Surface and sub-surface segregation at the Pt25Rh75(1 1 1) surface: a medium energy ion scattering study

Using 100 keV incident He+ ions, medium energy ion scattering (MEIS) has been used to determine the layer-by-layer composition of the outermost three atomic layers of a Pt25Rh75(1 1 1) crystal after annealing to temperatures in the range 900–1300 K. Highly layer-specific compositional information was obtained using three different double-alignment scattering geometries, although full analysis of the data included simulation of blocking curves recorded around the double alignment geometry. The results provide independent confirmation of the conclusions of an earlier quantitative low energy electron diffraction (LEED) investigation of the same crystal, notably strong Pt segregation to the outermost layer, enhanced average Pt segregation with increasing temperature, and Pt depletion of the second layer. MEIS also confirms the presence of a weak rumpling of the outermost atomic layer, with Pt atoms lying at slightly larger layer spacing than the surrounding Rh atoms. The quantitative agreement is significantly better than in the only previous comparison of LEED and MEIS for the determination of layer-dependent alloy surface segregation. Possible reasons for anomalously large surface vibrational amplitudes are discussed.