LOW-TEMPERATURE X-RAY-DIFFRACTION STUDY OF THE PHASES OF C-70

Results of X-ray diffraction study at 300K, 77K and 10K are presented for C-70 powder prepared by both solution route and by crushing a coll ection of single crystals. For the sample prepared by solution route t he room temperature hcp phase is quenched and retained up to 10K. Smal l changes however are observed in the c/a ratio. For the sample prepar ed via single crystal route the intensities of the Bragg peaks are sev eral orders of magnitude larger and the line widths considerably reduc ed enabling more accurate phase analysis to be carried out. At room te mperature the sample is predominantly of rhombohedral phase with an ad mixture of fee phase which is less than 6%. At 10K the structure is pr edominantly monoclinic with the fee fraction becoming vanishingly smal l: Upon warming back, the structure reverts back to rhombohedral + fcc (similar to 6%) at 300K. The results observed suggest that the equili brium structure at room temperature could be either rhombohedral or ic e depending on the microstructural defect state of the system and / or impurities in the system and also that the equilibrium structure at l ow temperature is monoclinic. The results also imply for the first tim e that there is a path in which the fee phase transforms directly to m onoclinic phase on cooling without the intermediary rhombohedral phase implying a simultaneous freezing of all orientations.