The origin of the first sharp diffraction peak in liquid Na-Pb alloys: ab initio molecular-dynamics simulations

Abstract

Ab initio molecular-dynamics simulations are carried out for liquid Na0.8Pb0.2 and Na0.5Pb0.5 alloys to investigate the ionic structure, the electronic states and especially the existence of the so-called `Zintl ion' Pb44- and the chemical complex Na4Pb. In our simulation, the existence of these chemical units is not confirmed, but rather the intermediate-range ordering of Pb ions is seen in the liquid Na0.8Pb0.2 alloy. It is found from the calculated partial and total structure factors that this ordering of Pb ions leads to the first sharp diffraction peak of the total structure factor, which agrees well with the results of the neutron diffraction experiment. The composition dependence of the electronic states is explained on the basis of the ionic configuration. A tendency towards ionicity or charge transfer is seen in both liquid alloys, though the valence-electronic charge distribution is not so localized around the ions.