DISORDERED AND ORDERED C-28 SOLIDS

Using tight-binding molecular dynamics, we have performed computer exp eriments to mimic the gas phase growth of a disordered solid composed of C-28 fullerenes. The growth has been simulated by repeated low ener gy collisions of molecules coming from random directions. The resultin g solid is composed of undamaged C-28 cages, with most fullerenes bein g three-and four-fold coordinated, similar to C atoms in amorphous mat erials. The system contains a high percentage of distorted sp(2) C sit es and only a small proportion of sp(3) sites. These results help clar ify the structure of disordered films obtained experimentally by small fullerene deposition on surfaces. Furthermore, we have compared the p roperties of the disordered C-28 solid (a-C-28) With those of ordered C-28 solids. We have found that the energy of a-C-28 is close to that of hyperdiamond (0.1 eV/atom higher) and differs by a few meV from tha t of other ordered structures, such as 2D-hypergraphite, hexagonal and clathrate solids. This indicates that in condensed phases C-28 molecu les can act as carbon superatoms, while showing more bonding flexibili ty than C atoms; in particular the capability of acting as six-fold co ordinated building blocks of hexagonal solids, which are as stable as a-C-28. (C) 1998 American Institute of Physics.