Geometry and bonding in alkali-metal-atomantimony (AnSb4) clusters

The structures and stabilities of A2Sb4, A4Sb4, and A6Sb4 clusters (A=Li, Na, K, and Cs), some of which were detected by Knudsen effusion mass spectrometry, are investigated in this work by Hartree-Fock pseudopotential optimization. For each system examined, we consider two structural alternatives: one involving a tetrahedral Sb4 cluster nucleus and another containing a square of four Sb atoms. The high electronegativity differences between antimony and alkali-metal-atom cluster constituents give rise to a sizable electron transfer from the alkali-metal atoms to the Sb4 unit, which tends to flatten out the Sb4 tetrahedron. From our calculations, the systems A2Sb4 and A4Sb4 appear to be highly polar compounds consisting of a symmetric arrangement of alkali-metal atoms above and below an Sb4 square. For A6Sb4 complexes, in contrast, no unique characterization can be given. While an Sb4 square variant is proposed for A6Sb4 clusters with A=Na, K, and Cs, for the Li6Sb4 cluster a structure that contains a tetrahedral Sb4 unit is more likely. In general, all the AnSb4 clusters studied here exhibit substantial polarity. The Cs6Sb4 system, however, is characterized by metallic bonding features.