Low temperature X-ray diffraction study of the phases of C₇₀

Abstract

Results of X-ray diffraction study at 300K, 77K and 10K are presented for C₇₀ powder prepared by both solution route and by crushing a collection of single crystals. For the sample prepared by solution route the room temperature hep phase is quenched and retained up to 10K. Small changes however are observed in the c/a ratio. For the sample prepared via single crystal route the intensities of the Bragg peaks are several orders of magnitude larger and the line widths considerably reduced enabling more accurate phase analysis to be carried out. At room temperature the sample is predominantly of Rhombohedral phase with an admixture of fcc phase which is less than 6%. At 10K the structure is predominantly monoclinic with the fcc fraction becoming vanishingly small. Upon warming back, the structure reverts back to rhombohedral + fcc (~6%) at 300K. The results observed suggest that the equilibrium structure at room temperature could be either rhombohedral or fcc depending on the microstructural defect state of the system and / or impurities in the system and also that the equilibrium structure at low temperature is monoclinic. The results also imply for the first time that there is a path in which the fcc phase transforms directly to monoclinic phase on cooling without the intermediary rhombohedral phase implying a simultaneous freezing of all orientations.