Anharmonic host-lattice dynamics enable fast ion conduction in superionic AgI

A basic understanding of the driving forces of ion conduction in solids is critical to the development of new solid-state ion conductors. The 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 fluidlike 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.