The adsorption geometry of the (2x1)-20 oxygen phase formed on the Co(10(1)over-bar-0) surface

The bond geometry of the (2x1)-20-p2mg overlayer on Co(<10(1)over bar 0>) was determined by analyzing low-energy electron diffraction (LEED) intensity data. Oxygen occupies the threefold coordinated hcp site along the densely packed rows on the unreconstructed surface. The O atoms are attached to two atoms in the first Co layer and to one Co atom in the second layer. The strong interaction between O and Co is indicated by the bond lengths of 1.83+/-0.10 Angstrom and 1.99+/-0.10 Angstrom to the top-layer Co and the Co atoms in the second layer, respectively. The most striking result of our work is that oxygen adsorption causes a marked expansion (by 25%) of the first Co layer spacing (0.90 Angstrom) with respect to the bulk value of 0.72 Angstrom. This strong expansion might offer diffusion channels for O atoms to penetrate further into the subsurface region.