Using a two-component lattice-gas model in a mean-field approximation, we calculate the surface tension and surface concentration of a model liquid alloy as a function of temperature and concentration. When the alloy has a negative heat of mixing (that is, has a tendency to phase separate), the concentration of the low-surface-tension component is much larger on the surface than in the bulk. This surface segregation decreases with increasing temperature, leading to a strongly negative surface entropy at small impurity concentrations and sufficiently low temperatures. The results are in agreement with an earlier density-functional calculation on ${\mathrm{Na}}_{\mathrm{x}}$${\mathrm{K}}_{1\mathrm{-}\mathrm{x}}$ as well as with experiments on ${\mathrm{Ga}}_{\mathrm{x}}$${\mathrm{Sn}}_{1\mathrm{-}\mathrm{x}}$. .AE

• Received 23 January 1986

DOI:https://doi.org/10.1103/PhysRevB.33.8048