Electronic structure and properties of designer clusters and cluster-assemblies S.N. Khanna and P. Jena Physics Department, Virginia Commonwealth University Richmond, VA 23284-2000, U.S.A. Received 28 June 1993 Abstract. Using self-consistent calculations based on density functional theory, we demonstrate that electronic shell filling and close atomic packing criteria can be used to design ultra-stable clusters. Interaction of these clusters with each other and with gas atoms is found to be weak confirming their chemical inertness. A crystal composed of these inert clusters is expected to have electronic properties that are markedly different from crystals where atoms are the building blocks. The recent observation of ferromagnetism in potassium clusters assembled in zeolite cages is discussed. PACS: 36.40.+d Small clusters containing two to few dozen atoms have emerged as a new state of matter. Extensive experiments and theoretical investigations on isolated clusters are showing that. the physical, electronic, magnetic or chemical properties of clusters [1] are often very different from individual atoms or bulk. The isolated clusters, however, have limited applications. For developing materials, the clusters have to be assembled together. When brought together, the clusters coalesce to form larger units and the properties characteristic of the smaller size evolve to bulk. The development of new materials using clusters, therefore, rests on the premise of designing clusters which will not coalesce when brought together. The properties of these materials will be dominated by those of individual clusters and are expected to be very different from those of the existing materials. The purpose of this paper is two fold. First we propose how by controlling size and composition, it should be possible to design clusters which will be very stable [2] as well as chemically inert [3]. These may provide ideal systems for generating cluster materials where each site would be occupied by' individual clusters. Secondly we offer an explanation to the recent discovery of ferromagnetism in Potassium clusters generated in zeolite cages. This part of our work relates to how the cluster assemblies can differ from bulk matter. We begin by discussing how to design clusters for forming cluster materials. That it is indeed possible to obtain clusterswhich will not merge when brought together is shown by two recent examples. The first relates to the discovery of bucky balls and the fullerene - solids [4]. Here, the isolated C60 clusters weakly interacting via Van der Waal's forces assemble together to form body centered cubic (BCC) or face centered cubic (FCC) arrangements with C60 occupying each lattice site. The properties of the pure or doped fullerides [5] are very different from other carbon solids and now constitute a major area of research. The second example is the twenty atom clusters MS C 12 containing 8 transition metal (M) atoms and 12 carbon atoms discovered recently by Castleman and co-workers [6]. These clusters called the metallo-carbohederenes or met- cars are proposed to have distorted dodecahedral shapes and join together by sharing common pentagonal faces maintaining individual identity. In both these examples, the stable clusters were discovered experimentally, by accident. We have been working to develop a general recipe for designing such clusters. Our arguments derive from two sets of experiments generating size selected clusters in beams. Firstly, experiments on smaller clusters of simple metals [7] show that clusters containing 2, 8, 18, 20, 40, ... atoms appear as dominant peaks in the mass spectra. These sizes called the magic numbers can be accounted for on the basis of a simple Jellium picture [7] where the cluster is modelled by a spherical Jellium. The magic numbers correspond to the sizes where the electronic shells are completely filled by the valence electrons. The added stability therefore reflects the electronic shell closure much in the same way as inert gas atoms in the periodic table. Secondly, experiments on larger simple metal clusters [8] showed that for clusters containing more than arounf 1000 atoms, the dominant cluster sizes correspond to clusters with complete geometric shells in an icosahedric or octahedric G. S. Anagnostatos et al. (eds.), Atomic and Nuclear Clusters © Springer-Verlag Berlin Heidelberg 1995