Structural studies of C60 transformed by temperature and pressure treatments Original Research Article

Raman spectroscopy, X-ray and electron diffractions have been applied to study samples of fullerene C60 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.