Equilibrium chemical order and segregation at alloy surfaces and nanoclusters
computed using tight-binding derived coordination-dependent bond energies

Micha Polak, mpolak@bgu.ac.il and Leonid Rubinovich, rubin@bgu.ac.il. Department
of Chemistry, Ben-Gurion University, 84105 Beer-Sheva, Israel

Surface-induced bond energy variations, computed by the self-consistent NRL
tight-binding (TB) method, are incorporated as an elemental interaction model in
the statistical-mechanical free energy concentration expansion method
(FCEM). First, segregation profiles computed for clean and sulfur-covered Pt-Rh
surfaces are used as a test case, yielding good agreement with reported
experimental data, and highlighting the role of subsurface tensions in the
emergence of oscillatory profiles even in the absence of "chemical mixing"
tendency. Secondly, 923 atom cuboctahedron clusters of Pd-Cu reveal core and
surface mixed-type chemical ordering, whereas Pd-Rh tends to separate into
clusters exhibiting distinct demixed order. At high temperatures the clusters
exhibit surface segregation and disordering followed by desegregation processes,
all reflected in characteristic Schottky-type configurational heat
capacity. Comparing Pd-Cu with Pd-Cu-Rh reveals distinct ternary alloying
effects. The role of the TB computed bond energy variations in the segregation
related order-disorder transitions is demonstrated.