Cu-capped surface alloys of Pt/Cu{100}

The room-temperature deposition of 0.5 monolayer (ML) Pt on Cu{100} followed by annealing to 525 K results in a sharp c(2 x 2) low-energy electron diffraction (LEED) pattern. The structure of this surface alloy is investigated by means of symmetrized automated tensor low-energy electron diffraction (SATLEED) analysis and ab initio plane wave density functional calculations. The results are then compared with those for the similar system 0.5 ML Pd/Cu{100}. SATLEED results for the Pt/Cu{100} show that it consists of an ordered c(2 x 2) Cu-Pt second layer alloy capped with a pure Cu first layer. The first and second interlayer spacings are found to be expanded by +5.1 +/- 1.7 and +3.5 +/- 1.7% respectively (relative to the bulk Cu interlayer spacing of 1.807 Angstrom) due to the insertion of the 8% larger Pt atoms into the second layer. The ordered mixed layer is found to be rippled by 0.08 +/- 0.06 Angstrom with Pt atoms rippled outwards towards the solid-vacuum interface. A smaller rippling of 0.03 +/- 0.11 Angstrom in the fourth pure Cu layer was also detected with Cu atoms directly underneath Pt atoms rippled towards the second layer Pt resulting in a Pt-Cu bond length of 2.52 Angstrom which compares with the sum of metallic radii of 2.67 Angstrom. A recent quantitative SATLEED analysis for 0.5 ML Pd/Cu{100} was performed in an earlier study (Barnes et al 2001 Surf. Sci. 492 55). A similar structure to the Pt/Cu{100} has been retrieved with slight differences in the interlayer spacings. The ab initio density functional results are fully consistent with the experimentally determined structures. However, they reveal an interesting difference between the stability of the Pd and Pt systems and highlight the fact that the Pd/Cu structure is best thought of as a metastable structure occurring as an intermediate step in the diffusion of the transitional metal ion into the bulk of the Cu substrate.