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

Ab initio molecular-dynamics simulations are carried out for liquid Na0.8Pb 0.2 and Na-0.5 Pb-0.5 alloys to investigate the ionic structure, the electr onic states and especially the existence of the so-called 'Zintl ion' Pb-4( 4-) and the chemical complex Na4Pb. In our simulation, the existence of the se chemical units is not confirmed, but rather the intermediate-range order ing 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 io ns 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 bas is of the ionic configuration. A tendency towards ionicity or charge transf er is seen in both liquid alloys, though the valence-electronic charge dist ribution is not so localized around the ions.