oxidizing species.

Hypernetted chain calculations of molten uranium dioxide: Comparison of rigid ion potentials

The first liquid state calculations within the hypernetted chain approximation for molten uranium dioxide using six different rigid ion model potentials are presented and discussed. Applicability of the existing rigid ion potentials to the molten phase of uranium dioxide is examined. Despite the minor differences, the radial distribution functions and static structure factors of all potentials are typical of a system which melted from a fast ion conducting phase. Only in one case, the breakdown in the local structure of oxygen ions in the solid state is reflected in the calculated coordination numbers. The behavior of static dielectric function is related to the onset of freezing and found to mimic that of the classical one component plasma at low values of plasma parameter. Finally, the enthalpy change is calculated and compared with molecular dynamic simulation results and experimental data.

The adsorption of CO2, H2CO3, HCO3− and CO32− on Cu2O (111) surface: First‐principles study

AbstractThe adsorption of CO2, and its derivatives, H2CO3, HCO, and CO, on Cu2O (111) surface has been investigated by first‐principles calculations based on the density functional theory at B3LYP hybrid functional level. The Cu2O (111) surface has been modeled using an embedded cluster method,in which the quantum clusters plus some ab initio ion model potentials were inserted in an array of point charges. On the surface, H2CO3 was dissociated into an H+ and an HCO ion. Among the CO2 species, HCO was the only activated species on the surface. The results suggest that the reduction of CO2 on Cu2O (111) surface can start from the form of HCO. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

The structure of molten AgCl, AgI and their eutectic mixture as studied by molecular dynamics simulations of polarizable ion model potentials

The structure of molten AgCl, AgI, and their eutectic mixture Ag(Cl(0.43)I(0.57)) is studied by means of molecular dynamics simulations of polarizable ion model potentials. The corresponding static coherent structure factors reproduce quite well the available neutron scattering data. The qualitative behavior of the simulated partial structure factors and radial distribution functions for molten AgCl and AgI is that predicted by the reverse Monte Carlo modeling of the experimental data. The AgI results are also in qualitative agreement with those calculated from ab initio molecular dynamics.

Transport Properties of Tetrahedral, Network-Forming Ionic Melts

Molecular dynamics simulations of liquid silica and beryllium fluoride are performed using the van Beest-Kramer-van Santen and transferable rigid ion model potentials, respectively, in order to compare transport properties. The ionic conductivity (sigma), shear viscosity (eta) and ionic self-diffusivities (D(+/-)) are computed over a fairly wide range of temperatures and densities and deviations from Arrhenius behavior along different isochores is studied. The Stokes-Einstein relation is shown to hold over the entire range of state points, though the effective hydrodynamic radius shows small variations due to thermal fluctuations, compression, and local tetrahedral order. Several alternative tests of the Nernst-Einstein relation are implemented which show that significant network-formation in the anomalous regime leads to a breakdown of this relationship. The relaxation times, tau(sigma) and tau(M), associated with the decay of the charge-flux and pressure ACFs respectively, are computed. In the anomalous regime, as the tetrahedral network formation progresses, tau(M) increases rapidly while tau(sigma) shows very little variation, indicating a decoupling of charge and momentum transport processes.

Ionic melts with waterlike anomalies: Thermodynamic properties of liquid BeF2

Thermodynamic properties of liquid beryllium difluoride (BeF(2)) are studied using canonical ensemble molecular dynamics simulations of the transferable rigid ion model potential. The negative slope of the locus of points of maximum density in the temperature-pressure plane is mapped out. The excess entropy, computed within the pair correlation approximation, is found to show an anomalous increase with isothermal compression at low temperatures which will lead to diffusional as well as structural anomalies resembling those in water. The anomalous behavior of the entropy is largely connected with the behavior of the Be-F pair correlation function. The internal energy shows a T(35) temperature dependence. The pair correlation entropy shows a T(-25) temperature dependence only at high densities and temperatures. The correlation plots between internal energy and the pair correlation entropy for isothermal compression show the characteristic features expected of network-forming liquids with waterlike anomalies. The tagged particle potential energy distributions are shown to have a multimodal form at low temperatures and densities similar to those seen in other liquids with three-dimensional tetrahedral networks, such as water and silica.

