  The thermodynamic properties of Bi-Sn were studied at 600 and 900K using a
quasi-lattice theory. After successful fitting of Gibbs free energies of mixing
and thermodynamic activities, the fitting parameters were used to investigate
the enthalpy of mixing, the entropy of mixing, concentration fluctuations,
Warren-Cowley short range order parameter, surface concentrations and surface
tensions of the binary systems. Positive and symmetrically shaped enthalpies of
mixing were observed in all composition range, while negative excess entropies
of mixing were observed. Bi-Sn showed a slight preference for like-atoms as
nearest neighbours in all composition range. The nature of atomic order in
Bi-Sn at 600 and 900K appeared similar. The highest tendency for
homocoordination exists at composition where mole fraction of Bi is about 40%.
It was also observed that Bi (whose surface tension is lower than that of Sn)
has the highest surface enrichment in the Bi-Sn systems. Unlike many previous
applications of the quasi-lattice theory where constant values were used to
approximate coordination numbers, temperature and composition-dependent
coordination numbers were applied in this work.
