Tensor LEED analyses for the (root 3x root 3)R30 degrees and c(4x2) structures formed by sulphur chemisorbed on the (111) surface of rhodium

Tensor LEED analyses have been made for the Rh(111)-(root 3 x root 3)R30 degrees-S and Rh(lll)-c(4 x 2)-S surface structures formed by S chemisorbed at 1/3 and 1/2 monolayer coverages respectively. For the lower-coverage form, S adsorbs on the regular three-coordinate sites which continue the fee packing sequence; the S-Rh bond lengths are indicated to equal 2.23 Angstrom, and relaxations in the metallic structure are negligible. In the c(4 x 2) form, the adsorption occurs equally on both types of three-coordinate site (fee and hcp), although some surface Rh atoms bond to two S atoms while others bond to just one, and this sets up some interesting relaxations. Specifically, the S atoms displace laterally from the centre of the three-fold sites by 0.20 to 0.29 Angstrom, and the first metal layer is buckled by about 0.23 Angstrom. The first-to-second interlayer spacing in the metal expands to 2.26 Angstrom from the bulk value of 2.20 Angstrom. The average S-Rh bond lengths equal 2.22 Angstrom, and so they are not significantly changed from that in the low-coverage form. The structural evolution for S chemisorbed on the (111) surface of rhodium with increasing coverage is compared with the corresponding evolution on the Rh(110) surface.