Relating the Fragility with the Decoupling Between Ionic Conduction and Structural Relaxation: A Study Based on the BSCNF Model

The degree of decoupling between ionic conduction and structural relaxation is a key quantity to characterize good ionic conductors. On the other hand, it is known that the fragility of the materials plays an important role in diverse properties of disordered materials. In the present report, the relation between the decoupling index and the fragility is studied based on the Bond Strength-Coordination Number Fluctuation (BSCNF) model developed by ourselves. The analytical expression derived from the model indicates that the decoupling index, defined as the ratio between the structural and conductivity relaxation times evaluated at the glass transition temperature, could depend or not on the fragility. Such kind of behavior have been reported for instance, in polymer electrolytes and in AgI-containing oxide glasses, respectively. These results indicate that the BSCNF model could provide a good reference to understand the fundamental properties of structurally disordered ionic conductors.