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

Dielectric spectroscopy is employed to probe the frequency-dependent conductivity and dipolar dielectric response of solid C60Br6. Below approximately 215 K, charge conduction is electronic and well described by Mott's variable-range polaron hopping model, with effective hopping activation energy Ea varying between 0.12 eV at 125 K and 0.16 eV at 220 K, and most probable hopping range varying between 100 and 125% of the decay length of the localized polaron's wavefunction. Above 215 K 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. (C) 2016 Elsevier B.V. All rights reserved.