Defect‐Topology Control of Anisotropic Lattice Flexibility for High‐Performance Fluoride‐Ion Electrolytes

Fluoride‐ion conductors show promise for next‐generation solid‐state ionics, but achieving high room‐temperature (RT) conductivity and strong thermal stability remains challenging, especially in systems where defect chemistry, clustering, and microstructure intricately influence ion transport. Here, we show that mechanochemical synthesis followed by hot pressing at 200–300°C yields a tysonite‐type Ca‐doped rare‐earth fluoride with high ionic conductivity at RT. Within this low‐temperature route, vacancy topology, c ‐axis lattice expansion, and grain‐boundary structure are optimized, resulting in a several‐order‐of‐magnitude increase in RT conductivity to ∼10 −4 −10 −5 S cm −1 and reduction of activation energy to ∼0.4 eV. Thermogravimetric analysis, combined with time‐resolved electrochemical impedance spectroscopy, indicates that conductivity fluctuations arise from changes in defect chemistry, particularly the formation of vacancies at the fluorine site. Hot pressing at 400°C results in vacancy clustering, c ‐axis collapse, and formation of intergranular glassy phases, sharply decreasing conductivity to ∼10 −9 S cm −1 and highlighting a trade‐off among processing, structure, and ion transport. Pellets pressed at 300°C show slow conductivity relaxation over 30–200 days under ambient conditions, indicating metastable defect dynamics and long‐term stabilization. These results demonstrate that near‐single‐crystal conductivity in polycrystalline tysonite fluorides can be approached by kinetic stabilization of anisotropic defect structure via low‐temperature thermomechanical processing.