Transferable Deformation-Dipole Model for Ionic Materials

A model for the ionic interactions in polyvalent metal halides was originally built for chloroaluminate clusters using an analysis of data on static and dynamic structure of their molecular monomers [for a review see M. P. Tosi, Phys. Chem. Liquids 43, 409 (2005)]. Recently, by continuing the deformation-dipole model calculations, the transferability of the halogen parameters was tested through the calculation of the structure of alkali halides and alkaline-earth halides. In this work we test the usefulness of the deformation-dipole model in the study of ionic materials by examining the transferability of the overlap parameters for the halogen ions across families of halide compounds. Following a comparative discussion of alkali and alkaline-earth halide monomers near equilibrium, results on alkaline-earth halides are given. By using the transferable ionic potential model we also calculate the equilibrium structure of the molecular clusters, as well as the vibrational frequencies of ACl4 compounds (where A = U, Np, Pu, Am and Th).

Classical molecular dynamics simulation of uranium monocarbide (UC)

Inter-atomic potential parameters of UC, for a partially ionic potential model, were found out by fitting method using experimental lattice parameter data in an improvised way. The inter-atomic potential used here is essentially a combined potential model that has earlier been used to simulate UO 2 system. The calculated lattice parameters, bulk modulus and coefficient of thermal expansion of UC, using the current potential parameters, are in good agreement with the experimental values. The model also shows that there is no Bredig transition in UC system as contrary to UO 2, where the same potential model was used.

Molecular-Dynamics Simulation of Lattice Thermal Conductivity in Pb1-xSnxTe and Pb1-xGexTe at High Temperature

Molecular-dynamics studies on lattice thermal conductivity (κ L ) of Pb 1- x Sn x Te and Pb 1- x Ge x Te are carried out by employing ion model potential for mutual ion-ion interactions at the average temperatures 300 and 500 K under non-equilibrium condition. Special emphasis of investigation is placed on the alloying effects of κ L calculated by changing the alloy content x in PbTe-based alloys. It has been shown that a remarkable reduction of κ L at finite x is demonstrated in Pb 1- x Sn x Te and Pb 1- x Ge x Te without relying on a phonon scattering description. We have compared our results with ones evaluated by the Abeles' formula of lattice thermal conductivity due to phonon scattering from alloy fluctuations.

Exploring simulation methods for self-healing oxide films

AbstractNew methods for modeling oxide film growth using MD (molecular dynamics) simulations are explored in order to study the atomic mechanism of the self-healing oxidation phenomenon in metal/oxide systems. A modified MEAM (modified embedded atom method) potential is proposed and extended by a variable charge ionic potential model from the literature, for which the PPPM (particle-particle-particle-mesh) was used as Coulomb solver. In this work the first results are reported. The ionic potential model is tested in combination with the PPPM solver. The results are excellent.

Molecular Dynamics Simulations of LiCoyMn2-yO4 Cathode Materials for Rechargeable Li Ion Batteries

Molecular dynamics (MD) simulations have been performed for the spinel solid−solution, LiCoyMn2-yO4 in combination with experimental methods, X-ray diffraction (XRD), and infrared spectroscopy. The partial ionic potential model was used and optimized in the present MD simulation study. It showed good accordance with the observed lattice parameter on various Co contents and temperature. From the analysis of simulated results, it was concluded that the local distortion was caused by the mismatch of ionic size between Mn3+ and Mn4+ via the 16c vacancy site, and the Co substitution suppressed the local distortion in the lattice. The dynamical properties based on the simulated results were also analyzed and compared with experimental infrared absorption spectra. The reinforcement of chemical bonds by Co substitution was indicated in the simulation results, and it was supported by infrared absorption spectra.

