LEED CRYSTALLOGRAPHIC STUDIES FOR THE RH(100)-P(2X2)-S AND RH(100)-C(2X2)-S SURFACE-STRUCTURES

New LEED intensity-versus-energy curves have been measured for the p(2 x 2) and c(2 x 2) surface structures formed by the chemisorption and annealing of H2S on to the (100) surface of rhodium, and the data analyzed with multiple scattering calculations to investigate structural details. Bucklings are indicated in the second metal layer of magnitudes 0.01 and 0.06 angstrom for the c(2 x 2) and p(2 x 2) surfaces respectively; for the latter structure the top layer metal atoms are laterally relaxed by around 0.05 angstrom. A surprising feature in the present work concerns the possibility that S is displaced laterally from the symmetrical centre site of 4-fold coordination, by around 0.27 angstrom, toward bridge sites. This shifting of S has a small, but definite, effect on the intensity curves of the higher order fractional beams. At this time, it does not appear as an artefact introduced by uncertainties in the direction of incidence, the second metal layer buckling or the surface Debye temperature.