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Leed crystallographic studies for chemisorbed surfaces Saidy, Madiba


Quantitative low-energy electron diffraction (LEED), using the tensor LEED approach, has been applied to determine the detailed structures for five systems in which electronegative atoms (S, P and I) interact with surfaces of (111) orientation for three face-centered cubic (fcc) metals (Cu, Ni and Pt). The surfaces studied are designated Cu(111)-(√7x√7)R19.1°-S, Ni(111)-(√7x√7)R19.1°-P, Pt(111)-(√3x√3)R30°-I, Pt(111)-(√7x√7)R19.1°-I, and Pt(111)-(3x3)-I. This work aims to contribute to the development of principles for surface structural chemistry. A detailed analysis for the (√7x√7)R19.1° structure formed by S at the Cu(111) surface indicates a new structural model which corresponds to a modified version of the copper sulphide overlayer model first proposed by Domange and Oudar. The topmost layer has 3/7 monolayer each of Cu and S atoms, but the modification involves one S atom per unit mesh moving down to displace a Cu atom from the second metal layer. Relaxations among the topmost Cu atoms result in one S atom per unit mesh being effectively three-fold coordinate while the other two are six-coordinate and twelve-coordinate; the averaged S-Cu bond lengths are indicated to equal 2.19 A, 2.47 A and 2.62 A respectively. Some discussion is included of factors that may influence this choice of surface structure. For the (√7x√7)R19.1° structure formed by 3/7 monolayer of P at the Ni(111) surface, a novel reconstructed structure has been identified in which each P atom bonds to seven neighbouring Ni atoms, four in the top layer and three in the second layer, at an average distance close to 2.39 A. Formally this reconstruction involves three neighbouring Ni atoms in a triangular arrangement per unit mesh of the original unreconstructed surface being replaced by three P atoms. A discussion is included of the structural relaxations which occur in this surface as the demands of the Ni-P bonding are balanced against those for the Ni-Ni and P-P contributions. Comparisons are made with the √7 reconstructions observed for related systems, including those for S/Cu(111), P/Rh(111), and S/Pd(111). Three distinct ordered iodine structures on a Pt(l 11) surface (in the coverage range 0.33 to 0.44 monolayers) have been studied. These surfaces have translational symmetries of the (√3x√3)R30°, (√7x√7)R19.1° and (3x3) types, and they all involve overlayer adsorption on the basically unreconstructed metal surface. The √3 surface phase is indicated to have the I atoms adsorbed at the regular 3-fold sites of fcc-type (i.e. 3f sites), with no significant involvement by the corresponding sites of hep-type (i.e. 3h sites). The √7 structure has one I on an atop Pt site, and one each at 3f and 3h sites per unit mesh, while the (3x3) surface has one I on an atop site and three on bridge sites per unit mesh. The I corrugation is about 0.5 A for the √7 structure, but is reduced to around 0.1 A at the (3x3) 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.

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