The crystal structure consists of layers of six-membered rings of sp2 carbon atoms arranged in hexagonal pattern and bound together by covalent bonds; see Fig. 3. These so-called graphene layers are stacked in the crystallographic c-direction bound together by weak van der Waals forces. When mechanical pressure is applied, graphite can be easily cleaved along the graphene sheets. This characteristic of graphite is the main reason for its good lubricating properties.
By stacking, the hexagonal symmetry is maintained. This is in contrast to the turbostratic order in which the carbon layers are parallel but rotated around the crystallographic c-axis. The thermodynamically stable crystal structure is hexagonal, where every third graphene layer has an identical position to the first layer resulting in a stacking sequence A-B-A-B. A rhombohedral structure exists in which only every fourth graphene layer has an identical position to the first layer resulting in a sequence of A-B-C-A-B-C. This rhombohedral structure appears as statistical stacking defects and can be formed by mechanical deformation of hexagonal crystals by shear forces. Rhombohedral defects can be highly dispersed in the hexagonal graphite crystal or segregated to an isolated rhombohedral phase. Heat treatment completely transforms the rhombohedral into the hexagonal structure.
The degree of graphitization (crystallinity) can be measured by X-ray diffraction (XRD) using the Scherrer or Maire and Mering equations (Feret 1998) and is directly related to the interlayer distance c/2. High crystallinity is a prerequisite for good electrical and thermal conductivity, and highly intrinsically conductive graphite powders usually show c/2 values between 0.3354 and 0.3360 nm and real densities of 2.24-2.27 g/cm3. Due to the layered crystal structure, graphite has strongly anisotropic properties. For example, it exhibits extremely high intrinsic electrical and thermal conductivity in the plane (up to 26,000 S/cm and 3000 W/mK), whereas perpendicular to the plane (“through plane”), the values are orders of magnitude lower.
The size of the crystals parallel to the graphite layers (La) and perpendicular to them (Lc) is important for the distinction of different graphite materials. Lc gives information about the average number of graphite layers stacked on each other in a single
Fig- 3 The hexagonal and rhombohedral graphite crystal structure (from Spahr 2010)
crystal and can be measured by XRD. La is more difficult to measure by XRD. As an alternative, Raman spectroscopy measurements can be used to measure the crystallinity in the direction of the graphite plane La (Tuinstra and Koenig 1970). The intensity and shape of the D- and G-band give additional information about the properties of graphite powders, like disorder and number of graphene layers (Ferrari 2007).