We present first-principles studies of the adsorption of Sb and Ag on
clean and Sb-covered Ag(111). For Sb, the substitutional adsorption si
te is found to be greatly favored with respect to on-surface fcc sites
and to subsurface sites, so that a segregating surface alloy layer is
formed. Adsorbed silver adatoms are more strongly bound on clean Ag(1
11) than on Sb-covered Ag. We propose that the experimentally reported
surfactant effect of Sb is due to Sb adsorbates reducing the Ag adato
m mobility. This gives rise to a high density of Ag islands which coal
esce into regular layers.