Segregation and Phase Transitions in Reduced Dimension: From Bulk to Clusters via Surfaces

Abstract

To describe the thermodynamics of bimetallic clusters, we use approaches that have been successfully employed for bulk alloys and their surfaces. We detail what happens for bulk and surface phase transitions when considering nanoalloys with a tendency to phase separation. A rigid-lattice approach allows us to analyze the behaviour of surface and core shells of the nanoalloys. We discuss the existence of bistabilities (or dynamical equilibrium) which are the analogous of surface and bulk phase transitions in semi-infinite alloys. Such dynamical equilibrium is susceptible to affect the cluster facets in an individual way (individual dynamical equilibrium), whereas the inner shells show a collective bistability (collective dynamical equilibrium). Then, we compare these bistabilities obtained in the semi-grand canonical ensemble with the results obtained in the canonical ensemble, before discussing the relation between experimental conditions and the two thermodynamic ensembles. Finally, a first attempt to establish generalized phase diagram for bimetallic clusters is proposed.