Migration-Coupled Local Fluoride Relaxation in Li2– x La(1+ x )/3Nb2O6F Single Crystals

Abstract High Li–ion conductivity is difficult to achieve in chemically robust solid electrolytes, especially oxides and oxyfluorides, because their rigid, less polarizable anion frameworks generally give rise to high migration barriers. The pyrochlore-type oxyfluoride Li2–xLa(1+x)/3Nb2O6F (LLNOF) is an unusual example that combines high bulk Li–ion conductivity with a low activation energy, but the origin of this behavior has remained unclear, largely because no single-crystal study has been available. Here we report the first growth of millimeter-sized LLNOF single crystals and establish nearly isotropic bulk Li–ion conduction. The bulk Li–ion conductivity increases with decreasing x, reaching 16.3 mS cm–1 at 25 °C, while the activation energy decreases systematically. Single-crystal X-ray diffraction and maximum entropy method analyses reveal that the cubic pyrochlore framework is retained, while the electron-density distribution around the F site contains a directional off-centered component toward neighboring 16d sites. This component is resolved as a density maximum at 150 K and remains as a shoulder at the same displacement at room temperature, indicating that the local off-centered component persists but is thermally averaged. Together, these observations support a local rearrangement of the fluoride environment coupled to Li/vacancy exchange. They identify migration-coupled local fluoride relaxation as a viable mechanism for low-barrier Li–ion transport in chemically robust solids lacking highly polarizable anion frameworks.