Electronic and ionic relaxations in oxide glasses

Abstract

A survey of relaxation processes in glasses exhibiting electronic and electronic–ionic conductivity mechanisms is presented. Electrical conductivity data on a range of transition metal oxide (TMO) glasses shows that a polaronic model of conductivity is generally applicable. It is shown that in TMO glasses both ac and dc conductivity processes are due to the same mechanism, which is hopping of small polarons between transition metal ions in different valence states. By applying scaling using the BNN relation it is possible to obtain universal conductivity curve and to find the relation between ac and dc activation energy. Complex plot of electric modulus gives a simple method of scaling analysis. An attempt of classification of mixed electronic–ionic behaviour in TMO containing alkali ions was performed on the basis of impedance spectroscopy. A deep minimum of conductivity at certain content of alkali ions is observed in glasses where TMO is a glass former. Two different behaviours can be distinguished in glasses where TMO is a glass network modifier. In glasses containing iron oxide electrical properties are only slightly influenced by alkali ions. A mixed electronic–ionic conduction is observed in glasses containing copper oxides. It is shown that the internal friction method is a powerful tool for the study of relaxation processes in glasses. In electronic and ionic conducting glasses internal friction spectra reveal a peak which activation energy is identical as for the dc conductivity. In mixed electronic–ionic conducting glasses the internal friction spectra reveal two peaks, which are attributed to the electronic hopping and the alkali ion migration. A comparison of internal friction and impedance spectroscopy investigations indicates that the internal friction is a more sensitive method for detecting local movement of ions or polarons in the glass network.