Structural study of the adsorption of Sb on Ag(111) using medium energy ion scattering

The structure of the Ag(1 1 1)(root3 x root3)R30degrees-Sb surface phase formed by a nominal 1/3 ML of Sb has been investigated by medium energy ion scattering (MEIS) using 100 keV H+ ions. This surface is widely accepted as involving an outermost layer comprising a substitutional alloy, but some recent studies have indicated that the atoms in this layer may occupy hcp hollow sites at the surface, directly above second layer Ag atoms of the underlying substrate, creating an effective stacking fault at the surface-alloy/substrate interface. The new MEIS study shows that it is possible to form both faulted and unfaulted surface phases, probably dependent on the previous Sb dosing history of the Ag(l 11) sample. The faulted surface phase may also be accompanied by sub-surface stacking faults in the Ag(I 1 1) substrate. The higher Sb coverage (0.6 +/- 0.1 ML) Ag(1 1 1)(2root3 x 2root3)R30degrees-Sb was found to be accompanied by multiple stacking faults in some 20 outermost layers of the Ag(l 11) substrate to an essentially hcp structure. The effect is tentatively attributed to a lowering of the stacking fault energy induced by low concentrations of dissolved Sb. The detailed structural parameters of the faulted surface Ag(1 1 1)(root3 x root3)R30degrees-Sb phase are in excellent agreement with those found in previous studies by electron and X-ray diffraction. (C) 2002 Elsevier Science B.V. All rights reserved.