Formation and geometry of a high-coverage oxygen adlayer on Ru(001), the p(2 × 2)-3O phase

A detailed low-energy electron diffraction (LEED)-IV analysis, complemented by scanning tunneling microscopy (STM) observations, was carried out for the apparent (2 × 2) structure of the oxygen-covered Ru(001) surface at a coverage of 0.75 ML. We present STM images of incomplete layers which allow one to define the symmetry of the ordered layer, in particular of the novel high density p(2 × 2)-3O phase. In the LEED-IV analysis we have tested 28 model structures; the results can be used for conclusions about the discrimination of this type of geometry determination. Our quantitative LEED analysis in connection with the STM results corroborates the model proposed before and shows that all of the oxygen atoms sit in the hcp sites with an averaged vertical distance to the outermost Ru layer of d_Ru-O^⊥ = 1.22 Å. This value falls into the general trend of increasing d_Ru-O^⊥ with oxygen coverage observed for the other ordered structures of adsorbed oxygen on Ru and is also predicted by recent total energy calculations. The O-Ru bonding distance of about 2.0 Å is essentially unchanged compared to the other structures. Considerable lateral and vertical displacements of both the O and the Ru atoms are found, with the O atoms being slightly displaced towards the fee hollow site located in the center of three oxygen atoms. The two uppermost substrate layers are buckled; in the first layer three out of four Ru atoms of the (2 × 2) unit cell are shifted away laterally from their bulk positions. These shifts, globally as well as locally, can be understood in terms of local electron density changes induced by the adsorbed oxygen atoms.