Formation and geometry of a high-coverage oxygen adlayer on Ru(001), the p(2 x 2)-30 phase

A detailed low-energy electron diffraction (LEED)-IV analysis, complemented by scanning tunneling microscopy (STM) observations, was carried out for t he apparent (2 x 2) structure of the oxygen-covered Ru(001) surface at a co verage of 0.75 ML. We present STM images of incomplete layers which allow o ne to define the symmetry of the ordered layer, in particular of the novel high density p(2 x 2)-3O phase. In the LEED-IV analysis we have tested 28 m odel structures; the results can be used for conclusions about the discrimi nation of this type of geometry determination. Our quantitative LEED analys is in connection with the STM results corroborates the model proposed befor e and shows that all of the oxygen atoms sit in the hcp sites with an avera ged vertical distance to the outermost Ru layer of d(Ru-O)(perpendicular to ) = 1.22 Angstrom. This value falls into the general trend of increasing d( Ru-O)(perpendicular to) 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 Angstrom is ess entially unchanged compared to the other structures. Considerable lateral a nd vertical displacements of both the O and the Ru atoms are found, with th e O atoms being slightly displaced towards the fee hollow site located in t he center of three oxygen atoms. The two uppermost substrate layers are buc kled; in the first layer three out of four Ru atoms of the (2 x 2) unit cel l are shifted away laterally from their bulk positions. These shifts, globa lly as well as locally, can be understood in terms of local electron densit y changes induced by the adsorbed oxygen atoms.