ORDERED AND DISORDERED RIPPLING IN THE COAL(110)-(1X1) SURFACE

A quantitative structural analysis of CoAl(110)-(1 x 1) by low energy electron diffraction shows that the topmost layer of the surface is ri ppled by an amplitude of 0.18 Angstrom, with aluminum atoms pulled out of the surface and the average distance to the second layer only slig htly expanded. In contrast to the perfect stoichiometry found for the top layer and the chemical order in the bulk, about 20% of the second layer Al atoms are substituted by Co. This is probably due to preferen tial sputtering during the course of sample preparation and incomplete subsequent reordering. A new version of Tensor LEED allows the detect ion of some disordered rippling induced by the substitutional disorder in this layer, with substitutional cobalt being 0.05 Angstrom above t he ideal aluminum position. The quality of the theory-experiment fit i s mirrored by a Pendry R factor value of R = 0.10.