SURFACE ALLOY FORMATION AND THE STRUCTURE OF C(2X2)-SN/NI(100) DETERMINED BY LOW-ENERGY ALKALI-ION SCATTERING

The growth of Sn on Ni(100) and the structure of the c(2 X 2)-Sn/Ni(100) surface were studied using 500-eV Li+ scattering, Auger-electron spectroscopy, and low-energy electron diffraction (LEED). The polar-angle dependence of the Li+ scattering from Sn and Ni of the c(2 X 2)-Sn/Ni(100) bimetallic surface prepared at 250 K reveals that an ordered two-dimensional surface alloy is formed. However, Sn is not coplanar with the first Ni layer, but buckled above the Ni plane by 0.44+/-0.05 Angstrom. These studies show that Sn and Ni mixing occurs at low temperatures. When the surface alloy prepared at 250 K was annealed to above 400 K, additional features appeared in the polar-angle dependence of Li+-Ni single scattering, suggesting that small surface reconstructions occur on the c(2 X 2) alloy surface. Further annealing to 900 K results in a LEED pattern characterized by a splitting of the half-order beams into quartets. Formation of out-of-phase domains may be responsible for this LEED pattern.