Proportion and dispersion of rhombohedral sequences in the hexagonal structure of graphite powders

Abstract

Using the Phase Matrix Method, we derive a calculation scheme giving the analytical form of the intensity diffracted by graphite crystallites containing rhombohedral stacking faults. The physical model accounts for all possible types of interstratification of the hexagonal and rhombohedral forms in graphite. The relevant statistical parameters are the proportion of the rhombohedral phase, its “dispersion” within the hexagonal structure and the average number of graphene layers in the crystallites. A mean-squares refinement program allows calculation of the (10ℓ) diffraction profiles and the X-ray data reduction into physically meaningful statistical parameters. In the case of powdered materials, the parameters describing the rhombohedral phase are found to depend on the material origin and on its preparation process. Reducing the grain size by milling always increases the proportion of the rhombohedral phase whereas its dispersion in the hexagonal structure can significantly vary, depending upon both raw material type and milling conditions. X-ray diffraction profile fitting also evidenced the fact that compacting a powder of crystallites in which the two phases are segregated (coexistence of large rhombohedral and hexagonal crystallographic domains) leads to the destruction of the large domains constituting both phases and results in strong inter-mixing of the two crystalline forms.