Density functional study of onion-skin-like [As@Ni 12 As 20 ] 3 and [Sb@Pd 12 Sb 20 ] 3 cluster ions Jijun Zhao a , Rui-Hua Xie b, * a Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA b Quantum Process Group, Atomic Physics Division, National Institute of Standards and Technology, Mail Stop 8423, 100 Bureau Drive, Gaithersburg, MD 20899, USA Received 28 January 2004; in final form 31 May 2004 Abstract A comprehensive study of the structural and electronic properties of the interesting onion-skin-like [As@Ni 12 As 20 ] 3 cluster ion, characterized by Moses et al. [Science 300 (2003) 778], was carried out using a plane-wave based density functional theory. The cal- culated interatomic distances agree well with experiment. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for [As@Ni 12 As 20 ] 3 are fivefold-degenerate with h u and h g symmetries, respectively, and its HOMO– LUMO gap is determined to be 0.2 eV lower than that of C 60 . The static dipole polarizability of [As@Ni 12 As 20 ] 3 is two times larger than that of C 60 . The optical gap of [As@Ni 12 As 20 ] 3 is redshifted by 1.4 eV relative to that of C 60 . The possibility of synthesis of [Sb@Pd 12 Sb 20 ] 3 is proposed. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction The sp 2 and sp 3 bonding characteristics of carbon give rise to a variety of pure carbon structures starting from the well-known planar graphite and tetrahedral diamond [1] to novel forms such as fullerenes [2], carbon onions and nanotubes [3]. They are not only able to dis- play the nature of their bonding and the novel mechan- ical, electronic, optical and magnetic properties [3,4], but also able to serve as vehicles for the synthesis of new structures by inserting atoms, molecules and clusters [5]. This has fueled much research into study of highly connected and symmetric molecular cages, especially, carbon-free fullerenes although experimental confirma- tions have been few [6–16]. For example, In 74 and Na 12 In 48 clusters in the Na 96 In 97 X 2 (X = Ni, Pd, Pt) phases [15] and Si 20 cages in the clathrate-II compound Cs 8 Na 16 Si 136 [16] are well-known fullerene-like cages. From the fundamental point of view, a cluster with a fullerene-like cage structure presents a microscopic form of matter that would exhibit unusual stability and/or reactivity [2,3,5]. On the other hand, a cluster with both closed atomic and electronic structures could be consid- ered as a stable superatom [17] and serve as building blocks for novel molecular solids [1,17,18] and nanoelec- tronic devices [19]. Recently, stimulated by the prediction that the fulle- rene-like cluster As 20 , unlike P 20 and N 20 clusters which are generally believed to be unstable with respect to the formation of P 4 and N 2 , respectively [20], is stable and isolable [8], Moses and co-workers [13] have synthe- sized and characterized a novel onion-skin-like [As@Ni 12 As 20 ] 3 cluster ion in both solid and gas phases. This ion contains interpenetrating reciprocal platonic polyhedron [13]: an undistorted As 20 pentago- nal dodecahedron and an arsenic-centered Ni 12 icosahe- dron. This ion is composed of 60 triangular faces and virtually undistorted molecular I h point symmetry. To 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.121 * Corresponding author. Fax: +1 301 990 1350. E-mail addresses: zhaojj@mailaps.org (J. Zhao), rhxie@nist.gov (R.-H. Xie). www.elsevier.com/locate/cplett Chemical Physics Letters 396 (2004) 161–166