Exploring the Sm_1–<i>x</i>Ca_<i>x</i>F_3–<i>x</i> Tysonite Solid Solution as a Solid-State Electrolyte: Relationships between Structural Features and F^– Ionic Conductivity

Pure tysonite Sm_1–<i>x</i>Ca_<i>x</i>F_3–<i>x</i> solid solutions for 0.05 ≤ <i>x</i> ≤ 0.17 have been prepared by the solid-state route. For the first time, the partial Sm_1–<i>x</i>Ca_<i>x</i>F_3–<i>x</i> solid solution is investigated on the basis of structural features and ionic conductivity measurements. Powder X-ray diffraction Rietveld refinements show an unexpected decrease of the hexagonal unit cell volume related to the creation of fluorine vacancies. The local environment of F1, which is mainly responsible for the ionic conductivity, changes with the Ca content: the distortion of the F1­(Sm,Ca)₄ tetrahedral site decreases with the Ca content. Fluoride ion exchanges have been qualitatively probed on two Sm_1–<i>x</i>Ca_<i>x</i>F_3–<i>x</i> (<i>x</i> = 0.05 and <i>x</i> = 0.15) samples thanks to ¹⁹F magic angle spinning NMR experiments at various spinning frequencies and temperatures. At room temperature, the ionic conductivity decreases exponentially with the Ca content and the activation energy increases monotonously with the Ca content. The highest conductivity is found for the lowest Ca content or the smallest fluorine vacancy content stabilized in the Sm_1–<i>x</i>Ca_<i>x</i>F_3–<i>x</i> tysonite network corresponding to Sm_0.95Ca_0.05F_2.95 (10^–4 S·cm^–1, <i>E</i>_a = 0.36 eV at room temperature). For this composition, the largest dispersions of F2–(Sm,Ca) and F3–(Sm,Ca) distances as well as (Sm,Ca)–F1–(Sm,Ca) angles are observed. The buckling of F2/F3 sheets around the <i>z</i> = 1/4 coordinate for low Ca content affects the large F1 tetrahedral site with the strongest distortion. The higher the buckling effect into the F2/F3 sheets, the higher the F1 local site distortion and the higher the ionic mobility and conductivity