Far-infrared dielectric response and hopping conductivity in quasi-one-dimensional (NbSe₄)₃I

Complex transmittance of the quasi-one-dimensional semiconductor (NbSe₄)₃In in the 8-30 cm^-1 range and reflectivity in the 15-650 cm^-1 range were measured as a function of temperature (10-300 K) around the structural phase transition at T_c=274 K. The spectra show predominantly dielectric characters with a rich phonon structure between 20 and 300 cm^-1. Several new lines gradually appear below T_c in qualitative agreement with the factor-group analysis. The submillimetre electronic conductivity along the chain direction is appreciably higher than the DC conductivity in the whole temperature range which suggests a hopping mechanism of the electron transport. This is even more directly documented by the strong increase in submillimetre permittivity with temperature. The important role of permittivity for proving the hopping transport is stressed.