Medium-energy ion scattering study of arsenic and sulfur segregation to the Fe-9% W(100) surface

High-resolution medium-energy ion scattering (MEIS) has been used to invest igate the segregation of arsenic and sulfur on the (100) oriented surface o f a body-centered cubic Fe-9% W(100) single crystal containing 53 wt-ppm As and 10 wt-ppm S. The sensitivities of Auger electron spectroscopy (AES), X -ray photoelectron spectroscopy (XPS), and MEIS to the presence of As on Fe or Fe-base alloy surfaces are discussed. At temperatures ranging from 800 to 1100 degrees C, both segregants compete for the available surface sites. Arsenic segregation dominates at temperatures around 800 degrees C, while S segregation is most pronounced at temperatures around 1000 degrees C. The total S and As coverage was found to be near 0.45 ML over the entire tempe rature range. Thermodynamic parameters, such as segregation free energies a nd interaction energies, have been determined utilizing the regular solutio n model proposed by Guttmann and McLean. Strong repulsive interactions betw een the segregants are found. MEIS structure analysis shows the segregated As and S atoms to be arranged 1.22-1.27 Angstrom above the topmost metal la yer (depending on surface composition). The metal-to-metal layer distances show an oscillatory relaxation behavior with an expansion of the first-to-s econd metal layer (+ 0.05 Angstrom). With As and S on the surface, there is a strong depletion of W in the topmost two metal layers. (C) 2000 Elsevier Science B.V. All rights reserved.