13C NMR and dielectric spectroscopy of strain relaxation in depolymerization of rhombohedral C60

Abstract The 13C nuclear magnetic resonance (NMR) and dielectric behavior of rhombohedral C60 polymer with (0 0 3) texturing have been studied. 13C NMR reveals that sp3 bonds break from 423 to 523 K. This is the temperature region in which depolymerization occurs, as deduced from temperature-resolved X-ray diffractometry. X-ray diffraction also shows the structural transition from rhombohedral to face-centered cubic structure at 530 K. This indicated a 0.1 A increment in the c-axis lattice parameter, attributable to the conversion of the elongated C60 cages to spherical C60 before the structural transition. The elongation arises from intermolecular sp3 bonds. Strain relaxation should take place when these break during depolymerization, and we give experimental proof that this occurs. The polymer’s AC impedance varies with temperature, and frequency-dependent peaks are found over the same temperature range of 423–523 K, these peaks disappearing in depolymerized samples. This dielectric relaxation has a thermal activation energy of 1.2 ± 0.1 eV, which may be interpreted as the potential barrier for the dissociation of cage-linking sp3 bonds. Possible origins of the electric dipoles responsible for dielectric relaxation are proposed.