BOND-LENGTH INEQUALITIES AND RELAXATIONS AT THE RH(110)-C(2X2)-S SURFACE-STRUCTURE STUDIED BY LEED CRYSTALLOGRAPHY

A tensor LEED analysis has been made for the Rh(110)-c(2 x 2)-S surface structure using intensity-versus-energy curves measured for twelve independent beams at normal incidence. Each S atom chemisorbs on a centre site of the Rh(110) surface. It bonds to the second layer Rh atom directly below, with a bond distance equal to about 2.27 angstrom, and to four neighbouring first layer Rh atoms at close to 2.47 angstrom. A significant feature of this structure is that the second metal layer is buckled; those Rh atoms directly below the S atoms relax down by about 0.11 angstrom compared with the other second layer Rh atoms. This buckling is apparently driven by the need to reduce the difference that would otherwise occur between these two types of S-Rh bond lengths. A component in the observed difference between the S-Rh distances appears to be dependent on the metallic coordination number for the Rh atoms; in this regard, a comparison is made with the structural details for O chemisorbed on reconstructed Ni(110).