TENSOR LEED ANALYSES FOR THREE CHEMISORBED STRUCTURES FORMED BY IODINE ON A Pt(111) SURFACE

Three distinct ordered iodine structures on a Pt(111) surface have been studied with LEED crystallography in the coverage range 0.33–0.44 monolayers. These surfaces have translational symmetries of the$(\sqrt{3}\times \sqrt{3})R30^\circ$,$(\sqrt{7}\times \sqrt{7})R19.1^\circ$and(3×3)types, and they all involve overlayer adsorption on the basically unreconstructed metal structures. The$\sqrt{3}$surface phase is indicated to have essentially all I atoms adsorbed at the regular threefold sites of the fcc type (i.e. 3f sites), with no significant involvement by the corresponding sites of the hcp type (i.e. 3h sites). The$\sqrt{7}$structure has one I on an atop Pt site, and one each at 3f and 3h sites per unit mesh, while the(3×3)surface has one I on an atop site and three on bridge sites per unit mesh. The I corrugation is about 0.5 Å for the$\sqrt{7}$structure, but is reduced to around 0.1 Å at the(3×3)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.