Static structure, collective dynamics and transport coefficients in the liquid Li-Pb alloy. An ab initio molecular dynamics study

M.M.G Alemany,Jaime Souto-Casares,Luis E González,David J González

doi:10.1016/j.molliq.2021.117775

Abstract

Several static and dynamic properties of the liquid Li-Pb alloy at diverse compositions, have been calculated by means of ab initio molecular dynamics simulation study. This alloy has attracted much attention because of the finding of fast sound at the Li0.80Pb0.20 composition and also the technological interest of the the eutectic composition, Li0.17Pb0.83, as a component of the blanket in fusion reactors.Results are reported for total static structure factors, which are compared with the available experimental data. An additional analysis of the structure allows the quantification of the heterocoordinating tendencies in this alloy, which at the Li0.80Pb0.20 composition are largest and lead to a Pb-centered polyhedral structure, where, however, Li4Pb units are not present.Regarding the collective dynamics, the calculated partial dynamic structure factors exhibit side peaks indicative of propagating density fluctuations, including density fluctuation modes with phase velocity greater than the hydrodynamic sound velocity. Also, the longitudinal and transverse dispersion relations have been calculated and its different branches analysed. We find all the high frequency branches to behave as optic-like modes, contrary to other interpretations in terms of an acoustic-like fast sound mode.Some transport coefficients such as self- and inter-diffusion coefficients, shear viscosities and adiabatic sound velocities, have also been calculated. Finally, the obtained results for the electronic density of states clearly indicate the metallic character of the liquid LixPb1-x alloy.

Highlights

In the last four decades, liquid binary mixtures have been extensively investigated, both experimentally and by computer simulations, with the main aim focused towards understanding the microscopic mechanisms behind the collective excitations and the interdifussion processes
Some more electron charge must be localized outside the clusters and this is what has been found in a recent ab initio study of this solid compound [46], which qualifies as an electride
This paper reports an ab initio MD (AIMD) simulation study of several static, dynamic and electronic properties of the liquid LixPb1Àx alloy at four concentrations

Summary

Introduction

In the last four decades, liquid binary mixtures have been extensively investigated, both experimentally and by computer simulations, with the main aim focused towards understanding the microscopic mechanisms behind the collective excitations and the interdifussion processes. The liquid alloy was modelled as a mixture of positive Li and negative Pb ions and the interactions among the ions were represented by interatomic pair potentials consisting of a hard core repulsion plus a screened Coulomb interaction They found that the Li-Li partial dynamic structure factor, SLiLiðq; xÞ, displayed a high-frequency side peak, which dispersed linearly with a velocity (% 7500 m/s) which is substantially greater than the hydrodynamic sound velocity of the mixture (cs % 2000 m/s). This feature was explained as a collective excitation supported by the light atoms and it was named the ‘‘fast sound” mode.

Technical details

Structural properties

Dynamic properties

Electronic density of states

Conclusions