Electrical and optical absorption studies of Cu7GeS5I fast-ion conductor

Abstract Electrical conductivity and fundamental absorption spectra of monocrystalline Cu 7 GeS 5 I were measured in the temperature ranges 95–370 and 77–373 K, respectively. A rather high electrical conductivity ( σ t =6.98×10 −3 Ω −1  cm −1 at 300 K) and low activation energy (Δ E a =0.183 eV) was found. The influence of different types of disordering on the Urbach absorption edge and electron–phonon interaction parameters were calculated, discussed and compared with the same parameters in Cu 7 GeS 5 I, Cu 6 P X 5 I ( X =S,Se) and Ag 7 Ge X 5 I ( X =S,Se) compounds. We have concluded that the P→Ge and Cu→Ag cation substitution results in an increase of the electrical conductivity and a decrease of the activation energy. Besides, P→Ge substitution, results in complete smearing and disappearance of the exciton absorption bands and in blue shift of the Urbach absorption edge, an increase of the edge energy width and an electron–phonon-interaction enhancement.