Structure ofO/Fe(001)-p(1×1)studied by surface x-ray diffraction

The geometric structure of O/Fe(001)-p(1x1) is studied by surface x-ray diffraction (SXRD). After dosing of about 4 L (1 L = 10(-6) Torr s) of oxygen at room temperature we find approximately one monolayer (ML) of oxygen in an FeO-like structure forming two layer thick islands covering about 40% of a roughened Fe(001) surface. Subsequent annealing up to 500 C results in surface flattening leading to a highly ordered structure. Very precise SXRD data reveal the presence of 1 ML of oxygen atoms located in fourfold hollow sites at d((O-Fe)) = 0.48 +/- 0.08 angstrom above the first Fe layer. The first Fe-Fe-interlayer spacing is expanded to d(12) = 1.66 +/- 0.02 angstrom corresponding to an increase of 16% relative to the bulk spacing (1.43 angstrom). Density-functional calculations confirm our findings and indicate a strong dependence of the (local) layer expansion on the oxygen coverage. Our results are important for understanding the surface magnetic properties of the O/Fe(001)-p(1x1) surface in general.