Medium range structure and power law conductivity dispersion in superionic glasses

The relationship between the exponent of the power law behavior in the ac conductivity and the wave number of first sharp diffraction peak (FSDP) in superionic glasses has been studied. For the case of (AgI) _x(AgPO ₃) _{1− x} glasses, it is shown that the exponent of the power law increases with the decrease of the FSDP wave number. Other superionic glasses such as (AgI) _0.2(0.6Ag ₂O–0.4V ₂O ₅) _0.8 and (AgI) _0.75(Ag ₂MoO ₄) _0.25 follow this trend. The relationship found is explained by using the energy barrier profile for the mobile ions in a disordered structure. The local energy barrier for the ion movement is determined by the local structure and bonding, and the ions flow through the low energy barrier sites. According to the model, the exponent of the power law contains information about the degree of connectivity of these low energy sites that extend at the intermediate length scale. The result suggests also that the universal aspect of the power law reflects the universal pattern of the potential barrier at intermediate length scales.