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.