Order-disorder phase transition of the Cu₃Au(100) surface studied by ToF-ion scattering

Cu₃Au(100) surface has been investigated with ToF-ISS at different temperatures below and above the bulk order-disorder transition temperature (T = 663 K). Grazing angle azimuthal scans on the surface reveal the crystallinity of the first monolayer. The polar scans measured by 2 keV Ne ion scattering aligned along the major orientations, i.e. 〈100〉 and 〈110〉, show that the surface is terminated at room temperature by an ordered and rippled CuAu layer. The 〈100〉 rows are alternatively pure Cu rows and pure Au rows while the 〈110〉 rows are made by an alternated sequence of Cu and Au atoms. When increasing the temperature close to T, the Au focusing peak at low incidence, along the 〈110〉 rows, broadens in a first time owing to thermal vibrations, then shifts rapidly to higher incident angles when we approach the transition. This effect is interpreted as the result of the order-disorder transition at the surface, which modifies the ordered atomic sequence constituting the rows. By plotting the Au scattering yield vs. temperature at fixed azimuthal and incident angles, a strong decrease of the signal near the transition is observed. However, the transition is not abrupt, indicating possibly a second-order phase transition at the first monolayer, in opposition to the first-order bulk transition.