Structural studies of C-60 transformed by temperature and pressure treatments

Raman spectroscopy, X-ray and electron diffractions have been applied to study samples of fullerene C-60 after they have undergone pressure and temperature treatments up to 8 GPa and 1073 K. It is confirmed that mixtures of rhombohedral and tetragonal structures are formed in the 2-4 GPa and 673-1073 K pressure-temperature domain. Traces of a hexagonal phase are also observed in the same range. Only the rhombohedral one is obtained in the 4-8 GPa range in the same temperature interval. The material obtained at temperatures between 473 and 673 K in the whole pressure range which has previously been described in terms of a cubic structure can be understood in terms of an orthorhombic one. It may also contain mixtures of different phases including a cubic one and even a cubic superstructure. The widened poorly resolved X-ray profiles indicate the existence of disorder within the materials formed and the increasing complexity of Raman spectra as temperature increases at fixed pressures (from 2 to 8 GPa) suggests that this disorder is related to phase mixtures in almost all samples. A rhombohedral sample formed at 6 GPa-873 K reverted to a mixture of rhombohedral and tetragonal phases at 2.5 GPa-873 K. Thus thermodynamic equilibrium between these two kinds of systems could exist. (C) 1997 Elsevier Science Ltd.