Interionic Potentials Research Articles

Oxygen Vacancy Ordering and the Conductivity Maximum in Y2O3-Doped CeO2

The defect structure and ionic diffusion processes within the anion-deficient, fluorite structured system Ce1–xYxO2–x/2 have been investigated at high temperatures (873 K–1073 K) as a function of dopant concentration, x, using a combination of neutron diffraction studies, impedance spectroscopy measurements, and molecular dynamics (MD) simulations using interionic potentials developed from ab initio calculations. Particular attention is paid to the short-range ion–ion correlations, with no strong evidence that the anion vacancies prefer, at high temperature, to reside in the vicinity of either cationic species. However, the vacancy–vacancy interactions play a more important role, with preferential ordering of vacancy pairs along the ⟨111⟩ directions, driven by their strong repulsion at closer distances, becoming dominant at high values of x. This effect explains the presence of a maximum in the ionic conductivity in the intermediate temperature range as a function of increasing x. The wider implications of these conclusions for understanding the structure–property relationships within anion-deficient fluorite structured oxides are briefly discussed, with reference to complementary studies of yttria and/or scandia doped zirconia published previously.

THERMOPHYSICAL PROPERTIES OF URANIUM DIOXIDE: A MOLECULAR DYNAMICS STUDY OF SOLID AND LIQUID PHASES OF STOICHIOMETRIC UO2

Thermodynamic and transport properties of solid and liquid uranium dioxide were studied using classical molecular dynamics simulation, with a newly parametrized interionic model potential. In addition to the static and transport properties which have been previously reported by the authors, this study further confirms that temperature dependence of the calculated thermophysical properties of uranium dioxide are in agreement with the available experimental data at both solid and liquid phases in providing an alternative rigid ion potential to the other model potentials in literature. Although lattice parameter and density have been underestimated, overall results give a fairly good description of the UO 2 system for wide range of temperature (0–4000 K). The transition to the superionic phase, Bredig transition, was successfully observed as a distinct λ-peak in specific heats at about 400 K below the experimental value. The results presented in our previous article and here show that the introduced alternative model potential for uranium dioxide is very promising and we are confident in the success of its use in future studies.

Electrical resistivity of the hole doped La 0.8Sr 0.2MnO 3 manganites: Role of electron–electron/phonon/magnon interactions

In this work, a quantitative analysis of reported metallic and insulating behaviour of resistivity in perovskite manganites La 0.8Sr 0.2MnO 3 is established. An effective inter-ionic interaction potential (EIoIP) with the long-range Coulomb, van der Waals (vdW) interaction and short-range repulsive interaction up to second-neighbour ions within the Hafemeister and Flygare approach was employed to estimate the Debye and Einstein temperature and was found to be consistent with the available experimental data. The electrical resistivity data in low temperature regime ( T < T MI ) were theoretically analyzed within the framework of the classical electron–phonon model of resistivity, for example, the Bloch–Gruneisen (BG) model. The Bloch–Gruneisen (BG) model and terms T 2, T 4.5 simplify the electron–phonon, electron–electron and electron–magnon scattering processes. On the other hand, in high temperature regime ( T > T MI ) the insulating nature is discussed with Mott's variable range hopping (VRH) model and small polaron conduction (SPC) model. For T > T MI SPC model is more appropriate than the VRH model. The SPC model consistently retraces the higher temperature resistivity behaviour ( T > θ D /2). The metallic and semiconducting resistivity behaviours of La 0.8Sr 0.2MnO 3 manganites are analyzed, to the knowledge, for the first time highlighting the importance of electron–phonon, electron–electron, electron–magnon interactions and small polaron conduction.

