The key role of the composition and structural features in fluoride ion conductivity in tysonite Ce1−xSrxF3−x solid solutionsElectronic supplementary information (ESI) available: Cell parameters, F-(Ce,Sr) distances and additional 19F MAS NMR spectra of Ce1−xSrxF3−x solid solutions. See DOI: 10.1039/c6dt04714a

Pure tysonite-type Ce 1− x Sr x F 3− 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 < x limit < 0.15). For 0 ≤ x ≤ 0.05, the F2-(Ce,Sr) and F3-(Ce,Sr) bond distances into [Ce 1− x Sr x F] (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 19 F 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 Ce 0.99 Sr 0.01 F 2.99 (22% at 64 °C) while this fraction linearly increases with x for La 1− x AE x F 3− x (AE = Ba, Sr). The highest conductivity found for Ce 0.975 Sr 0.025 F 2.975 (3 × 10 −4 S cm −1 at RT, E a = 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 AE 2+ and RE 3+ ionic radii and of the thickness of the slab buckling is established and could allow designing new ionic conductors. Evolution with x of RT ionic conductivity of RE 1− x AE x F 3− x showing that Ce 1− x Sr x F 3− x is the best F − tysonite conductor is discussed.