Structural investigation of ordered Sb adsorption phases on Ag(111) using coaxial impact collision ion scattering spectroscopy

The ordered overlayer structures formed following the adsorption of Sb on Ag(111) have been studied using coaxial impact collision ion scattering spectroscopy (CAICISS). At coverages up to several atomic layers there is evidence for layer-by-layer growth at room temperature, although the individual layers show no long-range order. Subsequent annealing gives rise to two ordered phases, a (√3 × √3)R30°Sb and a (2√3 × 2√3)R30°Sb. Neon ion CAICISS data have been used to distinguish between overlayer and substitutional structural models for the (√3 × √3)R30°Sb phase, and pure substitutional and mixed substitutional/overlayer models for the (2√3 × 2√3)R30°Sb phase. Despite the complexity of multiple scattering contributions, these data clearly favour the substitutional adsorption site for the (√3 × √3)R30°Sb phase giving a surface alloy of the form Ag ₂Sb. For the (2√3 × 2√3)R30°Sb phase, the data are best described by a model involving an ordered p(2 × 2)-Sb overlayer (periodicity relative to the substrate) superimposed on the (√3 × √3)R30°Sb surface alloy.