Abstract
Pt 25Rh 75(100) forms a p(3×1) reconstruction at saturation coverage of oxygen (23 L O 2, 600°C). A previous STM study on O/Pt 50Rh 50(100) suggests that every third row of the first substrate layer is shifted by half a lattice constant (“shifted rows”). We present a LEED I( E) analysis of Pt 25Rh 75(100) confirming the shifted-row model and find that oxygen resides in threefold-coordinated sites on both sides of the shifted rows. The adsorbate occupies those of the threefold-coordinated sites that are directly separated by the metal atom in the shifted row. Further I( E) calculations exclude the alternative threefold-coordinated adsorption site beside the shifted row as well as the fourfold-coordinated site symmetrically in between the shifted rows. We achieve 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 Å. The first substrate layer relaxes outward by 8.8% of the bulk value to 2.08 Å, but we do not observe any significant relaxations of deeper layer spacings. The shifted rows pop out of the surface by 0.38 Å. After determination of the oxygen adsorption site with LEED, we examine local adsorption structures on Pt 25Rh 75(100) at low 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 established by quantitative LEED. We determine chemical-composition depth-profiles by quantitative LEED for three surface preparations occurring during sample preparation. While the first substrate layer of clean and annealed Pt 25Rh 75(100) is enriched in Pt (76%) as compared to the bulk value (25%), that in p(3×1)-O/Pt 25Rh 75(100) is enriched in Rh (90%). Oxygen adsorption at a moderate temperature (600°C) and formation of the p(3×1) structure reverse segregation on Pt 25Rh 75(100). Finally, oxygen can be removed at room temperature by exposure of the surface to hydrogen. This lifts the reconstruction but keeps the Rh enrichment of the first substrate layer.
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