Structural and electronic properties of chemisorbed oxygen on Rh(111)

Using density-functional theory within the generalized gradient approximation, we investigate the adsorption of oxygen on Rh(lll) as a function of coverage Theta. At the coverages of 0.25 monolayer (ML) and 0.5 ML oxygen (occupying fee-hollow sites), the calculated atomic geometries (interlayer spacings, bond lengths, and lateral displacements) are in excellent agreement with those of previous low-energy electron-diffraction intensity analyses. We find a strong coverage dependence of the oxygen-induced buckling relaxations and a substantial overall expansion of the first interlayer spacing, reflecting a weakening of metal-metal bonds between the two outer substrate layers. The work functions of the relaxed structures are presented, and the stability of the adlayers is analyzed. We also predict that the existence of a dense O overlayer (Theta = 1 ML) with a (1 x 1) periodicity is possible, because oxygen will be incorporated as a subsurface adsorbate only when the (1 x 1) adlayer is (nearly) completed.