Equilibrium ordering properties of Au-Pd alloys and nanoalloys

Equilibrium configurations of bulk and surface Au-Pd alloys as well as of nanoalloy clusters are studied using Metropolis Monte Carlo importance sampling and the embedded atom method. The clusters contain about 1000 atoms. Three ordered bulk phases are predicted at low temperature, centred on compositions around 25%, 50%, and 75% Pd. The predicted order-disorder transition temperatures partially disagree with the available experimental results, but they are in good agreement with ab initio calculations. Surface enrichment in Au is systematically predicted, accompanied by partial subsurface enrichment in Pd, best enhanced around the equiatomic overall composition. The subsurface enrichment in Pd is suggested to play a decoupling role between surface and bulk conditions and, subsequently, ordered surface structures not induced by the order in the bulk are predicted at low temperatures. Clusters display similar segregation and ordering properties as flat infinite surfaces. However, the stability of the ordering at cluster surfaces is not globally characterized. The order-disorder transitions in the clusters occur at temperatures between 50 and 100 K lower than in the bulk. The disorder appears at the surface and proceeds to the core as the temperature is increased.