  Investigations of clathrate structures have gained a new impetus with the
recent discovery of room-temperature superconductivity in metal hydrides. Here
we report the finding, through density functional theory calculations, of a
clathrate phase in the fullerite C60 system. Intermolecular bonds of the type
5/5 2+3 cycloaddition are induced between each C60 molecule and its twelve
nearest neighbors in the face centered cubic lattice. Remarkably, this bonding
creates on octahedral sites new C60 cages, identical to the original ones, and
on tetrahedral sites distorted sodalite-like cages. The resulting carbon
clathrate has a Pm-3 simple cubic structure with half of the original face
centered lattice constant. Eighty per cent of its atoms are sp3-hybridized,
driving a narrow-gap semiconducting behavior, a moderate bulk modulus of 268GPa
and an estimated hardness of 21.6GPa. This new phase is likely to be prepared
by subjecting C60 to high pressure and high temperature conditions.
