XAFS Studies on the Ba1-xCaxF2 Fluoride Ion Conductor

Ba0.5Ca0.5F2 is a fluoride-ion conductor expected to be used in next-generation fluoride-ion batteries. We examined the local structures around Ba and Ca using K-edge X-ray absorption fine structure (XAFS) spectroscopy. Compared with pure BaF2 and CaF2, the local structures around both Ba and Ca are highly disordered in Ba0.52Ca0.48F2. The Ba–F and Ca–F bond lengths, which retain almost the same values found in pure BaF2 and CaF2, deviate significantly from Vegard’s rule. These deviations cause frustration in the ionic crystal and lattice disorder in Ba0.5Ca0.5F2, creating pathways for fluoride-ion conduction.