ORDER-DISORDER PHASE-TRANSITIONS OF OXYGEN ON RH(100)

The adsorption and ordering of oxygen atoms on the Rh(100) surface hav e been investigated by spot profile low-energy electron diffraction me asurements as a function of coverage and temperature. Three oxygen ind uced structures were observed: p(2X2), c(2X2), and (2X2)p4g. Using the intensity and the full width at half maximum of the diffraction spots , the (2x2)p4g-->c(2x2) and c(2X2)-->(1X1) order-disorder phase transi tions have been characterized and the phase diagram mapped out. The (2 X2)p4g-->c(2X2) transition is assigned to the two-dimensional Ising un iversality. The role of the adsorbate-adsorbate interactions in the fo rmation of the ordered phases and the kind of defects involved in the order-disorder phase transitions are discussed. [S0163-1829(97)06935-X ].