First-principles investigation on the geometry and electronic structure of the three-dimensional cuboidal $C60$ polymer

The structural stability and electronic properties of the recently characterized three-dimensional (3D) cuboid-shaped $C60$ polymer are studied using periodic ab initio density functional methods. It is shown that the experimentally observed structure is metastable and not fully relaxed from the high pressure state. A second polymorph, which is more stable than the experimental structure, is identified from the calculations. This new structure differs from the observed structure in the number of fourfold-coordinated atoms per $C60$ molecule. Both structures are found to be metallic with bulk moduli only about one-third that of diamond. The cuboidal $C60$ is not the long sought after superhard 3D carbon polymer; however, the two polymorphs studied here reveal unusual electronic band structures that might suggest interesting electronic properties.