Segregation and shape stability in small bimetallic particles

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Abstract
The equilibrium atomic spatial distribution of a 55-atom bimetallic particle is studied within the regular solution model. As a consequence of the large surface-to-volume ratio, the criterion for phase separation versus long-range ordering in small particles and in infinite systems differs considerably. With the comparison of the total energies in cubo-octahedral (C) and icosahedral (I) particles, a phase diagram for the ground state is obtained. As a function of temperature the particle may change its shape from C to I. We applied the theory, based on estimations from bulk properties, to the CuPd and the IrPd particles, and present results for the temperature dependence of the average concentrations at the different shells around the central atom. On the same basis some predictions for the NiPt, CuPt, AuPt, and the CuNi systems are made.