Role of optical phonon in fluoride-ion conductivity of LaF3

Fluoride-ion (F-) conductors have attracted much attention as solid electrolytes for all-solid-state fluoride-ion batteries with high energy densities surpassing those of conventional lithium-ion batteries. Ion conduction is mainly determined by the carrier amount (n) and diffusion coefficient (D), and progress is being made in understanding and controlling n. However, it is necessary to quantitatively evaluate not only D itself but also the factors that govern it. In this study, terahertz time-domain spectroscopy (THz-TDS), Fourier transform infrared spectroscopy, and first-principles calculations are used to address the effective jump attempt frequency that governs D. Phonons contributing to F- ion diffusion span a broad range of frequencies rather than a single vibrational frequency, indicating that more complex multi-phonon processes are at work. Empirical relations indicate that the mode frequency of 3 THz corresponds to an activation barrier of ∼0.5 eV. Phonon absorption around 5 THz for LaF3 involves the F vibration with the long La-F bond length. The THz-TDS conduction increases with the phonon absorption. The loose coupling between F- and counterions to soften the lowest optically active mode increases F- conductivity.