A QUANTITATIVE LEED ANALYSIS OF THE OXYGEN-INDUCED P(3X1) RECONSTRUCTION OF PT25RH75(100)

Pt25Rh75(100) forms a p(3 x 1) reconstruction at saturation coverage o f oxygen (23 L O-2, 600 degrees C). A previous STM study on O/Pt50Rh50 (100) suggests that every third row of the first substrate layer is sh ifted by half a lattice constant (''shifted rows''). We present a LEED I(E) analysis of Pt25Rh75(100) confirming the shifted-row model and f ind that oxygen resides in threefold-coordinated sites on both sides o f the shifted rows. The adsorbate occupies those of the threefold-coor dinated sites that are directly separated by the metal atom in the shi fted row. Further I(E) calculations exclude the alternative threefold- coordinated adsorption site beside the shifted row as well as the four fold-coordinated site symmetrically in between the shifted rows. We ac hieve a Pendry R-factor of 0.14 for the best-fit structure. Oxygen has equal bond lengths to its three metal neighbours, amounting to 1.95 A ngstrom. The first substrate layer relaxes outward by 8.8% of the bulk value to 2.08 Angstrom, but we do not observe any significant relaxat ions of deeper layer spacings. The shifted rows pop out of the surface by 0.38 Angstrom. After determination of the oxygen adsorption site w ith LEED, we examine local adsorption structures on Pt25Rh75(100) at l ow oxygen coverage with STM. We resolve the shifted rows in real-space , and for special tip conditions, we find maxima of apparent height at in-plane positions that coincide with the oxygen position as establis hed by quantitative LEED. We determine chemical-composition depth-prof iles by quantitative LEED for three surface preparations occurring dur ing sample preparation. While the first substrate layer of clean and a nnealed Pt25Rh75(100) is enriched in Pt (76%) as compared to the bulk value (25%), that in p(3 x 1)-O/Pt25Rh75(100) is enriched in Rh (90%). Oxygen adsorption at a moderate temperature (600 degrees C) and forma tion of the p(3 x 1) structure reverse segregation on Pt25Rh75(100). F inally, oxygen can be removed at room temperature by exposure of the s urface to hydrogen. This lifts the reconstruction but keeps the Rh enr ichment of the first substrate layer. (C) 1998 Elsevier Science B.V. A ll rights reserved.