Ordering and segregation of a Cu75Pt25(111) surface:: A first-principles cluster expansion study

Configurational thermodynamics for Cu75Pt25 (111) surfaces is examined by a first-principles calculation in conjunction with the cluster-expansion technique. The calculated surface segregation profile just below the bulk order-disorder transition temperature exhibits Pt segregation to the top and the third layers and Cu segregation to the second layer. No long-range ordered structure is found at the top layer. However, the simulated short-range-order parameter shows a small negative value, indicating a weak ordering tendency at the Cu75Pt25 alloy surface. This fact can be interpreted by the competition between the layer-confined spontaneous p(2x2) ordering tendency and the Pt segregation and interlayer surface ordering, which certainly disrupts the p(2x2) ordering. Five possible surface ground-state structures are found for the bulk L1(2) structure. The surface ground-state structures exhibit strongly localized electronic states composed of Pt and Cu d states at the topmost layer around 3 eV below the Fermi energy, which is consistent with the previous ultraviolet photoelectron spectroscopy measurement. One of the surface ground-state structures possesses additional surface states just below and above the Fermi energy, which are composed of Pt d states at the topmost layer. A significant ordering effect on the surface electronic states is confirmed for the Cu75Pt25 alloy.