An ab initio molecular dynamics exploration of associates in Ba-Bi liquid with strong ordering trends

Abstract

Abstracts:Fictive associates are widely used to describe and model liquid phases with strong ordering trends. However, little evidence is known about the assumed associates in most cases. In the present work, an ab initio molecular dynamics (AIMD) study is employed to investigate the characters of the Ba-Bi liquid, in which associates have been assumed in existing thermodynamic modeling. It is found that in the Ba rich melt, the Bi atoms are almost completely surrounded by Ba atoms. The Bi-centered coordination polyhedrons are strongly associated to crystalline structures of Ba5Bi3 and Ba4Bi3 with a longer lifetime than other polyhedrons during the AIMD simulations. In addition, these Bi-centered polyhedrons in Ba rich melt connect with each other through vertex, edge, face, and/or bipyramid sharing to form medium range orders (MRO). In the Bi rich melt, the Ba-centered polyhedrons also form MROs, but they are both structurally and compositionally diverse with a shorter lifetime. These findings from AIMD study provide evidences that there exist a strongly ordering Ba4Bi3 associate and a weakly ordering BaBi3 associate in the Ba-Bi liquid. The predicted enthalpy of mixing in the liquid agrees well with the results by the CALPHAD modeling in the literature. Keywords: Short range order, medium range order, molten structure, enthalpy of mixing of liquid, ab initio molecular dynamics 3 1.Introduction Recently, Lichtenstein et al. [1] reported the extremely low chemical activity of Ba (aBa) in the Bi rich melt by measuring the electromotive force (EMF) of the Ba-Bi alloys.For example, at the barium mole fraction xBa = 0.05, the aBa value is as low as 6.3×10 -16 at temperature T = 773 K; and aBa = 8.7×10 -12 at T = 1123 K. These EMF results indicate that there exist strong chemical affinity and short-range order between Ba and Bi, i.e., there are chemical associates [2][3][4] in the liquid.There are two major thermodynamic approaches to account for strong short-range ordering, i.e., (i) the quasichemical model using pair approximation [5] and (ii) the associate model assuming a mixture of liquid single element species and molecule-like associates [3,4]. In the associate model, the compositions of the associates are usually the same as those of the intermetallic compounds with high melting temperatures or highly negative values of enthalpy of mixing [3,4]. The associate model has been successfully applied to model thermodynamic properties of binary and multicomponent systems [2,3,[6][7][8][9][10][11][12].Atomic structures in molten or amorphous alloys are usually considered to be ordered in short up to medium range with the existence of hexagonal, pentagonal, and/or quadrangular bipyramid entities, such as in the molten Al [13], Ni [14], Zr [15], glass Zr80Pt20 [16], glass Al75Ni25 [16], and liquid/glass Cu-Zr [17]. Solute-solvent atomic network (e. g., in glass Ni63Nb37 [16] and liquid Ni3Al [18]) or connected pentagonal bi-pyramids (e.g., in molten Al3Cu[19]) can also form medium range orders (MRO). It has been observe...

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