Theoretical Predictions for a Two-Dimensional Rhombohedral Phase of Solid ${\mathrm{C}}_{60}$

We have calculated the properties of a rhombohedral phase of solid ${\mathrm{C}}_{60}$ using a C tight-binding potential. The solid is formed by layers of hexagonally packed ${\mathrm{C}}_{60}$, each of them connected to six neighbors by 2 + 2 cycloaddition of double bonds. We predict an equilibrium intermolecular distance of 9.17 Å, in agreement with experimental estimates of 9.2 Å. Rhombohedral ${\mathrm{C}}_{60}$ solid is higher in energy than free ${\mathrm{C}}_{60}$ molecules by 2.1 eV/molecule, and the barrier for dissociation to free ${\mathrm{C}}_{60}$ molecules is estimated to be 1.6 eV/molecule. Our results indicate that the lowest energy conformation of this new phase of solid ${\mathrm{C}}_{60}$ is a semiconductor, but defects in the intermolecular bonding pattern lead to semimetal properties. Results from similar calculations on tetragonal and orthorhombic ${\mathrm{C}}_{60}$ phases are also presented.