Thorium encapsulated caged clusters of germanium: Th@Ge_n, n = 16, 18, and 20

We report from ab initio calculations that thorium encapsulation can be used to stabilize highly symmetric cages of germanium with 16 and 20 atoms. The lowest energy structures of these clusters are different from the recently found silicon fullerenes and are similar to clusters found in bulk metallic alloys. The binding energies of these clusters are higher compared with the values for the elemental germanium clusters of comparable sizes, and this suggests a strong possibility of their experimental realization in large quantities. Also, Th@Ge₁₆ has a large highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gap of 1.72 eV that makes it interesting for optoelectronic applications.