  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.
