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 Tc=147 °C. At Tc, the crystal structure changes from the low-temperature wurtzite phase to a disordered-defect body-centered cubic structure (α phase) and the conductivity increases ≅ 104 (Ωcm)-1. Large abrupt reversible changes in the phonon spectra are observed at Tc. 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, σ(ω). This σ(ω) compares favorably with two published infrared results. However, the infrared results differ from each other in the low-frequency region (3-20 cm-1). The Raman results show much lower σ(ω) 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 σ(ω) varies very little to 400 °C, and suggest that in the α phase the Ag+ probably occupies the tetrahedral sites for a large fraction of the time. © 1977 The American Physical Society.