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 fre quency range from 10 Hz to 60 GHz. The frequency-independent and frequency- dependent conductivity data have respectively been analyzed. A non-Arrheniu s behavior of the dc conductivity is clearly observed. Such a behavior appe ars to be quite common in fast ionic conductors as if the conductivity woul d reach a limit value. The very complete conductivity versus frequency meas urements allowed the observation of distinct regimes and the exploration of various ion-relaxation processes within the material. The conductivity spe ctrum can be described by superposition of several contributions: the Unive rsal Dynamic Response due to diffusive motions, a linearly frequency-depend ent regime attributed to local motions, and finally, a vibrational contribu tion in the far infrared domain. (C) 2001 Elsevier Science BN. All rights r eserved.