A dipole polarizable potential for reduced and dopedCeO2 obtained from first principles

In this paper we present the parameterization of a new interionic potential for stoichiometric, reduced anddoped CeO2. We use a dipole polarizable potential (DIPPIM: the dipole polarizable ion model) andoptimize its parameters by fitting them to a series of density functional theory calculations.The resulting potential was tested by calculating a series of fundamental properties forCeO2 and by comparing them against experimental values. The values for all the calculatedproperties (thermal and chemical expansion coefficients, lattice parameters, oxygenmigration energies, local crystalline structure and elastic constants) are within 10–15% ofthe experimental ones, an accuracy comparable to that of ab initio calculations. This resultsuggests the use of this new potential for reliably predicting atomic scale properties ofCeO2 in problems where ab initio calculations are not feasible due to their size limitations.

Pressure dependent elastic properties of diluted magnetic semiconductors: Hg 1− xMn xS ( x=0.02 and 0.07)

A theoretical study of the elastic properties in diluted magnetic semiconductors Hg 1− x Mn x S ( x=0.02 and 0.07) using an effective interionic interaction potential (EIoIP) in which long-range Coulomb interactions, charge transfer mechanism (three body interaction) and the Hafemeister and Flygare type short-range overlap repulsion extending up to the second neighbor ions and the van der Waals (vdW) interaction is considered. Particular attention is devoted to evaluate Poisson's ratio ν, the ratio R S/ B of S (Voigt averaged shear modulus ) over B (bulk modulus), elastic anisotropy parameter, elastic wave velocity, average wave velocity and thermodynamic property as Debye temperature is calculated. By analyzing Poisson's ratio ν and the ratio R S/ B we conclude that Hg 1− x Mn x S is brittle in zinc blende (B3). To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of ductile (brittle) nature of Hg 1− x Mn x S compounds and still awaits experimental confirmations.

Semi-ab initio interionic potential for gadolinia-doped ceria

In the current work, a set of semi-ab initio interionic pair potentials in a concise functional form with parameters for gadolinia-doped ceria (GDC) systems is derived via the Chen–Mobius lattice inversion and ab initio quantum-chemical calculation. The quality of the proposed potentials is verified by molecular dynamics simulations of CeO 2 and A 2O 3 (A = Ce and Gd) on their static properties, doped concentrations and temperature dependence of lattice constants, mean-square displacements, pair correlation functions and elastic constants. Simulation results are consistent with corresponding experimental data, showing that the new form is valid over a wide range of interionic separations and applicable for describing structural properties of ionic solids.

Relationship Between Interatomic Separation and Pressure for Ionic Solids at High Temperatures

The equation of state derived from the Anderson approximation and the theory of interionic potentials based on the quantum mechanical form of the overlap repulsive energy are used to investigate the relationship between interatomic separation and pressure for three ionic solids (LiF, NaF, and CsC1) at high temperatures. The values of van der Waals dipole–dipole and dipole–quadrupole energies are also included in the model. The results obtained are in agreement with the available experimental data.

Pressure dependent mechanical and thermodynamical properties of Hg0.91Mn0.09Te semiconductor

The mechanical, thermodynamical and elastic properties of Hg0.91Mn0.09Te compound are calculated by formulating an effective interionic interaction potential. This potential consists of the long-range Coulomb, three body force parameter, the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbor ions and the van der Waals (vdW) interaction. The estimated values of phase transition pressure have revealed reasonably good agreement with the available experimental data on the phase transition pressure Pt = 11.5 GPa and the vast volume discontinuity in pressure-volume (PV) phase diagram indicate the structural phase transition from zincblende (B3) to rock salt (B1) structure. Later on, the Poisson’s ratio ν, the ratio RS/B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic anisotropy parameter, elastic wave velocity, average wave velocity and Debye temperature as functions of pressure is calculated. From Poisson’s ratio and the ratio RS/B it is inferred that Hg0.91Mn0.09Te is brittle in nature in both B3 phase and B1 phase. To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of ductile (brittle) nature of Hg0.91Mn0.09Te compounds and still awaits experimental confirmations.

