First-principles study of bulk ordering and surface segregation in Pt-Rh binary alloys

The cluster expansion technique in conjunction with first-principles calculations has been applied in Monte Carlo simulations to derive the configurational thermodynamics of the bulk and (111) surface of Pt-Rh alloys. Lattice-dynamics calculations reveal that the vibrational contribution to Pt-Rh bulk phase stability is fairly negligible. Calculated short-range-order parameter, ground state, and ordering transition temperature T-c of bulk Pt50Rh50 are in satisfactory agreement with experimental values in the literature. Calculated composition profiles of the (111) surface at T=1373 K show the enrichment of Pt at the top layer and Pt depleted at the second layer for the entire composition, which is in agreement with experimental observations. At low temperatures, a significant difference is found in the temperature dependence of the layer composition profile between Pt25Rh75 and Pt50Rh50. While Pt composition of the Pt25Rh75 subsurface shows positive temperature dependence, that of Pt50Rh50 has a minimum at T similar to 300 K. The former can be qualitatively interpreted by taking account of the on-site energy only. The latter is due to the occurrence of sublayer-confined phase transition from (root 3 x root 3) R30 degrees order to disorder alloys.