The structure of the Pd(111)-(√7x√7)R19.1°-S surface:: comparison with the corresponding P/Rh(111) surface

The structure of the Pd(lll)-(root 7 x root 7)R19.1<degrees-S surface has been determined by low-energy electron diffraction and has been shown to involve a reconstructed topmost metal layer based on a packed arrangement of Pd pentagons and triangles, with S atoms at 3/7 monolayer coverage occupying eight-coordinate sites at the centre of each Pd pentagon. Comparisons are made with the structure recently determined for the corresponding P/Rh(lll) system. In both cases, the atomic density in the reconstructed layer increases to five metal atoms and three chemisorbed atoms for every seven atoms in the regular metal surface. Some details are closely similar between the two systems, but an intriguing difference is also indicated. In S/Pd(lll), four metal atoms in each pentagonal ring appear close to hcp-type three-fold sites, while the other is close to a fee-type three-fold site. This contrasts with the situation for the topmost layer of the P/Rh(lll) system, for which the reversed lateral registry has been reported (i.e. the atoms of each pentagonal ring occupy three-fold sites where four are of the fee type and one of the hcp type). (C) 1997 Elsevier Science B.V.