Ordering and segregation of aCu75Pt25(111)surface: A first-principles cluster expansion study

Abstract

Configurational thermodynamics for ${\mathrm{Cu}}_{75}{\mathrm{Pt}}_{25}\phantom{\rule{0.3em}{0ex}}(111)$ 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 ${\mathrm{Cu}}_{75}{\mathrm{Pt}}_{25}$ alloy surface. This fact can be interpreted by the competition between the layer-confined spontaneous $p(2\ifmmode\times\else\texttimes\fi{}2)$ ordering tendency and the Pt segregation and interlayer surface ordering, which certainly disrupts the $p(2\ifmmode\times\else\texttimes\fi{}2)$ ordering. Five possible surface ground-state structures are found for the bulk $L{1}_{2}$ structure. The surface ground-state structures exhibit strongly localized electronic states composed of Pt and Cu $d$ states at the topmost layer around $3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ 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 ${\mathrm{Cu}}_{75}{\mathrm{Pt}}_{25}$ alloy.