GEOMETRY AND STABILITY OF GRAPHITIC ONION-LIKE STRUCTURES. Terrones, H., Instituto de Fisica UNAM, Apartado, Postal 20-364, C.P. 01000, Mexico, D.F. and Terrones M., School of Chemistry and Molecular Sciences, University of Sussex, Brighton BN1 9QJ, U.K

Using energy minimization with heptagonal and pentagonal rings of carbon in a graphite hexagonal mesh, we have produced quasi-spherical giant fullerenes which are suitable for forming stable bucky onions. A formation mechanism which explains the transformation, during high electron irradiation, of polyhedral graphite particles into graphitic onions is proposed. According to our mechanism, the most strained parts of the polyhedral particles (pentagonal rings and adjacent atoms) are destroyed during the irradiation generating flexible holey structures which can be moulded by energy minimization to obtain quasi-spherical giant fullerenes with holes. The holes are filled with heptagonal and pentagonal rings preserving the sphericity and avoiding the faceting characteristic of giant fullerenes with just 12 pentagonal rings. We also show that bucky-onions can be ordered (symmetric) or amorphous (non-symmetric) on the surface of a sphere. In general, the role of defects such as pentagons, heptagons and octagons in fullerenes, nanotubes and negatively curved graphite is analized. All these new structures open the field of a generalized crystallography in which atoms rest on surfaces with different curvatures: zero Gaussian curvature for nanotubes, positive curvature for fullerenes and negative curvature for cork-screw nanotubes and other hypothetical arrangements waiting to be discovered.