Theoretical model for the temperature dependence of Raman scattering in α -AgI

A new model for the distribution of the cations in α-AgI is proposed. This model explains the change in the anisotropic polarizability, as deduced by the temperature dependence of previous Raman data for temperatures below 700 K. We represent the Ag⁺ ion distribution on the "cages" of d sites around each I^- ion as a local fluctuation of charge density. At increasing temperature this arrangement vanishes progressively and the Ag⁺ distribution becomes homogeneous and isotropic. A simplified model of polarizability is developed to relate the theoretical dependences of Raman intensities and depolarization ratios to the experimental ones. Qualitative and quantitative agreement is obtained; hence we are confident that the Raman data indicate the existence of a cooperative correlation between silver-occupied positions which disappears at high temperature.