Structure determination of the ordered (√3 × √3)R30° phase of Ni₂Si and Ni₂Ge surface alloys on Ni(111) via low-energy electron diffraction

Abstract The 3×3R30°structures of 2-D nickel silicide and nickel germanide surface alloys on Ni(111) were investigated using quantitative low-energy electron diffraction analysis. The unit cells of the determined silicide and germanide structures contain two Ni atoms and one Si or Ge atom, corresponding to the chemical compositions Ni₂Si and Ni₂Ge, respectively. Both the Si and Ge atoms adopt substitutional face-centered cubic hollow sites to sit on the Ni(111) substrates, and the alloy surfaces exhibit a slight corrugation. The NiSi and NiGe distances were 2.44 and 2.56 Å, respectively, and corresponded well to their respective empirical radii. It was also found that the Ni atoms in the second and third layers moved toward the Si or Ge atom sites from their bulk crystal positions. These results indicate rather strong interactions of the Ni atoms with both the Si and Ge atoms at the surface, leading to trapping of the Si and Ge atoms at the substitutional sites without diffusion into the bulk.