A chemical approach to an understanding of fast ion conduction in silver iodide silver oxysalt glasses

Abstract

A number of AgI based fast ion conducting glasses, with a general formula AgI-Ag₂O-M_xO_y (M_xO_y-MoO₃, SeO₃, WO₃, V₂O₅, P₂O₅, GeO₂, B₂O₃, As₂O₃, CrO₃) have been studied. A chemical approach is made to investigate the origin of fast ion conduction in these glasses. An index known as structural unpinning number, SUN (S), has been defined for the purpose, based on the unscreened nuclear charge of silver ions and the equilibrium electronegativities of the halide-oxyanion matrix in these glasses. The variation of the glass transition temperature, T_g, conductivity, σ, and the energy of activation, E_a, with the concentration of AgI are discussed in the light of the structural unpinning number. Conductivities increase uniformly in any given glass series as a smooth function of S and level off at very high values. The entire range of conductivity appears to vary as ln σ₀/1-exp(-aS), where ln σ₀ corresponds roughly to the conductivity of the hypothetical AgI glass and "a" is a constant which could be obtained as the slope in the graph of ln E_a versus S. It is suggested that the increase in the concentration of AgI beyond 75-80 mole% in the glass is not advantageous from the conductivity point of view.