Dielectric properties and polaronic conductivity of WO3 and W x Mo1−x O3

An investigation of the Debye relaxations in WO3 and W x MO1−x O3 confirm the mechanism of polaronic conduction at low temperatures. The Mo positions act as trapping centres, and the relaxation times and activation energies depend largely on the crystallinity of the samples, Polarons are spin-paired in the ground state (bipolarons), and can be separated by optical irradiation giving rise to ‘inner’ photoconductivity. The transport mechanism of the bipolarons is found to take place in three steps : dissociation into two single polarons, their transport and subsequent recombination. This model is energetically favoured over the transport of the bipolarons as a whole.