Tensor LEED analyses for three chemisorbed structures formed by iodine on a Pt(111) surface

Three distinct ordered iodine structures on a Pt(lll) surface have been studied with LEED crystallography in the coverage range 0.33-0.44 monolayers. These surfaces have translational symmetries of the ((root 3 x root 3)R30 degrees, (root 7 x root 7)R19.1 degrees and (3 x 3) types, and they all involve overlayer adsorption on the basically unreconstructed metal structures. The root 3 surface phase is indicated to have essentially all I atoms adsorbed at the regular threefold sites of the fee type (i.e. 3f sites), with no significant involvement by the corresponding sites of the hcp type (i.e. 3h sites). The root 7 structure has one I on an atop Pt site, and one each at 3f and 3h sites per unit mesh, while the (3 x 3) surface has one I on an atop site and three on bridge sites per unit mesh. The I corrugation is about 0.5 Angstrom for the root 7 structure, but is reduced to around 0.1 Angstrom at the (3 x 3) surface. The surface I-Pt bond lengths from these analyses show a general tendency to follow trends expected with the varying I coordination numbers. A preliminary discussion is given for uncertainties associated with some relaxations indicated in the metallic structures.