Polarization and Structure of Small Clusters

In this paper we report the structure of the Li4Ge4 cluster as a function of charge transfer and polarizability. We find that for small charge transfer (Q < 0.5) this cluster has the expected cubic structure: a Ge4 tetrahedron with a Li ion attached at large distance to each face. With increasing charge transfer Q > 0.5 the structure of Li4Ge4 changes: for relative small polarizability of the Ge ion the Ge4 breaks up into two Ge2 pairs separated by the four Li. For larger polarizability there are three possibilities: 1) for full charge transfer the Li ions break up the Ge subcluster into four separated ions; 2) for smaller values of the charge transfer we still have the structure with two Ge2 pairs and 3) for intermediate values of, charge transfer the Ge sublattice forms a structure with two opposite bonds of the Ge4 tetrahedron cluster broken. These are the only stable geometries found. For large Ge polarizability we find that all structures become unstable: the size of the induced dipole moment becomes larger than the diameter of Ge. Based on this phase diagram of Li4Ge4 we discuss the structure of other A4M4 alkali(= A)–tetralide (= M) (= group 14) clusters, and related solid state structures. PACS: 36.40.Ei; 31.15.Ct; 31.10.+z. To be Submitted to J. Condens. Matter