Ab initiostudy of silicon-multisubstituted neutral and charged fullerenes

The electronic and structural properties of Si-doped fullerenes, obtained from C₆₀ by replacing up to 12 C atoms with Si atoms, are studied by means of first-principles density-functional theory calculations using numerical atomic orbitals as basis sets. We have analyzed the relative stability of several C_60-xSiₓ isomers, with $x=1,2,3,6,$ and $12.$ We find that, for $x=3$ and 6, the Si atoms strongly ``prefer'' to be first neighbors of each other, in order to minimize the number of Si-C bonds. However, these configurations of lower energy present a few relatively weak Si-C and Si-Si bonds that could indicate possible breaking paths. For a better comparison with the experimental measurements, we have also considered some positively charged ions, and report the differences between properties of these ions and the corresponding neutral molecules.