The key role of the composition and structural features in fluoride ion conductivity in tysonite Ce 1-x Sr x F 3-x solid solutions

Pure tysonite-type Ce Sr F 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 < 0.15). For 0 ≤ x ≤ 0.05, the F2-(Ce,Sr) and F3-(Ce,Sr) bond distances into [Ce Sr F] 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 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 Sr F (22% at 64 °C) while this fraction linearly increases with x for La AE F (AE = Ba, Sr). The highest conductivity found for Ce Sr F (3 × 10 S cm at RT, E = 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 and RE ionic radii and of the thickness of the slab buckling is established and could allow designing new ionic conductors.