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

Abstract The structure and composition of a disordered Pt 25 Cu 75 (1 1 1) sample was studied by low energy electron diffraction. The r -factor, D 1 =0.096, illustrates a very good agreement between experimental and theoretical spectra. The Pt concentration versus depth oscillates around the bulk value (C 1 ,C 2 ,C 3 ,C 4 )=(28,8,48,8 at. % Pt ) . The most striking result is the quite large and unusual Pt enrichment in layer 3 ( C 3 =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 L1 1 phase, which exists only in the Cu–Pt system.