An unusual composition profile: a LEED–TBIM study of Pt25Cu75(1 1 1)

The structure and composition of a disordered Pt25Cu75(1 1 1) sample was studied by low energy electron diffraction. The r-factor, D1=0.096, illustrates a very good agreement between experimental and theoretical spectra. The Pt concentration versus depth oscillates around the bulk value (C1,C2,C3,C4)=(28,8,48,8 at.% Pt). The most striking result is the quite large and unusual Pt enrichment in layer 3 (C3=48%) followed by the still important Pt depletion in layer 4 as opposed to the damped oscillations observed for similar Pt–Ni(1 1 1) and Pt–Co(1 1 1) alloys. This result, which is in contradiction with a previous LEIS study, is interpreted within the Tight Binding Ising Model and is related to a termination of the L11 phase, which exists only in the Cu–Pt system.