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.

Figure 1

(a) Conventional ion transport mechanism: A mobile ion (green circle) hops (green arrow) across barriers in the potential landscape due to the host lattice (blue circles) and surrounding mobile ions. (b) $\alpha \text{−}\mathrm{AgI}$ Raman spectrum at 443 K. Inset: $\alpha \text{−}\mathrm{AgI}$ crystal structure, with I atoms (purple) located at the bcc positions and Ag atoms (dark gray) hopping (black dashed arrows) between empty tetrahedral sites (white). (c) Vibrational density of states (VDOS) of $\alpha \text{−}\mathrm{AgI}$ calculated from AIMD at 500 K. The VDOS is decomposed into contributions from each atomic species. The gray circle on the $y$ axis marks the zero-frequency finite VDOS from which a diffusion coefficient of $3.1\times {10}^{-5}{\mathrm{cm}}^2$/s was calculated.

Figure 2

PO Raman measurements of $\alpha \text{−}\mathrm{AgI}$. (a) PO intensity maps of Raman spectra taken at 443 K, at polarization orientation angles of $0^{\circ }\text{–}{360}^{\circ }$ in parallel ($\parallel$) and perpendicular ($\perp$) polarization configurations, including both Stokes and anti-Stokes scattering. The region between $-10$ and 10 ${\mathrm{cm}}^{-1}$ is cut off by a notch filter. Spikelike features seen near 10 ${\mathrm{cm}}^{-1}$ are due to interference effects from the notch filter. (b) Representative Raman spectra and their fit components and envelope, for both polarization configurations, taken from the dashed lines in (a). (c) Plots of the normalized intensity coefficient vs polarization orientation angle for all four features, for each of the three temperatures measured. The expected response of the “local tetrahedral oscillator” model is also included (gray dashed line).

Figure 3

(a) Diagram of the ${\mathrm{AgI}}_4$ tetrahedral site of the bcc lattice, with the I-I-I bond angle $\theta$ analyzed in AIMD simulations indicated. Lines of the same texture have the same length. (b) Time series of $\theta$ for $\alpha \text{−}\mathrm{AgI}$ AIMD simulation at 500 K with all atomic motions allowed and (c) when ${\mathrm{Ag}}^{+}$ is fixed to zero velocity at tetrahedral sites. (d) Mean-square displacement as a function of time for ${\mathrm{Ag}}^{+}$, and calculated diffusion coefficient $D_{{\text{Ag}}^{+}}$, for the following cases: All atomic motions allowed (black line), and I fixed at bcc lattice positions (yellow).