LEIS and AES on sputtered and annealed polycrystalline Pt-Ru bulk alloys

Low energy ion scattering spectroscopy (LEIS) and Auger electron spectroscopy (AES) were performed on sputtered and UHV annealed polycrystalline Pt-Ru bulk alloys over the entire compositional range. LEIS spectra were acquired with an estimated sputter damage of less than 0.5% of a monolayer by employing ⁴He ⁺ ions with an energy of 2 keV and a current density of 20 nA cm². The partial overlap of the scattering peaks of Pt and Ru made it necessary to devise a method to numerically fit the data. LEIS showed that sputter-cleaning with 0.5 keV Ar⁺ ions at 63° incidence does not effect any preferential sputtering. Annealing at 800°C in UHV causes a strong surface enrichment in Pt for equilibrated Pt-Ru alloys. The alloy with a hcp bulk lattice (9.5 at% Pt) exhibited a larger segregation than the fcc alloys (> 35 at% Pt), which we rationalized with the lattice mismatch between the bulk and the surface face structure, resulting in a loosely packed surface. Second-layer compositions of annealed specimens were extracted from AES and LEIS data, utilizing a calibration method based on Auger signals of the continuously sputtered surface. A smooth decay of the enriched outermost layer composition towards the bulk composition was assessed. The platinum surface enrichment of annealed bulk alloys was contrasted with the segregation observed for bimetallic supported clusters of Pt and Ru. Ideal solution thermodynamics, utilizing surface free energy data and molar surface area data from the literature, was able to predict the measured segregation for the UHV annealed bulk alloys.