Au enrichment and vertical relaxation of the Cu3Au(111) surface studied by normal-incidence x-ray standing waves

We have investigated the Cu3Au(111) surface, prepared under ultrahigh vacuum conditions by sputtering and annealing, by low energy electron diffraction (LEED), scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy, and normal incidence x-ray standing waves (NIXSW). We find the surface to be depleted with Cu and enriched with Au at the same time, yielding a nominal Cu: Au ratio of 0.61:0.39 in the topmost layer. The STM images reveal that the first layer is nearly closely filled with atoms and contains a small amount of vacancies with an area concentration of about 5%. Together with the Au enrichment, these cause local short-range disorder of the Au p(2 x 2) reconstruction. From this data, the average stoichiometry of the p(2 x 2) surface unit cell is estimated at Cu2.22Au1.44 square(0.20) (instead of Cu3.00Au1.00 square(0.00) of the ideal surface; square denotes an atomic vacancy site). From NIXSW we find a significant outward relaxation of both the Cu and Au atoms of the topmost layer by 0.28 angstrom and 0.33 angstrom, which corresponds to 13% and 15% of the (111) bulk layer spacing of Cu3Au. We suggest that this originates from a widening of the first/second layer spacing, by 6.8% and 8.8% for the Cu and Au atoms, respectively, plus an additional rigid increase in the second/third layer spacing by 6.2%. We explain this by steric repulsions between Au atoms of the topmost layer, replacing smaller Cu atoms, and Au atoms in the second layer in combination with disorder. Finally, a lateral reconstruction, similar to that on the Au(111) surface, but with a much larger periodicity of 290 angstrom, is identified from LEED.