Lattice dynamics of the superionic conductor AgI

We present results of harmonic lattice-dynamics calculations of the density of states, infrared conductivity, and Raman scattering for the superionic conductor AgI. The calculations do not incorporate the low-frequency, diffusive aspects of the ionic motion, but they do contain the oscillatory aspects in some detail. We find that the intrinsic disorder of the α phase accounts for the broad spectra which are observed. In particular, the density of states and the calculated spectra display a lower-frequency transverse-acoustic-like feature as well as an upper-optic-mode peak at ≈ 100 cm^-1 in agreement with experiment. We also consider effects of possible defect sites on the spectra of the wurtzite and α phases of AgI.