Variable-range electron hopping, conductivity cross-over and space-charge relaxation in C60Br6

[Display omitted] •The electronic conductivity of solid C60Br6 obeys Mott’s variable-range hopping below 215K, with a typical hopping range of the order of the molecular radius.•A conductivity cross-over is observed at 215K, likely due to a solid-solid transition.•A structured space-charge relaxation loss is observed by dielectric spectroscopy, possibly due to the existence of two different inhomogeneity lengthscales.•The main relaxation feature obeys the Barton-Nakajima-Namikawa condition. Dielectric spectroscopy is employed to probe the frequency-dependent conductivity and dipolar dielectric response of solid C60Br6. Below approximately 215K, charge conduction is electronic and well described by Mott’s variable-range polaron hopping model, with effective hopping activation energy Ea varying between 0.12eV at 125K and 0.16eV at 220K, and most probable hopping range varying between 100 and 125% of the decay length of the localized polaron’s wavefunction. Above 215K a new contribution appears in the conductivity, accompanied by a weak endothermic feature in the calorimetric thermogram, which might signal a solid-solid phase transition. A broad dielectric loss feature is observed in the imaginary part of the permittivity. Such feature stems largely from a conductivity-related frequency-dependent loss associated with polaron hopping, as confirmed by the validity of the Barton-Nakajima-Namikawa condition.