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 conductivity and strong thermal stability remains a significant challenge, especially in rareearth fluoride systems, where vacancy formation, clustering, and microstructure intricately influence ion transport. Here, we show that mechanochemical synthesis followed by moderate hot pressing within the effective temperature range of 200-300 °C yields a tysonite-type Ca-doped rare-earth fluoride with high room-temperature conductivity. Within the low-temperature processing route, vacancy topology, c-axis lattice expansion, and grain-boundary structure are collectively optimized, resulting in a 5-orderof-magnitude increase in room temperature (RT) fluoride-ion conductivity to approximately 10 −4 S cm −1 , while also lowering the activation energy to approximately 0.4 eV. Thermogravimetric analysis, combined with time-resolved Electrochemical Impedance Spectroscopy (EIS) data, indicates that fluctuations in conductivity can be attributed to 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, the formation of intergranular glassy phases, and a sharp decrease in conductivity to approximately 10 −9 S cm −1 , highlighting a trade-off among processing, structure, and ion transport. Notably, pellets pressed at 300 °C show slow conductivity relaxation over approximately 30 days under ambient conditions, indicating metastable defect dynamics and long-term stabilization. We demonstrate that near singlecrystal ion conductivity in polycrystalline tysonite fluorides can be attained not via densification or equilibrium defect optimization, but by kinetically stabilizing an anisotropic defect network through low-temperature thermomechanical processing.