Experimental Visualization of F-Ion Diffusion Pathways and Geometric Frustration-Induced Positional Disorder in CaF₂–BaF₂ Solid Electrolytes

Metastable CaF 2 –BaF 2 compounds synthesized via high-energy ball milling or using thermal plasma are promising candidates for the fluoride-ion conducting solid electrolytes of all-solid-state fluoride batteries. The ionic conductivity of (CaF 2 ) x (BaF 2 ) 1– x with x ≈ 0.5 is over 5 orders of magnitude greater than those of CaF 2 and BaF 2 . However, the transport mechanism of fluoride ions in (CaF 2 ) x (BaF 2 ) 1– x materials has not yet been fully elucidated. Herein, we have determined the conduction pathways of fluoride ions and local atomic configurations for (CaF 2 ) 0.48 (BaF 2 ) 0.52 via a neutron diffraction analysis. Among various (CaF 2 ) x (BaF 2 ) 1– x samples, (CaF 2 ) 0.48 (BaF 2 ) 0.52 exhibits the highest conductivity and lower activation energy. The “–F1–□ F –F1–” diffusion pathways of fluoride ions are identified via Rietveld refinement and a maximum entropy method, where F1 indicates a regular site for a fluoride ion (i.e., the (1/4, 1/4, 1/4) atomic position) within the T -unit composed of four Ca and/or Ba atoms, and the □ F interstitial site is located at an off-center position in the O -unit composed of six Ca and/or Ba atoms. Moreover, reverse Monte Carlo modeling conducted using an atomic pair distribution function reveals the existence of geometric frustration-induced positional disorder with respect to Ca, Ba, and F atoms even at 150 K, which increases the mobility of fluoride ions in the “–F1–□ F –F1–” diffusion pathways. The findings of this work may facilitate the development of solid electrolytes for next-generation all-solid-state rechargeable batteries with large-scale applications such as electric vehicles and residential energy storage systems.