Results from Raman spectra of the superionic conductor AgI

Raman measurements are reported in the superionic conductor AgI far below and above the transition temperature $T_c=147\mathrm{}$°C. At $T_c$, the crystal structure changes from the low-temperature wurtzite phase to a disordered-defect body-centered cubic structure ($\alpha$ phase) and the conductivity increases ≅ ${10}^4$ ${\left(\mathrm{\Omega }\mathrm{c}\mathrm{m}\right)}^{-1\mathrm{}}$. Large abrupt reversible changes in the phonon spectra are observed at $T_c$. Assuming a frequency-independent matrix element, which seems reasonable for a disordered-defect structure, we have interpreted the experimental data in terms of a frequency-dependent conductivity, $\sigma \left(\omega \right)$. This $\sigma \left(\omega \right)$ compares favorably with two published infrared results. However, the infrared results differ from each other in the low-frequency region (3-20 ${\mathrm{cm}}^{-1\mathrm{}}$). The Raman results show much lower $\sigma \left(\omega \right)$ in this low-frequency region, in agreement with one of the infrared results, and some theories (but in disagreement with other theories). The measurements show that $\sigma \left(\omega \right)$ varies very little to 400°C, and suggest that in the $\alpha$ phase the ${\mathrm{Ag}}^{+}$ probably occupies the tetrahedral sites for a large fraction of the time.