Identification of the polymerized orthorhombic phase of C-60 fullerene

It is established by x-ray diffraction and Raman scattering that the polymerization of C-60 fullerene at 1.5 GPa and 723 K leads to the formation of an orthorhombic phase that is different from the previously identified high-pressure orthorhombic phase. It is determined by a calculation of the optimal packing of linear C-60 polymers by the method of atom-atom potentials that the energetically favorable structure of the orthorhombic phase belongs to the space group Pnnm and not the previously proposed group Immm. The computed value of the rotation angle of the polymer chains that corresponds to the minimum packing energy was equal to 61 degrees. The mechanisms leading to the formation of the polymerized phases are discussed on the basis of the results obtained. (C) 1997 American Institute of Physics.