New Fullerene Materials Obtained in Solution and by High Pressure High Temperature Treatment

Sammanfattning: Crystallization of C60 and C70 from organic solution often leads to the formation of new solvates and other fullerene compounds. In the present thesis, a number of such solvates were obtained and their phase transitions studied using in situ "in solution" techniques. New fullerene materials can be also obtained using High Pressure High Temperature (HPHT) treatment. The formation of C60 polymers in thin films and bulk samples has been studied in situ over a wide pressure-temperature range.New methods for single-crystal growth of fullerenes and their compounds have been developed. It was found by in situ "in solution" XRD and Raman spectroscopy that solvate C60 crystals with benzene, toluene and hexane are stable only in equilibrium with their solution. Their melting points coincide with the maximum in the temperature dependence of solubility. C70 solvates grown from these solutions are stable out of solution, and decompose above the boiling points of the solvents. Vibrational signatures were found for the C60 and C70 solvates which are very similar to these for fullerene-sulfur compounds obtained as thin films and single crystals. A new C70S8 compound was obtained as relatively large single crystals. C60 polymerisation under HPHT conditions was studied on thin films and showed a thickness effect on the phase transition around 20 GPa. Superhard and superelastic films were obtained by treatment at 23 GPa and 570K. In situ Raman and XRD studies were performed on bulk samples at pressures up to 27 GPa and temperatures up to 850K. Below 13 GPa, only one- and two-dimensional polymers were found to form during the heating. The observed polymerisation pathway suggests a gradual increase in polymerisation. Above 18 GPa, the in situ Raman spectra obtained during heating remained almost unchanged. The XRD study showed that heating at 830K and 13 GPa leads to the formation of a rhombohedral phase with a volume per C60 molecule of 560-570 Å3/M, which is below the value for two-dimensional polymers. Nevertheless, no superhard, highly dense phases were observed under these conditions, in contrast to previous studies.