Phonon-induced diffuse x-ray scattering inβ-AgI

The one-phonon diffuse x-ray scattering of $\beta$-AgI has been computed from a lattice-dynamical model with parameters derived from neutron scattering data. The computed diffuse scattering is compared to experimental results and excellent agreement is found. The complex and intense diffuse scattering of the ordered $\beta$ phase originates from a band of low-frequency dispersionless lattice modes. There are strong indications for the existence of similar low-frequency dispersionless (local) lattice excitations in the disordered $\alpha$ phase of AgI. Thus we argue that the diffuse x-ray scattering of the $\alpha$ phase is to a large extent of thermal origin, and that intense phonon-induced x-ray scattering may in certain superionic conductors completely preclude the extraction of information about static short-range order from diffuse x-ray scattering.