Persistence of Graphite Phase in G-Ball Under High Pressure Showing Metallic Behavior

Graphitic-carbon polyhedrons G-balls are supposed to have a closed-shell framework of graphite according to TEM image. It is expected that high-pressure structural-change of G-balls is topologically different from that of graphite. High-pressure x-ray powder-diffraction measurements of G-balls has been carried out at room temperature. In addition, the temperature dependences of electrical resistances were obtained in order to investigate the relationship between structures and electronic properties. The graphite phase in G-balls is very stable and does not have the phase transition even if the pressure is higher than 40 GPa. The volume reduces by 29 % at 43 GPa. The value of bulk modulus, B0, is determined to be B0=41.5 GPa. G-balls are geometrically difficult to cause the side-splitting between graphene sheets in comparison with graphite, resulting in absence the phase transition. Electrical resistance measurements exhibit the metallic behavior of G-ball at the pressure above 20 GPa at room temperature. The c-axis length shortens by 18 % at 20 GPa, which means the extraordinary overlapping between carbon pi-orbitals.