Ionic Conduction in Sr-Doped CeF₃ as Studied by ¹⁹F NMR and AC Impedance Measurement

Abstract

Among the three inequivalent fluorides in tysonite CeF3 (F1, F2, and F3 in the ratio of 12:4:2 per unit cell), we show by 19F solid-state NMR that F1 is solely responsible for the ion conductivity in Ce1–x Sr x F3–x (x = 0.001) at 0 °C. It is further shown that the observed conductivity can be explained quantitatively by using the Nernst–Einstein relation with the F1–F1 exchange rate (ca. 6 × 105 s–1) estimated from lineshape analysis with the carrier–F1 concentration. As for an alternative method to obtain the hopping rate of a carrier, we adopt the AC impedance method, for which the identity of the “carrier” is rather elusive. The observed AC impedance gives the carrier hopping rate of 3.5 × 107 s–1, which is ca. 60 times of the F1–F1 exchange rate determined by NMR. The slower F1–F1 hopping rate is ascribed to the result of the long-time average of the faster carrier hopping rate. For the AC impedance analysis, the concentration of the carrier to realize the observed conductivity is much larger than that of the fluoride ion vacancy introduced by Sr doping. For explanation, we postulate that what influences AC impedance includes not only the vacancy but also fast-exchanging fluoride ions around the vacancy.

Composite anode for fluoride-ion batteries using alloy formation and phase separation in charge and discharge processes

“…24 LaF 3 and CeF 3 have relatively high uoride-ion conductivity (LaF 3 : 1 × 10 −3 S cm −1 at 423 K, 27 CeF 3 : 4 × 10 −4 S cm −1 at 500 K (ref. 32)), therefore, they are the most promising materials at present. However, using these materials faces another problem: most electrolytes, whether solid [33][34][35] or liquid, 6,9,22,[36][37][38][39][40] are at risk of decomposition because their cathodic limits are located almost the same or higher than the RPs of La/LaF 3 and Ce/CeF 3 .…”