  Basic understanding of the driving forces of ion conduction in solids is
critical to the development of new solid-state ion conductors. Physical
understanding of ion conduction is limited due to strong deviations from
harmonic vibrational dynamics in these systems that are difficult to
characterize experimentally and theoretically. We overcome this challenge in
superionic AgI by combining THz-frequency Raman polarization-orientation
measurements and ab-initio molecular dynamics computations. Our findings
demonstrate clear signatures of strong coupling between the mobile ions and
host lattice that are of importance to the diffusion process. We first derive a
dynamic structural model from the Raman measurements that captures the
simultaneous crystal-like and fluid-like properties of this fast-ion conductor.
Then we show and discuss the importance of anharmonic relaxational motion that
arises from the iodine host lattice by demonstrating its strong impact on ion
conduction in superionic AgI.

Materials Science