Effect of the ordering potential on the structure of liquid alloys

The concept of “ordering or alloying potential” (J. Hafneri: from Hamiltonians to phase diagrams: Springer Berlin 1987 and R. N. Singh and F. Sommerii Rep. Prog. Phys. 60 (1997) 57–150) enables the understanding of the different kind of alloys: hetero-coordinated one’s leading to compounds, homocoordinated ones leading to miscibility gap systems and substitutional alloys. The ordering potential is based on the comparison of identical atom interionic potentials (V11 and V22) and different atom interionic potential (V12) It allows the description of the demixing properties of some alloys. In order to understand the concepts, we developed our calculations by using a Lennard-Jones potential, the atomic structure being calculated by molecular dynamics simulation. We obtained surprising and unexpected results putting in evidence the time of simulation and the strength of the ordering potential.

Elastic properties of alkaline earth oxides under high pressure

In this article, we have investigated the high-pressure structural phase transition of alkaline earth oxides using the three-body potential (TBP) model. Phase transition pressures are associated with elastic constants. An effective inter-ionic interaction potential (TBP) with long-range Coulomb interactions and the Hafemeister–Flygare type short-range overlap repulsion and the vdWl interaction is developed. The present calculations have revealed reasonably good agreement with the available experimental data on structural transition (B1–B2 structure). The phase transition pressures Pt of MgO, CaO, SrO, and BaO occur at 220, 45, 40, and 100 GPa, respectively. Further, the variations of the second-order elastic constants with pressure have followed a systematic trend, which are almost identical to those exhibited by the observed data measured for other semiconducting compounds with rocksalt (B1)-type crystal structure. It is found that TBP promises that we would be able to predict phase transition pressure and elastic constants for other chalcogenides as well. The results may be useful for geophysical study.

Electrical resistivity behaviour of sodium substituted manganites: electron-phonon, electron-electron and electron-magnon interactions

The present paper focuses on a quantitative analysis of the metallic and semiconducting behaviour of electrical resistivity in perovskite manganites La1- x Na x MnO3 (x = 0.1, 0.17). An effective interionic interaction potential (EIoIP) with the long-range Coulomb, van der Waals (vdW) interaction and short-range repulsive interaction up to second neighbour ions within the Hafemeister and Flygare approach was formulated to estimate the Debye and Einstein temperature and was found to be consistent with the available experimental data. For both doping concentration x = 0.1 and 0.17 the electron-phonon, electron-electron and electron-magnon interactions are effective to describe the resistivity behaviour for temperatures less than the metal-insulator transition (T P ). For temperatures, T > T P , the semiconducting nature is discussed with Mott's variable range hopping (VRH) model and small polaron conduction (SPC) model. The fitted density of states as revealed from VRH differs drastically from the experimental value and therefore means the VRH model is not a viable option for describing the resistivity behaviour in high temperature region, T > T P . The SPC model consistently retraces the higher temperature resistivity behaviour (T > θ D /2). The metallic and semiconducting resistivity behaviour of sodium substituted manganites are analysed, to our knowledge, for the first time highlighting the importance of electron-phonon, electron-electron, electron-magnon interactions and small polaron conduction.

Ab initio interionic potentials for UN by multiple lattice inversion

Based on the Chen–Möbius lattice inversion and a series of pseudopotential total-energy curves, interionic pair potentials for UN were derived. By means of molecular dynamic (MD), we have examined this interionic potentials. Comparing with the experimental data, the thermal expansion coefficient and the compressibility were well reproduced by this potentials.

High pressure behaviour of heavy rare earth mono antimonides

We have investigated theoretically the high-pressure structural phase transition and cohesive properties of two heavy rare earth mono antimonides (RESb; RE = Dy and Lu) by using two body interionic potential with necessary modifications to include the effect of Coulomb screening by the delocalized 4f electrons of the RE ion. The peculiar properties of these compounds have been interpreted in terms of the hybridization of f electrons with the conduction band and strong mixing of f states of RE ion with the p orbital of neighboring pnictogen ion. These compounds exhibit first order crystallographic phase transition from their NaCl (B1) phase to CsCl (B2) phase at 23.6 GPa and 25.4 GPa respectively. The bulk modulii of RESb compounds are obtained from the P-V curve fitted by the Birch equation of state. We also calculated the RE-RE distance as a function of pressure. Elastic properties of these compounds have also been studied and their second order elastic constants are calculated.

