On the existence of monoxides on close-packed surfaces of lanthanide metals

Oxygen adsorption on Yb(111) and Lu(0001) was studied by 4f and valence-band photoemission. For both systems, ordered surface oxides were found which are different from the well-known sesquioxides. They are characterized as monoxides that are not stable as bulk compounds. For Yb, such a surface oxide is only observed below room temperature where diffusion of oxygen into the bulk is reduced. Lu monoxide is stable in the presence of a localized d-like surface state which does not contribute to chemical bonding. In their electronic structure, these surface oxides resemble the lanthanide monosulfides which exist as bulk compounds.