Tensor LEED analysis of theNi(111)(3×3)R30°−Pb

Abstract

The structure of the Ni(111)()ϫ))R30°-Pb surface has been determined by quantitative low-energy electron diffraction ͑LEED͒, 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 Ni 2 Pb ͑with all atoms in ''fcc'' sites relative to the underlying Ni͒ and clearly excludes a surface/subsurface stacking fault ͑with occupation of ''hcp'' sites͒ like that found for similar phases of Sb on Cu͑111͒ and Ag͑111͒. Within the surface alloy layer the Pb atoms are 0.73Ϯ0.05-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.There are now many examples of the observation that the deposition on metal surfaces of atoms in the submonolayer coverage range can lead to these adsorbate atoms occupying substitutional sites in the outermost atomic layer to produce a single-layer surface alloy. 1 This can even occur for some adsorbate/substrate element combinations immiscible in the bulk, reflecting the different energetics of the surface. One question that has attracted some interest in these systems is what the role of effective atomic radii of the adsorbate atoms is in these surface alloys, which determines the degree of ''rumpling'' of the alloy layer. If the deposited atoms have larger atomic radii than that of the substrate atoms they replace, then because the periodicity of the surface alloy parallel to the surface is fixed at the value of the underlying substrate ͑i.e., the surface alloy layer is pseudomorphic͒, a simple hard-sphere picture would predict that the adsorbate atoms would have a larger layer spacing relative to the underlying second layer of the substrate than that of the undisplaced outer-layer substrate atoms. This simply reflects the inability of a larger atom to be fully accommodated in the vacant site produced by the removal of a smaller substrate atom from the surface layer. Recent quantitative structure determinations of quite a number of surface alloy phases, however, indicate that the rumpling observed is almost always less than that predicted by this simple hard-sphere model, and this has been suggested to be a consequence of the valence electron depletion in the surface layer due to spillover into the vacuum, allowing the surface-layer atoms to approach one another more closely than in a bulk solid. 2 One system for which this effect appears to be especially large is the Ni(111)()ϫ))R30°-Pb surface formed by 0.33 ML of Pb on Ni͑111͒. A structural investigation of this surface was made by low-energy ion scat...

LEEDI–Vand DFT structure determination of the (\surd 3\times \surd 3)\mathrm {R}30^{\circ } Pb–Ag(111) surface alloy

“…This amplitude of the 'rumpling' is smaller by about 1 Å than that predicted by a simple touching hard-spheres model based on bulk metallic radii (1.44 Å for Ag and 1.80 Å for Pb). This suggests a reduction in the effective radii similar to that found by Quinn et al [6] in their LEED study of the substitutional surface alloy structure for the Pb-Ni(111) system. It has been suggested that the reduction in the effective radii may be attributed to the influence of valence electron charge smoothing and associated surface stress effects [5,24,6,13].…”

“…This suggests a reduction in the effective radii similar to that found by Quinn et al [6] in their LEED study of the substitutional surface alloy structure for the Pb-Ni(111) system. It has been suggested that the reduction in the effective radii may be attributed to the influence of valence electron charge smoothing and associated surface stress effects [5,24,6,13]. A similar effect is possibly the case for the Pb-Ag(111) structure in our study.…”

Adsorption structure determination of a large polyaromatic trithiolate on Cu(111): combination of LEED-I(V) and DFT-vdW

“…It is commonly agreed that R P values below 0.2 indicate an excellent agreement between experimental and theoretical I(V) curves, whereas values above 0.3 are interpreted as mediocre fits. Excellent refinements yielding very low R P values in the range 0.11-0.24 were reported for complex inorganic surface structures such as CoO(111), 48 GaN(0001), 49 and BaFe 2 As 2 (001) 50 as well as surface alloys or metal superstructures on metals like Pb/Ni(111), 51 Sn/Ni(110), 52 Sn/Ni(111), 53 Sb/Cu(110), 54 and Au/Pd(100). 55 Moreover, relatively simple atomic superstructures could also be refined with high accuracy, examples comprise hydrogen on Ir(110), 56 (R P = 0.10) and halogens on metal surfaces such as Cl/Ru(0001) 57 and Br/Pt(110) 58 with R P = 0.19 and 0.23, respectively.…”

Lead overlayer dynamics on Ni(111)

“…The study of the growth and morphology of thin and submonolayer films composed of heavy elements of group IV on semiconductor and metal substrates receives much attention from both experimental and theoretical perspectives. 1–13 To date, it has been established using various methods that the submonolayer adsorption of heavy elements on the substrates results in the formation of two-dimensional surface phases. 1–3,14,15 This can be either an adlayer consisting only of adatoms or a surface alloy.…”