Interionic pair potential and entropy of mixing of Al–Mg compound forming binary molten alloys

The observed asymmetric behaviour of mixing properties of AlMg liquid alloys around equiatomic composition with smaller negative values for excess free energy of mixing (−0.15 kJ mol −1) has aroused our interest to undertake a theoretical investigation of this system. A simple statistical mechanical theory based on compound formation model ( μA + νB ⇌ A μ B ν ) has been used to investigate the energetics of formation of intermetallic compound Al 3Mg 2 in the melt, through the steady of thermodynamic property such as entropy of mixing. Besides, the interionic interactions between component atoms (Al and Mg) of the alloy have been understood through the study of interionic pair potential ϕ ij ( r), calculated from pseudopotential theory in the light of CF model. Our study of ϕ ij ( r) suggests that the effective interaction between Al–Al atom in the pure state is found to be smaller than those obtained in Al 0.425Mg 0.575 and Al 0.625Mg 0.375 alloys. Φ Al–Al in the case of Al 0.625Mg 0.375 alloys lies in between pure state (Al) and Al 0.425Mg 0.575 .The nearest neighbour distance does not change appreciably on alloying, whereas the effective interaction between Mg–Mg atoms increase in the order Al 0.625Mg 0.375 < Al 0.425Mg 0.575 < Mg pure, being minimum in CF alloys. The computed values of S M from pseudopotential theory are positive at all concentrations, but the agreement between theory and experimental is not satisfactory. This might be happening due to parameterization of σ 3 and Ψ compound.

Pressure dependent elastic and structural ( B3– B1) properties of Ga based monopnictides

By formulating an effective interionic interaction potential that incorporates the long-range Coulomb, the covalency effects, the charge transfer caused by the deformation of the electron shells of the overlapping ions, the Hafemeister and Flygare type short-range overlap repulsion extended up to the second neighbour ions and the van der Waals (vdW) interaction, the pressure dependent elastic and thermodynamical properties of the III–V semiconductors as Ga Y ( Y = N, P, As) are studied. The estimated values of phase transition pressure of Ga Y ( Y = N, P, As) are in reasonably good agreement with the available data on the phase transition pressures ( P t = 41, 22, 17 GPa). The vast volume discontinuity in pressure–volume phase diagram identifies a structural phase transition from zinc-blende ( B3) to rock salt ( B1) structure. Later on, the Poisson's ratio ν, the ratio R S/ B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic anisotropy parameter, elastic wave velocity, average wave velocity and Debye temperature as functions of pressure is calculated. From Poisson's ratio and the ratio R S/ B it is inferred that Ga Y ( Y = N, P, As) is brittle [ductile] in zinc-blende ( B3) [Sodium Chloride ( B1)] phase. To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of ductile (brittle) nature of Ga Y compounds and still awaits experimental confirmations.

Pressure induced mechanical properties of boron based pnictides

The elastic and thermodynamical properties of the III–V semiconductors as BY (Y = N, P, As) are calculated in zincblende and NaCl phases by formulating an effective interionic interaction potential. This potential consists of the long-range Coulomb, the Hafemeister and Flygare type short-range overlap repulsion extended up to the second neighbour ions and the van der Waals (vdW) interaction. The variations of elastic constants with pressure follow a systematic trend identical to that observed in other compounds of ZnS type structure family and the Born relative stability criteria is valid in boron monopnictides. From the elastic constants the Poisson's ratio ν, the ratio R S / B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic wave velocity, average wave velocity and thermodynamical property Debye temperature are calculated. By analyzing the Poisson's ratio ν and the ratio R S / B we conclude that at low pressures the boron monopnictides are brittle in nature in ZnS phase and ductile nature at high pressures in both ZnS and NaCl phases. To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of ductile (brittle) nature of BY compounds.

