Structural analysis of a short range ordered layer with several adsorption sites : O/Ni(111)

A detailed structural analysis of O/Ni(111) around a coverage of 0.3 ML was performed where a disordered phase was formed in thermal equilibrium. The analysis of LEED and DLEED-I(E) curves showed that both hcp and fcc sites were occupied simultaneously at these coverages, whereas oxygen exclusively adsorbs on fcc sites in the ordered p(2x2) and (root 3xroot 3)R30 deg structures, which were completed at coverages of 0.25 and 0.33, respectively. This information, and the fraction of hcp and fcc occupation as a function of coverage, can already be obtained from an analysis of the integral order spots, as shown in the present analysis. Integrated intensities around the ( 1/2,0) and (3/2,0) positions were evaluated. In order to determine the short range order correlations between the adsorbed atoms, which govern the intensity distribution in reciprocal space, the complex scattering amplitudes, especially the phases, were needed. They were determined for selected K-points from full dynamical scattering calculations of ordered structures with fcc site and hcp site occupation. It turned out that a successful kinematic simulation of the LEED patterns could be carried out only for energies where the phase difference between hcp and fcc sites was small at all K-points of the p(2x2) structure. At these energies, the experimentally observed complex angular distribution of diffracted low energy electrons could be reproduced in all details with a model of small p(2x2) domains, with oxygen in fcc sites separated by a random distribution of domain walls (wall concentration fixed by coverage) in which the hcp sites were occupied. For other energies, the fast variation of these phase differences in reciprocal space highly complicates the situation and makes simulations unreliable.