Diffuse x-ray scattering under grazing incidence was measured of a $\text{Ni-}23.2\text{at}\text{.}\mathrm{\%}\text{Pt}\left(110\right)$ surface at 923 K. The modulation of in-plane and out-of-plane scattering is characterized by the maxima in short-range order scattering located at positions of the X type (as for the bulk microstructure). The Warren-Cowley short-range order parameters from diffuse scattering are in good agreement with theoretical simulations based on bulk effective pair interaction parameters from first-principles theory. The nearest-neighbor interaction parameter is dominant also when determined from short-range order scattering. Its value is larger than for the bulk microstructure of Pt-rich alloys, what might reflect the difference in composition and is not compellingly due to the near-surface microstructure. Allowing for the presence of a tetragonal site occupation, indications for a segregation profile are weak in diffuse scattering and hardly resolvable for the Warren-Cowley short-range order parameters. This is consistent with the findings for layer-resolved short-range order parameters from theory. Theoretical simulations also demonstrated that the strong Ni enrichment of the surface layer has the same origin as the segregation reversal for the (110) surface in the case of $\text{Ni-}50\text{at}\text{.}\mathrm{\%}\text{Pt}$.

• Received 7 September 2009

DOI:https://doi.org/10.1103/PhysRevB.81.054205