Competition of arsenic and sulfur segregation on Fe-9%W(100) single crystal surfaces

High-resolution medium-energy ion scattering (MEIS) was used to investigate the segregation of arsenic and sulfur on the (100) oriented surface of a body-centered cubic Fe-9 wt%W(100) single crystal containing 53 wt-ppm As and 10 wt-ppm S. At temperatures ranging from 800 to 100°C, both segregants compete for the available surface sites. Arsenic segregation dominates at temperatures around 800°C where maximum As surface concentrations of θ^max_As=0.40 were found. S segregation is most pronounced at temperatures around 1000°C with θ^max_S=0.43. MEIS shows the segregated As (S) atoms to be arranged 1.27 Å (1.22 Å) above the topmost metal layer. The metal-to-metal layer distances show an oscillatory behavior with an expansion of the first to second metal layer (+0.05 Å). With As and S on the surface, there is a strong W depletion of the topmost three metal layers.