Non-Arrhenius ionic conductivity in solid electrolytes: A theoretical model and its relation with the bonding nature

Some solid electrolytes exhibit the non-Arrhenius type ionic conductivity whose origin is still not well understood. In the present study, a model for the non-Arrhenius ionic conductivity is proposed by exploiting the formulation of the bond strength-coordination number fluctuation (BSCNF) model developed originally to describe the transport properties of supercooled liquids. According to the present model, the origin of the non-Arrhenius ionic conductivity as described by the VFT equation traces back to the bonding energy fluctuations of the diffusing ions within the solid. The model suggests that good ionic conductors exhibit a non-Arrhenius behavior in the ionic conductivity. The present study reveals also that the degree of the non-Arrhenius behavior of different materials is separated roughly into two groups depending on the nature of the chemical bonds. One of these groups consists mainly of compounds such as Ag ion conductors, and the other group contains materials such as Li ion conductors.