Effect of iron oxide on the electrical conductivity of soda-lime silicate glasses by dielectric spectroscopy

A survey of relaxation processes in glasses exhibiting ionic and electronic–ionic conductivity mechanisms is presented. Electrical conductivity and dielectric properties are investigated using complex impedance spectroscopy in a frequency range from 0.1 to 106 Hz and a temperature range from 253 to 423 K. The results reveal that ionic conduction depends on the alkali concentration and ion mobility while electrical conduction is only slightly influenced by alkali ions and a mixed electronic–ionic conduction can occur. The Jonscher’s expression of ac electric conductivity is modified by adding a new term taking into account the displacement current density associated with the dielectric relaxation. The change in the activation energy depends upon the chemical composition indicating a changeover of the predominant conduction mechanism from ionic to polaronic. Quantum mechanical tunneling (QMT) model was suggested to describe the conduction mechanism of alkali-silicate glass G1 where frequency exponent s < 1. However, small polaron tunneling (SPT) model was applied to describe conductivity of semiconductor glass G2 (alkali-silicate glass with iron ions) with s > 1, whose conduction mechanism may be considered in terms of the optical phonon assisted hopping of small polarons between overlapping states. The electrical modulus exhibited relaxation character.