Stability of charged Si-doped heterofullerenes: A first-principles molecular dynamics study

We study the structural stability of the singly and doubly charged (positively and negatively) Si-doped heterofullerene C30Si30 via density-functional theory calculations combined with first-principles molecular dynamics. Geometry optimization aimed at establishing the most stable configurations at T=0 K shows that C30Si30 undergoes very limited changes in the bond lengths after addition or extraction of one or two electrons. Consideration of thermal motion reveals that the dynamical stability is not significantly altered in C30Si30- with respect to the neutral case. On the contrary, Si-Si and C-C bond stretching followed by rapid fragmentation, occurring in less than 1 ps, are observed for C30Si30--. This effect is encountered in two sets of calculations performed with the periodic cell and the isolated cell boundary conditions for the heterofullerene. In the periodic case, fragmentation is due to the predominance of Si atoms carrying charges of equal sign in the Si-rich portion of the cage. In the isolated case, the number of neighboring charges of equal sign is reduced but the strength of the residual repulsive interaction is sufficient to destabilize the network at finite temperature. The lack of stability of doubly charged Si-doped heterofullerenes confirms that the observed charged species are the singly charged ones.