  We have carried out an extensive search for the SiC Bucky-diamond structure
to confirm not only that a pair of Si and C atoms can form sp2- as well as
sp3-type bonds but also that these two types of bonds can co-exist in the same
SiC-based structure. The successful and surprising discovery of the SinCm
Bucky-diamond structure at the specific composition of n=68 and m=79 is the
result of the relaxation of the truncated bulk 3C-SiC network to yield a SinCm
cluster with m +n=147. It highlights the important role played by the
composition in determining the structure and hence other properties of
SiC-based nano-structures. We have also shed light on the mechanism behind the
formation of the Bucky-diamond structure. The formation process is initiated by
the induced bonds between pairs of surface carbon atoms of the initial
configuration of the Si68C79 cluster obtained by truncating bulk 3C-SiC
network. This action then continuously incorporates atoms in the six outer
shells of the Si68C79 clusters to form the 112-atom Fullerene shell through
nearest neighbor Si-C interactions. Because the 35-atom inner core with five
completely filled shells only interacts weakly with the Fullerene shell through
the six atoms on its "surface", the diamond-like inner core is barely perturbed
and is suspended inside the Fullerene shell. We have also suggested a likely
route of synthesizing the SiC Bucky-diamond structure based on the result of
our simulation.
