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Leed crystallographic studies of structures formed on the (110) and (111) surfaces of Rhodium

University of British Columbia

Surface crystallographic analyses have been undertaken with the tensor LEED approach in low-energy electron diffraction (LEED) in order to contribute to the development of the principles of surface structural chemistry. For the Rh(110)-c(2x2)-S surface structure, each S atom chemisorbs on a center site of the Rh(110) surface. It bonds to the second layer Rh atom directly below, with a bond distance equal to about 2.27 A, and to four neighboring First layer Rh atoms at close to 2.47 A. A significant feature of this structure is that the second metal layer is buckled; those Rh atoms directly below the S atoms relax down by about 0.11 A compared with the other second layer Rh atoms. This buckling is apparently driven by the need to reduce the difference that would otherwise occur between these two types of S-Rh bond lengths. A component in the observed difference between the S-Rh distances appears to be dependent on the metallic coordination number for the Rh atoms. For the corresponding higher coverage Rh(110)-(3x2)-S surface, the result supports an arrangement of chemisorbed S atoms at 2/3 monolayer (ML) coverage on a basically unreconstructed metallic structure. Alternating [110] channels in the metal surface are occupied differently, although each has two S atoms per unit mesh. In one set, the S atoms occupy long-bridge and center sites with a constant separation along the channel of 4.03 A. In the other set of channels, all S atoms occupy equivalent positions, displaced from regular center sites by 0.39 A, to give successive S to S separations of 3.47 and 4.60 A. The long-bridge site bonding is a novel feature which is facilitated by the neighboring topmost Rh atoms relaxing laterally by about 0.27 A perpendicular to the [110] row. Bucklings of 0.20 and 0.10 A are indicated to occur in the first and the second Rh layers respectively; the latter value essentially equals that (0.11 A) in the corresponding c(2x2) surface. For S atoms at or near center sites in the (3x2) structure, the average S-Rh bond distances to the first and the second Rh layer atoms are 2.42 and 2.29 A respectively; the corresponding values at the long-bridge sites are 2.20 and 2.27 A. New analyses for the Rh(111)-(>^x>/3)R30o-S and Rh(111)-c(4x2)-S surface structures indicate S coverages of 1/3 and 1/2 M L respectively. For the lower-coverage form, S adsorbs on the regular three-coordinate sites which continue the fcc packing sequence; the S-Rh bond lengths are indicated to equal 2.23 A, and relaxations in the metallic structure are negligible. In the c(4x2) form, the adsorption occurs equally on both types of three-coordinate site (fcc and hcp), although some surface Rh atoms bond to two S atoms while others bond to only one, and this sets up some interesting relaxations. Specifically, the S atoms displace laterally from the center of the threefold sites by 0.20 to 0.29 A, and the first metal layer is buckled by about 0.23 A. The first-to-second interlayer spacing in the metal expands to 2.26 A from the bulk value of 2.20 A. The average S-Rh bond length equals 2.22 A, and is not significantly changed from that in the low-coverage form. The structural evolution with increasing coverage for S chemisorbed on the (111) surface of Rh shows a close relation to the corresponding evolution on the Rh(110) surface. ln the Rh(111)-(2x1)-0 surface structure formed by chemisorption of O on the Rh(111) surface, O atoms chemisorb close to the regular fee sites. The structure shows significant relaxations; for example, a buckling of about 0.07 A is indicated in the first metal layer and O appears to displace laterally by about 0.05 A. The individual O-Rh bond lengths are around 2.01 and 1.92 A to top layer Rh atoms which bond to two and one O atoms respectively. Comparison is made with O-Rh bond lengths determined recently by another group for the Rh(110)-p2mg(2x1)-0 surface structure.

Thesis/Dissertation

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2009-03-19

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http://hdl.handle.net/2429/6219

Chemistry

University of British Columbia

UBCV

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