Structure and Growth of Small Palladium Clusters
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STRUCTURE AND GROWTH OF SMALL PALLADIUM CLUSTERS Y. S. Li*, Y. Cai**, and J. M. Newsam* *Biosym Technologies, Inc., 9685 Scranton Road, San Diego, CA 92121 "**Departmentof Physics and Astronomy, Rutgers University, Piscataway, NJ 08855 ABSTRACT We study the structure and growth sequence of small palladium clusters Pdn using the Many-Body Alloy (MBA) potential and simulated annealing techniques. Our results show the preference of compact polyhedral structures. These equilibrium structures are compared with the bulk Pd crystal in terms of cohesive energies and nearest neighbor distances. Both the cohesive energy and the nearest neighbor distance show a slow convergence to bulk behaviors. By analyzing the detailed structures and cohesive energies, we find that Pd4, Pd7 and Pdl3 are magic number structures, which are the consequence of their high symmetry and large coordination number. INTRODUCTION Geometrical structures of atomic clusters have attracted a large amount of attention recently [1-4]. By studying how the atoms are packed together in these clusters, one hopes to elucidate the evolution of small cluster into bulk material. From a technological point of view, this will provide insight into their physical and chemical properties and help us to develop fundamental understanding of their applications to catalysis, magnetic storage, photographic and electronic imaging. Quantum theory such as density functional theory, combined with various optimization methods, has been successfully utilized to the study of small clusters. Research on Si, Na, Al, and Mg has shown many interesting results. For example, the electronic shell structures lead to the extreme stability of alkali metal clusters with magic numbers. Semiconductor clusters have magic numbers associated with bonding rules. Rare gas clusters show magic numbers that can be interpreted in terms of filled geometrical shells and subshells of icosahedra. However, for transition metal clusters, our knowledge is limited by the complex nature of bonding and the heavily increasing computational demand. Many empirical potential methods have been used to study the geometrical structure of transition metal clusters. Despite their questionable applicability to small clusters, these models nevertheless can be used to presearch the 3n-6 dimensional configuration space for possible equilibrium structures of an atomic cluster. This presearch will provide useful information about the cluster structures and identify a limited number of geometries for a more sophisticated ab initio approach. Examples include the embedded atom method (EAM) [5],the effective medium method (EM) [6], and the corrected effective medium method (CEM) [7]. Unlike quantum theory with solid physical origin, the empirical potential methods require extreme care in choosing an interaction form. EAM atom potential improves over simple pair potential by taking the many body effect into consideration. In this paper, we intend to shed more light onto this problem by determining the detailed geometrical structure
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