Polygonization and anomalous graphene interlayer spacing of multi-walled carbon nanofibers

The graphene interlayer spacing in pure graphite is known to have a minimum value of d(min)=0.3354 nm, while defective graphites typically have larger interlayer spacings. Using x-ray diffraction, we find that the graphene interlayer spacing in multi-walled carbon nanofibers heat treated above approximate to 2800 K is distinctly smaller than d(min). To explain this unusual observation, we investigate the structural properties of carbon nanotubes using a multiscale approach rooted in extensive first-principles calculations, specifically allowing the nanotube cross sections to polygonize. We show that, whereas normal nanotubes are favored energetically at low temperatures, the configuration entropy associated with Stone-Wales defect creation at high temperatures makes the polygonal shape of large nanotubes or nanofibers thermodynamically stable, accompanied by a reduction in the graphene interlayer spacing. These unique predictions are confirmed in further experimental tests.