Order-disorder phase transitions of oxygen on Rh(100)

The adsorption and ordering of oxygen atoms on the Rh(100) surface have been investigated by spot profile low-energy electron diffraction measurements as a function of coverage and temperature. Three oxygen induced structures were observed: p(2×2), c(2×2), and (2×2)p4g. Using the intensity and the full width at half maximum of the diffraction spots, the (2×2)p4g→c(2×2) and c(2×2)→(1×1) order-disorder phase transitions have been characterized and the phase diagram mapped out. The (2×2)p4g→c(2×2) transition is assigned to the two-dimensional Ising universality. The role of the adsorbate-adsorbate interactions in the formation of the ordered phases and the kind of defects involved in the order-disorder phase transitions are discussed.