The key role of the composition and structural features in fluoride ion conductivity in tysonite Ce_1−xSr_xF_3−x solid solutions

Abstract

Pure tysonite-type Ce_1-xSr_xF_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-xSr_xF]^(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 Ce_0.99Sr_0.01F_2.99 (22% at 64 °C) while this fraction linearly increases with x for La_1-xAE_xF_3-x (AE = Ba, Sr). The highest conductivity found for Ce_0.975Sr_0.025F_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 AE²⁺ and RE³⁺ ionic radii and of the thickness of the slab buckling is established and could allow designing new ionic conductors.