Abstract
After annealing at 970 K low energy ion scattering (LEIS) reveals the topmost atomic layer of Pt 25Ni 75(100) to be enriched in Pt to a value of 39 at%, whereas for Pt 25Ni 75(110) the first monolayer is found to consist nearly exclusively of Ni atoms and the second layer shows strong Pt enrichment. LEIS measurements performed after ion bombardment at room temperature indicate the superposition of the effects of preferential sputtering of Ni atoms and segregation. Segregation is found to start at rather low annealing temperatures, whereas high annealing temperatures are necessary to allow thermodynamic equilibration by bulk diffusion. Most of the theoretical models fail to reproduce the orientation dependent change in the segregating component of the PtNi system. One of the few successful approaches is a thorough thermodynamic description which is used here to calculate the composition profile and to discuss the segregation behaviour. Good agreement between measurement and calculation is found.
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