Pressure induced structural phase transition and elastic properties in BSb, AlSb, GaSb and InSb compounds

By formulating an effective interionic interaction potential that incorporates the long-range Coulomb, the covalency effects, the charge transfer caused by the deformation of the electron shells of the overlapping ions, the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbour ions and the van der Waals (vdW) interaction, the pressure dependent elastic and thermodynamical properties of the III–V antimonide semiconductors as YSb (Y=B, Al, Ga, and In) are investigated. Estimated values of phase transition pressure of YSb antimonides are consistent with the available data on the phase transition pressures. The ratio R S / B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic wave velocity, average wave velocity and Debye temperature as functions of pressure is calculated. From the ratio R S / B it is inferred that YSb (Y=Al, Ga, and In) are ductile and BSb is brittle in zinc blende (B3) phase. To our knowledge this is the first quantitative theoretical prediction of the ductile (brittle) nature of YSb antimonides and still awaits experimental confirmations.

A molecular dynamics simulation of lithium fluoride: Volume phase and nanosized particle

The PC GAMESS package was used to obtain interionic pair potentials for lithium fluoride. A molecular dynamics simulation of the volume phase and nanosized LiF particle was performed. The temperatures of fusion and self-diffusion coefficients of the volume phase and lithium fluoride nanoparticle were found; for the volume phase, they were close to the experimental data. The temperature of fusion of a particle ∼2 A in diameter decreased by ∼600 K. The possibility of a considerable increase in ionic conductivity over the temperature range 520–1122 K was demonstrated for nanosized LiF.

Interpretation of metallic and semiconducting temperature-dependent resistivity of La1−xNaxMnO3 (x=0.07, 0.13) manganites

Abstract In this paper, we undertake a quantitative analysis of the reported metallic and semiconducting behaviour of electrical resistivity in perovskite manganites La1−xNaxMnO3 (x = 0.07, 0.13). We have formulated an effective inter-ionic interaction potential (EIoIP) with the long-range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction up to second neighbor ions within the Hafemeister and Flygare approach to deduce Debye (θD), and Einstein temperature (θE). The temperature-dependent resistivity for temperatures less than metal–insulator transition (TMI), is theoretically analysed within the framework of the classical electron–phonon model of resistivity. Due to inherent low-frequency acoustic phonon as well high-frequency optical phonons, the contributions to the resistivity have been estimated. For low doping (x = 0.07; TMI = 194 K), the electron–phonon interaction is sufficient to describe the resistivity behaviour. However, additional electron–electron scattering (power temperature dependence) contribution to resistivity is required for higher doping as x = 0.13 (TMI = 301 K). For temperatures, T > TMI, the semiconducting nature is discussed with Mott’s variable range hopping (VRH) model and small polaron conduction (SPC) model. The SPC model consistently retraces the higher temperature resistivity behaviour (T > θD/2). The metallic and semiconducting behaviour of resistivity is theoretically discussed for the first time to our knowledge for La1−xNaxMnO3 in various compositions.

PHONON DYNAMICS AND THERMODYNAMICAL PROPERTIES OF ALKALI METAL DOPED C60 COMPOUNDS

The present paper deals with the comprehensive studies of phonon dynamics in alkali metal (M) doped C 60 ( MC 60) solids in fcc phase using a rigid ion model with pairwise interionic interaction potential. The investigations include the computation of the phonon density of states, the specific heat, the Gruneisen parameters, thermal expansion and thermal expansion coefficient and the phonon-dispersion curves of RbC 60. The analysis incorporates the van der Waal's and Coulomb interactions for alkali metal– C 60 as well C 60– C 60 molecule and show fairly good agreement with reported inelastic neutron scattering results for RbC 60. We have found that thermal expansion coefficient at room temperature is in the range 4.3–6.2 × 10-5 K -1, which is of the order of pure C 60 solid (6.1 × 10-5 K -1). Comparing the thermal expansion coefficient of different alkali-doped solids, we conclude that doping decreases anharmonicity in general, and further it decreases with increase in size of the alkali metal. The results have been found to present an overall consistent interpretation of the available experimental data.