Colloquium: Understanding ion motion in disordered solids from impedance spectroscopy scaling

ac impedance measurements of ion-conducting glasses provide considerable insight into the nature of the ionic motions in disordered solids. However, interpreting the ac impedance has been a matter of considerable debate, particularly in regards to how best to represent the relaxation process that is the result of a transition from correlated to uncorrelated ion hopping. Although interpretations based upon the electric modulus have featured prominently in the earlier literature, direct analysis of the complex conductivity in the frequency domain is gaining popularity as it provides direct (via Fourier transform) information regarding the microscopic mean-squared displacement of ions. Here many recent findings are summarized that emphasize the scaling features present in the impedance spectra to demonstrate how the casual researcher might interpret ac impedance results without elaborate curve fitting.