THE SURFACE-STRUCTURE AND COMPOSITION OF (111) AND (100) ORIENTED SINGLE-CRYSTALS OF THE ORDERED ALLOY PT3SN

The surface composition and structure of [111] and [100] oriented single crystals of the ordered alloy Pt3Sn (Ll2 or Cu3Au-type) were determined using a combination of low-energy electron diffraction (LEED) and low-energy ion scattering spectroscopy (LEISS). The clean annealed surfaces displayed p(2 x 2) and c(2 x 2) LEED patterns, respectively, and the Sn/Pt LEISS intensity ratios at normal incidence were consistent with the surface compositions expected for bulk termination surface structures. In the case of the [100] crystal, preferential termination in the mixed (50% Sn) layer was indicated, which appears to be the consequence of the thermodynamic tendency for Sn to be at the surface. A strong variation in Sn/Pt LEISS intensity ratios with incidence angle was observed on both surfaces, indicating an outward displacement of tin atoms from the plane of platinum atoms.