Thermodynamic and structural properties of Ni-Al cluster assembled materials are investigated at the atomic scale. Model predictions are available for elemental systems but the field of bimetallic nanostructured systems remains close to unexplored. The aim of the present work is to model at the atomic scale the structural and segregation properties in the ${\mathrm{Ni}}_x{\mathrm{Al}}_{1-x}$ bimetallic cluster assembled materials that are synthesized in two different ways. In the first, isolated clusters are compacted at high pressure. We consider the ${L1\mathrm{}}_2$ and $B2\mathrm{}$ phases of the initial free clusters. Compaction of clusters at thermodynamic equilibrium is modeled by classical molecular dynamics combining isobaric and isothermal schemes. After compaction, interface segregation is computed by Metropolis Monte Carlo importance sampling in the semigrand canonical ensemble. After this model treatment, clusters are found to keep their identity, and their structural and segregation states do not differ much from those in the initial free clusters. The cluster cores keep the stable bulk phases while segregation may take place at the interfaces. The second method is low-energy cluster beam deposition. Cluster impact is found to influence chemical and structural order in the films formed. This is shown and discussed on the example of ${L1\mathrm{}}_2$ cluster deposition. Molecular dynamics is used therefore, which accounts for electron-phonon coupling in the equations of motion. The slowing down of a single cluster is examined in detail. It is found that the expitaxial accommodation of the cluster with the substrate and chemical order in the cluster depend on the mechanical properties of the substrate material. Competition between chemical order and epitaxy is observed. The harder the material, the higher the epitaxy and the lower the chemical order. The cluster impact induces significant chemical disorder but the clusters forming the cluster assembled film keep their initial identities. Similarly to the sample obtained by compaction, this one displays partial structural and chemical order at its interfaces. The film density is particularly low and the open volumes form a fully interconnected network of pores.

• Received 28 March 2002

DOI:https://doi.org/10.1103/PhysRevB.66.195408