Adsorbate-induced faceting of a nearly close-packed surface: Te-Pd(100)

The phase diagram of Te on face-centred cubic Pd(100) has been determined b y means of high-resolution spot profile analysing low-energy electron diffr action. We found two types of (root 5 x root 5)(n)R27 degrees structure wit h one (n=1) and two (n=2) atoms per unit cell, which develop successively t hroughout the coverage range from 0.06 to 0.5 ML. Below 0.20 ML the structu re coexists with a lattice gas and forms extended domain walls between its islands with a p(2x2) periodicity. At coverages between 0.2 and 0.4 ML both types of root 5 structure coexist, and metastable domain walls with c(2x2) structure are formed. Above 0.4 ML Te adsorption induces faceting to pyram idal nanostructures with (210)-oriented facets. A quantitative low-energy e lectron diffraction analysis has been carried out for Te(1x1) on Pd(210), w hich allows a microstructural explanation for the faceting process on Pd(10 0). We found adsorption on a sixfold-coordinated sire on this surface, and substantial Te-induced changes in vertical substrate relaxations up to 11% compared with the clean surface structure. Relaxations parallel to the surf ace and to the single mirror plane have been found to be below the detectio n limit for the substrate layers, whereas Te shows a significant shifting o f 0.1 Angstrom away from the highest coordinated adsorption site. Besides h igher Te adsorption energies on Pd(210), the reduction of surface stress on the adsorbate-covered surface, mainly due to the strong repulsive interact ion between the Te atoms, is supposed to play an important role in the face ting mechanism. (C) 2000 Elsevier Science B.V. All rights reserved.