THEORETICAL PREDICTIONS FOR A 2-DIMENSIONAL RHOMBOHEDRAL PHASE OF SOLID C-60

We have calculated the properties of a rhombohedral phase of solid C60 using a C tight-binding potential. The solid is formed by layers of hexagonally packed C60, each of them connected to six neighbors by 2 + 2 cycloaddition of double bonds. We predict an equilibrium intermolecular distance of 9.17 , in agreement with experimental estimates of 9.2 . Rhombohedral C60 solid is higher in energy than free C60 molecules by 2.1 eV/molecule, and the barrier for dissociation to free C60 molecules is estimated to be 1.6 eV/molecule. Our results indicate that the lowest energy conformation of this new phase of solid C60 is a semiconductor, but defects in the intermolecular bonding pattern lead to semimetal properties. Results from similar calculations on tetragonal and orthorhombic C60 phases are also presented. © 1995 The American Physical Society.