The study of dynamics and phase transitions of small Ag/Pd motifs using molecular dynamics and histogram methods

Abstract

The structural and thermodynamic properties of bimetallic PdnAg(13−n) (n⩽13) have been carried out using the Molecular Dynamics (MD) with Sutton–Chen potential and histogram methods, respectively. Thirteen particle icosahedral clusters of neat palladium and silver atoms were first reproduced accordingly with the results in literature. Then in the silver icosahedra, each silver atom has been successively replaced by palladium atom. Calculation is repeated for both palladium-centered and silver-centered clusters. It is found that the palladium-centered clusters are more stable than the silver-centered clusters and cohesive energy increases along the silver end to palladium end. Phase transition of each cluster species is studied by means of caloric curve, root mean square bond fluctuation and heat capacity. Trend in variation of melting temperature is accordance with the energy trend. Silver centered cluster shows a premelting at low temperature due to onset of more facile motions of center particle (silver) and a transformation to a low energy palladium-centered configuration in the cluster depending on the cluster compositions. Species-centric order parameters and velocity autocorrelation functions are used to understand the dynamic behavior in the solid and liquid phases. Mixing of silver–palladium occurs in the liquid phase with higher silver concentration and palladium-core/silver-shell cluster formation would occur at low silver concentration.