A coupled-mode model relating Raman line shape to high ionic conductivity

A phenomenological model, based on the generalized Langevin equation scheme first developed by Bruesch, Zeller, and co-workers, is employed to calculate the Raman band shapes for metal–halogen stretching modes in ionic conductors of the Ag2HgI4 class. The observed strong broadening of the Hg–I4 stretch mode near 122 cm−1 is explained as arising from coupling to the mobile ion diffusive mode. Below the β→α phase transition, the Ag–I mode is oscillatory, and the coupling effect is negligible on either Ag–I or Hg–I. In the conducting phase, the long time diffusive character of the Ag–I motion results in a strong effective damping of the Hg–I motion, leading to the observed broadening. The coupling effects are strong only when one of the two modes is indeed diffusive, thus explaining the apparent value of the Raman linewidth as a screening device for possible new ionic conductor crystals.