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

Metastable CaF2–BaF2 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 (CaF2)x(BaF2)1–x with x ≈ 0.5 is over 5 orders of magnitude greater than those of CaF2 and BaF2. However, the transport mechanism of fluoride ions in (CaF2)x(BaF2)1–x materials has not yet been fully elucidated. Herein, we have determined the conduction pathways of fluoride ions and local atomic configurations for (CaF2)0.48(BaF2)0.52 via a neutron diffraction analysis. Among various (CaF2)x(BaF2)1–x samples, (CaF2)0.48(BaF2)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.