Numerical simulation study of a low freezing point metallic alloy: Na-K-Cs.

A molecular dynamics study is performed of liquid Na0.139K0.435Cs0.426alloy known to have the lowest reported freezing point (195.2 K). Using Fiolhais's pair potential, we study the temperature dependence of the atomic structure of this alloy from 100 to 1000 K. Na atoms appear to play an important role in the lowering of its melting temperature. The self-diffusion coefficients as well as the viscosity are also computed. Their temperature dependence does not show any unusual behaviour. However, as temperature is nearing the melting point, diffusion and viscosity reach uncommon values for this kind of materials.

Viscosity of liquid Na-K alloys from molecular dynamics simulations.

The shear viscosity of liquid Na-K alloys is computed from molecular dynamics simulations using the Green-Kubo formalism. Interatomic interactions are described using effective pair potentials obtained from Fiolhais pseudo-potential and self-consistent screening. The composition dependence of the viscosity is first investigated at 373 K, then its temperature dependence at three different compositions, namely Na10-K90, Na50-K50, and Na90-K10. Simulation results are first compared with available experimental data. The evolution of the viscosity of the alloys versus temperature is similar to that of a pure one component fluid. This is discussed in connection with the chemical order of the mixtures.

The role of chemical order in the temperature and composition dependence of the viscosity of liquid alloys.

The influence of the chemical order on the viscosity of liquid alloys is investigated by numerical simulation of molecular dynamics. The temperature and composition dependence is discussed in the case of two contrasting alloys: K-Cs and Li-Bi. These two mixtures have different chemical orders, the first one being random and the second one having strong heterocoordination tendencies. In the case of K-Cs, the behavior of the mixture vs temperature is similar to a pure system and its viscosity varies monotonically with the composition. It is not the case for Li-Bi due to its marked chemical order and the heterocoordination tendency is accompanied by a maximum of the viscosity of the mixture when the composition is changed. For the first time, estimates of the temperature dependence of the viscosity of three representative Li-Bi alloys are given.

Viscosity of Lennard-Jones mixtures: A systematic study and empirical law.

A systematic study of the viscosity of the binary Lennard-Jones (LJ) mixtures is carried out by equilibrium molecular dynamics simulations via the Green-Kubo relation. The effects of mass, size, and energy-parameter asymmetries on the viscosity and the self-diffusion coefficients are examined separately, both in equimolar mixtures and by varying the molar fractions. The systems are mapped into an effective one-component model according to the van der Waals one-fluid (vdW1) model. Furthermore, using an empirical law for pure LJ liquids, similar to the one proposed recently for liquid sodium, it is shown that the viscosity of the mixtures studied here are well-predicted by the combination of vdW1 fluid and empirical law. The Stokes-Einstein relation in the mixtures has also been investigated. A possible simple extension of this relation, from pure liquids to mixtures, has been proposed and tested.

A search for manifestation of two types of collective excitations in dynamic structure of a liquid metal: Ab initio study of collective excitations in liquid Na.

Using a combination of ab initio molecular dynamics and several fit models for dynamic structure of liquid metals, we explore an issue of possible manifestation of non-acoustic collective excitations in longitudinal dynamics having liquid Na as a case study. A model with two damped harmonic oscillators (DHOs) in time domain is used for analysis of the density-density time correlation functions. Another similar model with two propagating contributions and three lowest exact sum rules is considered, as well as an extended hydrodynamic model known as thermo-viscoelastic one which permits two types of propagating modes outside the hydrodynamic region to be used for comparison with ab initio obtained time correlation functions and calculations of dispersions of collective excitations. Our results do not support recent suggestions that, even in simple liquid metals, non-hydrodynamics transverse excitations contribute to the longitudinal collective dynamics and can be detected as a DHO-like spectral shape at their transverse frequency. We found that the thermo-viscoelastic dynamic model permits perfect description of the density-density and current-current time correlation functions of the liquid Na in a wide range of wave numbers, which implies that the origin of the non-hydrodynamic collective excitations contributing to longitudinal dynamics can be short-wavelength heat waves.

Reply to Comment on 'An effective fitting scheme for the dynamic structure of pure liquids'.

We reply to the comment on our paper by Bafile et al (2014 J. Phys.: Condens. Matter 16 168002).

An effective fitting scheme for the dynamic structure of pure liquids.

A scheme of analysis for the dynamic structure functions in pure liquids is presented which can be implemented with both experimental and simulation data. Expressions for contributions of relaxing and propagating modes proposed earlier in the framework of the generalized collective modes approach are optimized in order to strictly fulfil three among the required sum-rules. The method is applied to simulation data for liquid cesium, the description of which appears to only require one relaxing and one propagating mode in the investigated wavevector range. These expressions are able to account for the dynamics in both the hydrodynamic and the kinetic regimes, being quantitatively accurate up to the onset of the first peak of the static structure factor and qualitatively beyond. Features of the modes can thus be obtained easily, without resorting to heavy formalism. The scheme of analysis can be straightforwardly extended to account for a higher number of relaxing and propagating modes.

