| dc.description.abstract | In this chapter, the preparation and characterization of a number of AgIbased FICs with the general formula AgI-AgO-MO is presented. Efforts have been made to understand the variation of molar volumes, glasstransition temperatures and heat capacities in the light of established models of glass structure and bonding.
An essentially chemical model has been developed to understand the fastion conduction (FIC) in AgIbased glasses. It is suggested in the model that the incidence of FIC in AgI-Ag(oxysalt) glasses arises naturally from their chemistry. New FIC systems can be engineered from the considerations discussed in the model. Better FIC glasses must have higher values of S, the structural unpinning number. It is demonstrated that conductivity varies directly with S, being higher for higher values of S. Spectroscopic and dielectric studies supporting this model are discussed in the forthcoming chapters.
While S provides insight into the chemical origin of conductivity, it is still unable to explain why a large jump in conductivity occurs at the - transition of AgI [1]. At the present stage, the model cannot differentiate between glassy and crystalline materials. Although it is well known that the presence of connected (ordered) vacant positions in a lattice plays an important role in the conductivity of crystalline materials, we still need to explain this feature satisfactorily.
III6. Conclusions
IR studies of AgIbased FIC glasses indicate that all these glasses consist of discrete anions along with Ag and I ions, except for the germanate and borate glasses, which contain condensed anions formed from combinations of GeO-GeO and BO-BO groups, respectively. Raman spectra of AgI-AgPO glasses do not indicate the presence of AgI clusters. Both IR and Raman results are consistent with the structural unpinning model proposed for AgIbased FIC glasses in Chapter III.
The a.c. conductivity and dielectricrelaxation behaviour of a few representative AgIbased FIC glasses have been examined. Dielectric relaxation has been analysed using the complex electricmodulus formalism because of the high ionic conductivity of these materials. The relaxation behaviour is consistent with a stretchedexponential decay function using the available method of analysis. The stretching exponent, , is itself shown to be covariant with the structural unpinning number (S).
The structural unpinning number, together with the cluster-tissue model, provides a better and more direct explanation of the correlated states that give rise to Kohlrausch-Williams-Watts (KWW) behaviour. The physical origin of the correlated states is possibly the “tissue region,” where vibrationpotential walls merge at higher energies and enable ionic motion, giving rise to intercationic (manybody) interactions relevant to the formation of correlated states. | |
