The key role of the composition and structural features in fluoride ion conductivity in tysonite Ce1-xSrxF3-x solid solutions.

Pure tysonite-type Ce1-xSrxF3-x solid solutions for 0 ≤ x < 0.15 were prepared by a solid-state route at 900 °C. The cell parameters follow Vegard's laws for 0 ≤ x ≤ 0.10 and the solubility limit is identified (0.10 < xlimit < 0.15). For 0 ≤ x ≤ 0.05, the F2-(Ce,Sr) and F3-(Ce,Sr) bond distances into [Ce1-xSrxF](2-x)+ slabs strongly vary with x. This slab buckling is maximum around x = 0.025 and strongly affects the more mobile F1 fluoride ions located between the slabs. The 19F MAS NMR spectra show the occurrence of F1-F2,3 exchange at 64 °C. The fraction of mobile F2,3 atoms deduced from the relative intensity of the NMR resonance is maximum for Ce0.99Sr0.01F2.99 (22% at 64 °C) while this fraction linearly increases with x for La1-xAExF3-x (AE = Ba, Sr). The highest conductivity found for Ce0.975Sr0.025F2.975 (3 × 10-4 S cm-1 at RT, Ea = 0.31 eV) is correlated to the largest dispersion of F2-(Ce,Sr) and F3-(Ce,Sr) distances which induces the maximum sheet buckling. Such a relationship between composition, structural features and fluoride ion conductivity is extended to other tysonite-type fluorides. The key role of the difference between AE2+ and RE3+ ionic radii and of the thickness of the slab buckling is established and could allow designing new ionic conductors.