Classical molecular dynamics simulation of UO 2 to predict thermophysical properties

Interatomic potential parameters of UO 2, for a partially ionic potential model, were found out by a fitting method using isothermal compressibility data up to 1600 K. The inter-atomic potential used here is essentially a combined potential model comprising of Bushing–Ida and Morse-type potential to simulate UO 2 system using classical molecular dynamics. It has been shown that the deviation in the estimated lattice parameter from the experimental data as reported in literature is due to the inadequacy of the potential parameter. The calculated deviations in lattice parameter, due to the generation of point defects, with variation of temperature have been discussed. The calculated lattice parameters, thermal expansivity, isothermal compressibility of UO 2, using the current potential parameters are in close agreement with the experimental values. The model also successfully predicts Bredig transition.

Static Structure of a Group of Trivalent Molten Salts: An Alternative Rigid Ion Model Potential

The total structure factors of eleven 3:1 molten salts and the partial structure factors of molten YCl 3 have been calculated using rigid ion model potentials with the functional form originally proposed by Vashishta and Rahman (VR) /18/. The calculations were performed by solving numerically the hypernetted chain approximate theory of liquids. The results of these calculations were compared with neutron diffraction data and those obtained by using the rigid ion potentials proposed by Tatlipinar et al. /16/ (TAPT). The results obtained by using the VR potentials follow the main features of the experimental total structure factors better than those obtained using the TAPT potentials.

Modification of structural and chemisorption properties of small palladium clusters supported on the MgO(0 0 1) surface from density functional calculations

We have performed gradient corrected density functional calculations on Pd tetramers and pentamers deposited on the regular MgO(0 0 1) surface as well as on the chemisorption of CO molecules on such Pd clusters. These systems can be considered as models of supported metal catalysts on a fully dehydroxylated MgO surface. The oxide surface has been represented by MgO clusters embedded in total ion model potentials and an array of point charges. The geometrical structures of Pd, Pd 4, and Pd 5 supported on the MgO substrate have been optimized partially and the corresponding electronic properties have been analyzed. Both two- and three-dimensional Pd clusters have been considered. Changes in the electronic structure of the clusters induced by interaction with the substrate have been analyzed by adsorbing a CO probe molecule on free and supported Pd clusters. Perturbation of the metal cluster due to interaction with the support, although not large, has two important consequences: a weakening of the metal–metal bonds which results in an elongation of the Pd–Pd distances within the cluster, and a change in the preferred adsorption site. On the other hand, the geometrical and vibrational properties of the adsorption complexes CO–Pd/MgO and CO–Pd do not differ significantly.

A quantum-chemical study of Pd atoms and dimers supported on TiO 2(110) and their interaction with CO

The interaction of Pd atoms and dimers with the (110) surface of TiO 2 has been studied by means of gradient corrected density functional theory (DFT) calculations. (TiO 2) n ( n=3–15) clusters have been used to simulate the surface of rutile. The role of embedding in point charges and total ion model potentials has been analyzed in detail. Adsorption sites along the protruding oxygen and the basal five-fold coordinated Ti rows have been compared. A considerable stability of the results versus cluster size has been achieved. At low coverage we found a preference for Pd adsorption on the two-fold coordinated protruding oxygens where the bond strength is about 1 eV. Considerable adhesion is found also along the Ti rows, with interaction energies of 0.5–0.6 eV. A coverage of 0.25 monolayers (ML) of Pd has been simulated by performing periodic supercell DFT calculations. In this case there is an indication for a similar stability of the two sites, with the Ti rows slightly preferred. Adsorbate-induced reconstructions are not included in these models. Pd dimers adsorbed on the TiO 2 surface lose most of the Pd–Pd interaction due to the relatively strong bond with the substrate. Adsorption of a CO molecule on the supported Pd dimers shows that the major effect of the bond at the interface is not a modification of the ability of the metal to donate electrons to CO but that of preventing a polarization of the metal electrons away from the CO molecule, thus increasing the Pauli repulsion contribution to the CO–Pd bonding.

