Results from Raman spectra of the superionic conductor AgI

Abstract

Raman measurements are reported in the superionic conductor AgI far below and above the transition temperature ${T}_{c}=147$\ifmmode^\circ\else\textdegree\fi{}C. At ${T}_{c}$, the crystal structure changes from the low-temperature wurtzite phase to a disordered-defect body-centered cubic structure ($\ensuremath{\alpha}$ phase) and the conductivity increases \ensuremath{\cong} ${10}^{4}$ ${(\mathrm{\ensuremath{\Omega}}\mathrm{c}\mathrm{m})}^{\ensuremath{-}1}$. 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, $\ensuremath{\sigma}(\ensuremath{\omega})$. This $\ensuremath{\sigma}(\ensuremath{\omega})$ compares favorably with two published infrared results. However, the infrared results differ from each other in the low-frequency region (3-20 ${\mathrm{cm}}^{\ensuremath{-}1}$). The Raman results show much lower $\ensuremath{\sigma}(\ensuremath{\omega})$ 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 $\ensuremath{\sigma}(\ensuremath{\omega})$ varies very little to 400\ifmmode^\circ\else\textdegree\fi{}C, and suggest that in the $\ensuremath{\alpha}$ phase the ${\mathrm{Ag}}^{+}$ probably occupies the tetrahedral sites for a large fraction of the time.