Density-functional study of the compressed (2√3 × 2√3)R30°-CO overlayer on the ferromagnetic Co(0001) surface

By using the first-principles pseudopotential density-functional calculations, we study the possible structures of the experimentally observed compressed ( 2 3 × 2 3 ) R 30 ∘ -CO overlayer (θ = 0.58) on the ferromagnetic Co(0001) surface. As the most stable structure, the geometry with one CO in the atop site and six other ones tilted by 12∘ in approximately bridge positions per the elementary cell is derived. Besides that, we find two very similar (differing by occupation of the hcp or fcc hollow site) metastable geometries. Here, one hollow, and two triples of tilted molecules shifted away from atop and bridge positions, respectively, are found. The average magnetic moment per surface Co atom is always about 1 μB. CO stretching frequencies, local bond-lengths and local magnetic moments correlate roughly with those derived previously for the CO θ = 1/3 coverage. There exists analogy to compressed CO overlayers at Ru(0001), Ir(111) and Ni(111) that deserves further attention.