Structural and thermodynamic properties of elemental and bimetallic nanoclusters: an atomic scale study

Structural and thermodynamic properties of elemental and bimetallic nanoclu sters are studied at the atomic scale. The modelling is achieved by means o f molecular dynamics (MD) and Metropolis Monte Carlo (MC) sampling in the s o-called transmutational ensemble. The cohesion model used is based on the second moment approximation of the tight binding model. Copper elemental an d NixAl1-x binary alloy clusters are selected as case studies. Particles co ntaining less than n = 201 atoms are predicted to be structureless, except when elemental, formed by n = 13, 55, 135 and 147 atoms. These so-called ma gic numbers allow icosahedral geometry. Binding energies are not found to b e significantly dependent on morphology, suggesting the coexistence of seve ral isomers. As far as NixAl1-x clusters are concerned, phase stability is systematically studied as a function of x, ranging from 0 to 1 and discusse d with reference to the bull: ordered alloy. Except in one special case, an d in contrast to elemental clusters, no stable phase at all is found in the smallest clusters (IE < 201) as they are structureless. In the larger ones , consistently with a recent study with another cohesion model (Campillo J M, Ramos de Dibiaggi S and Care A 1999 J. Mater. Res. 14 2849), a partition shows up between a core where the bulk stable L1(2) and B2 phases are retr ieved and a mantle which may be subjected to aluminium segregation. In the range of cluster sizes considered (n = 13-10000), the results suggest that, because of the easy surface segregation, the martensitic metastable phase occurring in bulk Ni-Al systems does not take place in free clusters. The s egregation efficiency is found to decrease with increasing cluster size whi le the relative mantle thickness is size independent. This may be the reaso n why the martensitic phase only occurs in systems larger than currently in vestigated.