On the correlation between phonon spectra and surface segregation features in Ag-Cu–Ni ternary nanoalloys

Atomic scale characterization of chemical ordering, compositional distribution and microstructure is of tremendous importance for applications such as catalysis which is primarily dominated by processes occurring at surface and is strongly influenced by the subsurface layers. Phonon spectra obtained from molecular dynamics simulations of single metals as well as their bimetallic and ternary alloy nanoclusters can be used to obtain new insights into the atomic scale distribution in the nanoclusters, their microstructure and dynamical properties. Monte-Carlo (MC) simulations are used to obtain the minimum energy configurations of various Ag–Cu–Ni ternary alloys in which the Ag content is systematically varied from 0 to 50%Ag while keeping the relative composition of Cu and Ni constant. Detailed compositional analyses of the final MC configurations are carried out. The generated microstructure comprised of surface segregated structures in which Ag atoms occupy low coordination sites such as corners, edges and faces. As the Ag content in the ternary alloy is increased, the surface sites get increasingly occupied with the lowest coordination sites being populated first. The Cu and Ni compositions in the interior of the cluster show compositional oscillation. The final alloy microstructure is dictated by the competition between the various entropic and energetic factors. Our analysis of the phonon density of states identifies various surface (low frequency) and bulk (high frequency) modes which is determined by their location in the nanocluster and the local environment. Systematic trends in the observed peak intensities and frequency shifts at the low and high frequency ends of the spectrum for the various alloy compositions are explained on the basis of bond-lengths, local coordination, extent of alloying, and neighboring elemental environment. We find that the characteristic microstructural features observed at the atomic scale are strongly correlated to the vibrational densities of states of the constituent atoms in nanoalloys. Comparisons with experimental investigations are made where possible. Such a characterization method provides a predictive tool for materials which are extremely important for catalytic applications and emerging energy technologies.