Exploring the Sm_1–xCa_xF_3–x Tysonite Solid Solution as a Solid-State Electrolyte: Relationships between Structural Features and F^– Ionic Conductivity

Abstract

Pure tysonite Sm1–x Ca x F3–x solid solutions for 0.05 ≤ x ≤ 0.17 have been prepared by the solid-state route. For the first time, the partial Sm1–x Ca x F3–x 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 Sm1–x Ca x F3–x (x = 0.05 and x = 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 Sm1–x Ca x F3–x tysonite network corresponding to Sm0.95Ca0.05F2.95 (10–4 S·cm–1, E 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 z = 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.

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

“…This variation is odd compared to that observed for Sm1−xCaxF3−x in which the fluoride anionic vacancies segregate around Ca 2+ . 17 In Ce1−xSrxF3−x, the Sr 2+ cations exert a chemical pressure, weaker than that of larger Ba 2+ in La1−xBaxF3−x that contributes to the enlargement of the interslab space only ( Fig. 1 and 3).…”

Halide‐Based Materials and Chemistry for Rechargeable Batteries

“…[295] This technique achieved ah igh ionic conductivity comparable to that of the single-crystal samples.T he microstructural study indicated that the F1 site (Figure 21 a) with the anion vacancies is the main contributor to fluorine mobility.T he same group used as imilar method to prepare solid solutions of the pure tysonites Sm 1Àx Ca x F 3Àx and Ce 1Àx Sr x F 3Àx ,w hich possessed the highest room-temperature ionic conductivities of 1 10 À4 and 3 10 À4 Scm À1 for the optimum compositions of Sm 0.95 Ca 0.05 F 2.95 and Ce 0.975 Sr 0.025 F 2.975 ,r espectively. [296,297] However,t heir application in the full battery is limited by their mechanical incompatibility with electrode powder materials.…”

Halogenid‐basierte Materialien und Chemie für wiederaufladbare Batterien

“…Die gleiche Gruppe verwendete eine ähnliche Methode, um die Sm 1− x Ca x F 3− x ‐ und Ce 1− x Sr x F 3− x ‐Tysonit‐Mischkristalle zur präparieren, welche die bei Raumtemperatur höchsten Ionenleitfähigkeiten von 1×10 −4 bzw. 3×10 −4 S cm −1 für die optimalen Zusammensetzungen von Sm 0.95 Ca 0.05 F 2.95 und Ce 0.975 Sr 0.025 F 2.975 besaßen . Allerdings ist ihre Anwendung in der aufgeladenen Batterie aufgrund ihrer mechanischen Inkompatibilität mit Elektrodenpulvermaterialien begrenzt.…”

“…Aufsätze optimalen Zusammensetzungen von Sm 0.95 Ca 0.05 F 2.95 und Ce 0.975 Sr 0.025 F 2.975 besaßen. [296,297] Allerdings ist ihre Anwendung in der aufgeladenen Batterie aufgrund ihrer mechanischen Inkompatibilitätm it Elektrodenpulvermaterialien begrenzt.…”