Magic compositions in Pd-Au nanoalloys.

We identified new magic compositions of Pd-Au nanolloys of simple symmetries according to the icosahedral and the cubic truncated octahedral structures. Chemically ordered structures as well as segregated ones are characterized using Monte Carlo simulations in semi-grand canonical ensemble and tight-binding semi-empirical potential fitted to ab initio calculations for the Pd-Au system. Ordering and surface (or core) segregation phenomena can be distinguished by their behavior as a function of the temperature: chemical ordering at surface or in the core disappears above a critical temperature (which is lower or equal to the bulk alloy order-disorder temperature), whereas surface/core segregation (core-shell structures) remains at high temperature, although the interfacial profile is less abrupt. [ABSTRACT FROM AUTHOR]