THE STRUCTURE OF DENSE SULFUR LAYERS ON RU(0001) .1. THE C(2X4) STRUCTURE

The detailed geometrical structure of the c(2 x 4)-2S phase of S/Ru(00 01) was investigated by LEED-IV analysis using the energy range betwee n 30 and 400 eV. After averaging, 18 non-equivalent beams with a total energy range of 3600 eV were analyzed. The S atoms occupy sites close to the hcp and fcc threefold sites, but are shifted away from the sym metric position laterally by 0.16 angstrom in opposite directions alon g the mirror plane, which was maintained in our analysis. Significant buckling for whole rows of atoms vertical to the mirror plane was foun d in the first substrate layer. The rows coordinated with 2 sulphur at oms are located 0.19 angstrom further out than those which are directl y coupled to only 1 S atom. Slight pairing of these rows (by 0.1 angst rom) was also observed, but no further positional modulation within th e rows. In the second layer only a small buckling was found with the a tom below the hcp site moving closer to the bulk by 0.05 angstrom. The se positional changes take place so that the S-Ru bond length to the n earest-neighbour atoms varies very little (2.24-2.29 angstrom) and is close to the one found at lower coverage. Also the average substrate l ayer distances are close to the bulk values with a small expansion (0. 03 angstrom) of the first to second layer distance.