Formation of titanium-carbide in a nanospace of C78 fullerenes

Geometric and electronic structures of C78 fullerenes encapsulating two Ti atoms and a C2 molecule are studied by the first-principle total energy calculations. We find that the substantial hybridization between the 3d state of Ti atoms and the 2p state of C atoms results in the formation of weak covalent bonds between them and that the Ti atoms and the C2 molecule form a zigzag chain inside the cage under the optimized geometry. Encapsulated C2 molecule decreases the Coulomb repulsion energy between positively charged Ti atoms and rotates inside the cage. A plausible candidate for the structure of Ti2C2@C78 extracted in a recent experiment is assigned by the density of states for the occupied electronic states.