Collective dynamics in binary liquids: a molecular dynamics study of the composition dependence of the spectra of collective excitations.

The spectra of longitudinal and transverse collective excitations in liquid binary metallic Na(c)K(1-c) alloys are studied for pure components and four different concentrations. A theoretical generalized collective modes approach is used to analyze the concentration dependence of the dispersion of acoustic and optic branches in a wide region of wavenumbers. The dispersion of longitudinal collective excitations in binary alloys is estimated from the eight-variable thermo-viscoelastic dynamic model with full account of thermal fluctuations. It is found that the longitudinal and transverse branches show different dependences on concentration in the short-wavelength region. The issue of 'positive dispersion' of acoustic excitations in liquid binary alloys on the boundary of the hydrodynamic regime is discussed. It is shown that the coupling between longitudinal acoustic and optic modes is responsible for an increase of the 'positive dispersion' close to equimolar composition.

Origin of large Landau-Placzek ratio in a liquid metallic alloy.

Dynamic structure factors for Na(c)K(1-c) liquid metallic alloys and pure components are studied by molecular dynamics simulations. Large values of Landau-Placzek ratio for four compositions of the liquid alloy are analyzed by wave-number dependent contributions from relaxation and propagating processes within the generalized collective modes method. The origin of the large Landau-Placzek ratio for liquid alloys is discussed.

Evolution of the liquid-vapor coexistence of the hard-core Yukawa fluid as a function of the interaction range.

The present work is devoted to the study of the liquid-vapor coexistence curve of hard-core Yukawa fluids for range parameter lambda, going from 0.5 to 7 by means of an integral equation approach. Both binodal and spinodal lines are computed and compared to available simulation data, and the integral equation used appears to be accurate. We also compare two methods for determining the coordinates of the critical point. The first one, using the rectilinear diameter law, appears to be less accurate than the second one based on the heat capacity at constant volume. It is found that the critical temperature decreases as the range of the interactions increases and that the liquid-vapor coexistence disappears for lambda greater than 6.

Collective dynamics in a liquid polyvalent metal: liquid thallium at the melting point.

Collective dynamics in liquid thallium at the melting point in a wide range of wave numbers and frequencies is studied by molecular dynamics simulations and a theoretical analysis of time correlation functions within the approach of generalized collective modes. The heat fluctuations were explicitly treated in the theoretical scheme within the thermoviscoelastic dynamic model. We report dispersion and damping of generalized longitudinal sound excitations, nonhydrodynamic shear and heat waves, as well as wave number dependence of main relaxation processes. Generalized wave number-dependent thermodynamic quantities and transport coefficients in liquid Tl are discussed.

Melting scenario in metallic clusters.

The isothermal Brownian-type molecular dynamics simulation has been applied to study the melting behavior of bimetallic clusters. It was found that the specific heat and Lindermann-like parameter customarily used in bulk system to describe solid-liquid transition show incongruity in the predicted melting temperature T(melt). The underlying mechanisms that lead to the incompatibility of T(melt) separately deduced from these two quantities were analyzed further. To gain insight into the melting behavior, we calculated in addition the velocity autocorrelation function and its Fourier transform, the power spectrum, and extracted from them the T(melt). It appears that the T(melt) inferred from the latter quantities is closer to that deduced from the principal peak position of specific heat. Two bimetallic clusters, namely, Ag(1)Cu(13) and Au(1)Cu(13), were selected for a thorough investigation. In the context of cluster morphology, we scrutinized the atomic distributions of Ag(1)Cu(13), Au(1)Cu(13), and Cu(14) and effected a comparative study between a bimetallic cluster and a pure cluster so as to learn from comparison the differences in the thermal reaction of atoms, in particular, the impurity atom in the bimetallic cluster. On analyzing the dynamical data, we observed at a lower temperature (T<

Transport properties of liquid nickel near the melting point: An ab initio molecular dynamics study.

The authors have investigated the dynamic properties of liquid nickel near the melting point by means of first principles molecular dynamics simulations in the framework of the density functional theory. Single-atom as well as collective dynamic properties are determined and transport coefficients are deduced. The calculation of the shear viscosity from the transverse current-current correlation function is examined in detail, and finite size effects are analyzed through a reference model to show the feasibility of this approach from first principles. The role played by the anharmonic character of the interactions is discussed.

Phase diagram of the hard-core Yukawa fluid within the integral equation method.

In this study, the integral equation method proposed recently by Sarkisov [J. Chem. Phys. 114, 9496 (2001).], which has proved accurate for continuous potentials, is extended successfully to the hard sphere potential plus an attractive Yukawa tail. By comparing the results of thermodynamic properties, including the liquid-vapor phase diagram, with available simulation data, it is found that this method remains reliable for this class of models of interaction often used in colloid science.