Surface composition of Pt-Rh alloys : the role of lattice vibrational entropy

The bulk and surface composition of a polycrystalline Pt-Rh alloy, equilibrated at 1000 K., have been analysed by use of Auger electron spectroscopy. The results are discussed in relation to experimental data in the literature. Although the enthalpy of sublimation and the atomic diameters of Pt and Rh do not differ very much, a significant enrichment of Pt in the surface is observed. It is shown that the surface segregation behaviour in atomically clean Pt-Rh alloys can be understood quantitatively by taking into account the difference in vibrational entropy of bulk and surface atoms of the constituent metal