Anharmonic Host Lattice Dynamics Enable Fast Ion Conduction in Superionic AgI

TL;DR

This paper reveals how anharmonic lattice dynamics in superionic AgI facilitate rapid ion conduction, combining experimental Raman data and ab-initio simulations to uncover the coupling between ions and the host lattice.

Contribution

It introduces a dynamic structural model based on Raman measurements and highlights the role of anharmonic relaxational motion in enhancing ion conduction.

Findings

Strong coupling between mobile ions and host lattice identified

Anharmonic relaxational motion significantly impacts ion conduction

A new dynamic structural model captures both crystal-like and fluid-like properties

Abstract

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.