Paper SS1+NC-TuA10
Alloy Nanocluster Distinct Surface Segregation Phenomena Related to the Coordination Dependence of Bond-Energies: Introduction and Application of a New Theoretical Approach

Tuesday, October 21, 2008, 4:40 pm, Room 208

This work introduces some novel aspects concerning alloy nanocluster equilibrium surface segregation phenomena and their modeling. The first involves the extraction of the Coordination dependence of Bond Energy Variations from DFT computed surface energies (the CBEV method). In particular, polynomial functions in terms of pair coordination numbers were fitted to DFT data reported recently for energies of pure Pt, Pd and Rh surfaces and small clusters. These functions yield elemental bond energy variations pertinent to any site at the cluster surface and subsurface layers. The second novel aspect, revealed when using the bond energy variations as input in statistical-mechanical computations based on the "free-energy concentration expansion method" (FCEM¹), concerns the finding that preferential pair-bond strengthening can lead to quite unique surface segregation characteristics. In particular: (i) In spite of the general tendency of Pd to segregate at Pt-Pd alloy surfaces, in cuboctahedron clusters certain (111) surface sites are populated by Pt, namely, a segregation reversal is predicted; (ii) Distinct Pd segregation profiles with maximal concentration at the subsurface layer are obtained for the Pt-Pd cluster (100) face, compared to subsurface oscillation in the Rh-Pd cluster (100) face. Further computations for Rh-Pd revealed inter-cluster separation involving “magic number” Pd segregated structures exhibiting various types of order. At higher temperatures atomic exchange among surface sites as well as surface-core desegregation processes are reflected in distinct configurational heat capacity peaks of the Schottky type.² As a test case, CBEV/FCEM computations done for the Pt-Rh(111) two-layer (oscillatory) profile show better agreement with previously reported experimental data as compared to our earlier work based on the NRL-TB/FCEM approach.³ Using the highly efficient CBEV/FCEM method enables to explore also compositional structures and properties of Rh-Pt-Pd ternary nano-clusters. Compared to current other approaches, this method is highly transparent, yielding better insight into the origin of surface segregation in bulk alloys and nanoclusters.

¹ M. Polak and L. Rubinovich, Surf. Sci. Rep. 38 (2000) 127.
² M. Polak and L. Rubinovich, Phys. Rev. B 71 (2005) 125426.
³ M. Polak and L. Rubinovich, Phys. Rev. B 75 (2007) 045415.