Structural metastability of endohedral silicon fullerenes

Endohedrally doped ${\text{Si}}_{20}$ fullerenes appear as appealing building blocks for nanoscale materials. We investigate their structural stability with an unbiased and systematic global geometry optimization method within density-functional theory. For a wide range of metal-doping atoms, it was sufficient to explore the Born-Oppenheimer surface for only a moderate number of local minima to find structures that clearly differ from the initial endohedral cages but are considerably more favorable in terms of energy. Previously proposed structures are thus all metastable.

Figure 1

The first and third columns show the lowest energy structures with a 20 silicon atom cage for all $M@{\text{Si}}_{20}$ systems studied. The dopant atom $M$ is brighter than the silicon atoms. These structures are in general already quite distorted and of low symmetry. The second and fourth columns show the lowest energy configurations found by minima hopping for the same dopants. In these structures, the transition metals stay inside the silicon atoms, but the silicon atoms form cages of less than 20 atoms with some peripheral silicon atoms. For the other metal dopants, the cage structure is entirely lost.