Microstructural mapping of C60 phase transformation into disordered graphite at high pressure, using X-ray diffraction microtomography

An extended use of synchrotron-based X-ray diffraction microtomography (XRD-µCT) to study simultaneously the phase distribution and microstructure in phase-transformation processes is proposed. This three-dimensional non-invasive imaging approach has been applied to understand the phase transformation of C60 rhombohedral polymer (C60R) into disordered graphite (DG) at high pressure and high temperature. The heterogeneous sample was synthesized (5 GPa, 1100 K) using a Paris-Edinburgh cell and selective image reconstructions were achieved for all different phases present in this sample. The XRD-µCT analysis evidences elongated DG domains with a fiber texture where nested (002)_DG planes show ±70° preferential orientation relative to the compression axis. In contrast C60R domains are found to be small and spotty, preferentially in the middle of the sample. The parent and product phases are mutually interpenetrative and exhibit a crystallographic relationship. This study evidences that formation of (002)_DG planes occurs parallel to {111}_C60C pseudo-cubic planes. Among these four possible alignments, uniaxial pressure favors one [111]_C60C direction. Transmission electron microscopy observations validate these nondestructive XRD-µCT results.