Transition metals supported on oxides: density functional cluster studies of palladium deposition on MgO(001)

The interaction of isolated Pd atoms and of a square Pd4 cluster with the (001) surface of MgO is investigated by means of density functional (DF) calculations. The oxide surface is represented by various model clusters and the effect of the surrounding is taken into account by embedding the cluster in point charges and total ion model potentials. The calculations are performed at the relativistic level using the Becke–Perdew exchange-correlation functional. The adsorption properties determined with this computational scheme are compared with other DF results. The bonding of the Pd atoms and clusters with the surface is analyzed in terms of charge density difference plots. It is found that the polarization of the metal adsorbate due to the surface electric field provides an important contribution to the metal–oxide adhesion energy.

High‐Pressure Phase Behavior of Alumina: Predictions of a Transferable Ionic Potential Model

A recently introduced potential model for alumina has been used to study a wider range of structures than previously attempted. The model contains both the spherical relaxation of, and induced dipoles and quadrupoles on, the oxide anions. The model is applied to the possible high‐pressure phase transition from the ground‐state corundum structure to the Rh2O3‐II structure. A transition pressure of 194 GPa has been predicted, which is in excellent agreement with the most‐recent experimental results. The spherical relaxation and induced quadrupoles are crucial in effectively modeling the transition. The shortcomings of the model are assessed by comparison with experiment and both ab initioand model calculations. Other structures common in this stoichiometry have also been considered.

Integral Equations Calculations and Computer Simulations of the Static Structure and Ionic Transport in Molten Nickel Halides

We present the first integral equations calculations and computer simulations of the static structure and ionic transport properties of molten nickel dihalides near melting. The calculations have been carried out using the hypernetted chain theory of liquids (HNC), and for the simulations we have used molecular dynamics (MD). The potentials used for the calculations have a similar functional form as the semiempirical potentials originally proposed by Vashishta and Rahman in 1978 (Phys. Rev. Lett., 40, 1337) to study α-AgI. The results for both the pair distribution functions and partial structure factors are in fair agreement with experiment, except for the cation-cation partials where agreement is poor, particularly at small momentum transfer. Most of, but not all, the differences with experiment are likely to be due to the use of a pairwise additive rigid ion model potential. There is, to our knowledge, no experimental information on the ionic transport properties for the NiX 2 melts (X = Cl, Br, I). We believe that our predicted results for the self-diffusion coefficients and ionic conductivity are likely to overestimate the experimental values when these become available. The results for the velocity autocorrelation functions suggest a transport mechanism akin to that present in the alkali halides. We find no evidence of remnants of superionic behaviour in molten NiI 2 .

The interpretation of vibrational spectra of ionic melts

Computer simulations of the short-time, vibrational dynamics of the network-forming ionic melts, LaCl3 and ZnCl2, and of their mixtures with network-breaking alkali halides are described. In the mixtures, high frequency peaks in the vibrational density of states are shown to be describable in terms of the normal coordinates of vibration of transient molecular ion species, like LaCl63− and ZnCl42−. Novel simulation methods are presented which allow this association to be established. In the pure melts, the vibrational motions retain a strong aspect of this local polyhedral unit vibrational character, but the effects of network-induced coupling between the vibrations of different units become pronounced, particularly in ZnCl2. The calculated vibrational spectra are compared with extensive Raman data on these systems, and with infrared and neutron spectra in pure ZnCl2. For the mixtures, remarkably good agreement with experiment is found, confirming the high quality of the representation of the interionic interactions obtained with the polarizable ion model potentials used. For the melts, there are discrepancies between the peak frequencies observed in the vibrational DOS and the Raman spectra. These discrepancies are likely to be due to the network-induced couplings, whose effect on the Raman (and infrared) spectra is not fully included in the calculated DOS.

Adsorption of Pd atoms and Pd 4 clusters on the MgO(001) surface: a density functional study

We performed gradient-corrected density functional calculations on the adsorption of Pd atoms and Pd 4 clusters on the MgO(001) surface. The surface was represented by clusters embedded in point charge arrays and total ion model potentials. Pd atoms adsorb on top of the O ions with a binding energy of 0.9±0.1 eV and a distance of 2.2 Å. A Pd 4 square cluster is almost perfectly accommodated to the MgO substrate, suggesting a preference for pseudomorphic growth of large Pd particles. PdPd distances are substantially elongated compared to the bulk metal, in agreement with observations on small supported Pd clusters.