Tensor LEED analysis of the Ni(111) (root3xroot3)R30o-Pb Surface

The structure of the Ni(111)(√3×√3)R30°−Pb surface has been determined by quantitative low-energy electron diffraction (LEED), using multiple-scattering simulations of the measured diffracted beam intensities with a tensor-LEED program. The results confirm that the surface comprises a single-layer substitutional alloy of stoichiometry Ni2Pb (with all atoms in “fcc” sites relative to the underlying Ni) and clearly excludes a surface/subsurface stacking fault (with occupation of “hcp” sites) like that found for similar phases of Sb on Cu(111) and Ag(111). Within the surface alloy layer the Pb atoms are 0.73±0.05−Å higher above the surface than the surrounding Ni atoms in the alloy layer. This magnitude of rumpling is in excellent agreement with a recent medium-energy ion scattering investigation of this surface, but is significantly larger than that of an earlier low-energy ion scattering investigation. Compared to the rumpling amplitude of 1.67 Å expected from a simple hard-sphere model based on bulk metallic radii, however, it confirms a strong reduction of the effective atomic radii in this surface alloy.