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 by means of high-resolution spot profile analysing low-energy electron diffraction. We found two types of (root 5 x root 5)(n)R27 degrees structure with one (n=1) and two (n=2) atoms per unit cell, which develop successively throughout the coverage range from 0.06 to 0.5 ML. Below 0.20 ML the structure 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 pyramidal nanostructures with (210)-oriented facets. A quantitative low-energy electron diffraction analysis has been carried out for Te(1x1) on Pd(210), which allows a microstructural explanation for the faceting process on Pd(100). 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 surface and to the single mirror plane have been found to be below the detection limit for the substrate layers, whereas Te shows a significant shifting of 0.1 Angstrom away from the highest coordinated adsorption site. Besides higher Te adsorption energies on Pd(210), the reduction of surface stress on the adsorbate-covered surface, mainly due to the strong repulsive interaction between the Te atoms, is supposed to play an important role in the faceting mechanism. (C) 2000 Elsevier Science B.V. All rights reserved.