Ion dynamics in the argyrodite compound Ag7GeSe5I: non-Arrhenius behavior and complete conductivity spectra

The conductivity of the superionic conductor argyrodite compound Ag7GeSe5I has been measured for temperatures between 123 and 473 K and in a broad frequency range from 10 Hz to 60 GHz. The frequency-independent and frequency-dependent conductivity data have respectively been analyzed. A non-Arrhenius behavior of the dc conductivity is clearly observed. Such a behavior appears to be quite common in fast ionic conductors as if the conductivity would reach a limit value. The very complete conductivity versus frequency measurements allowed the observation of distinct regimes and the exploration of various ion-relaxation processes within the material. The conductivity spectrum can be described by superposition of several contributions: the Universal Dynamic Response due to diffusive motions, a linearly frequency-dependent regime attributed to local motions, and finally, a vibrational contribution in the far infrared domain.