Electronic structure and energetics of pressure-induced two-dimensionalC60polymers

We report on the electronic structure and energetic stabilities of two-dimensional ${\mathrm{C}}_{60}$ polymers, in both tetragonal and rhombohedral phases, studied by using the local-density approximation in the framework of the density-functional theory. Owing to hybrid networks of ${\mathrm{sp}}^2$-like (threefold coordinated) and ${\mathrm{sp}}^3$-like (fourfold coordinated) carbon atoms, the electronic structure of these phases is considerably different from that of face-centered-cubic (fcc) ${\mathrm{C}}_{60}.$ Both systems are found to be elemental semiconductors having small indirect gaps. Furthermore, since the interlayer distance between adjacent polymerized planes for both phases is small, these systems are found to have three-dimensional electronic structures. From structural optimizations under the experimental lattice parameters, we reveal energetic high stabilities of these phases. In particular, the tetragonal phase is found to be considerably more stable in energy than the fcc phase. Its high stability is caused by the formation of intercluster bonds whose energy gain is larger than the energy loss due to the distortion of the carbon networks of ${\mathrm{C}}_{60}$ units upon polymerization.