Fluorosulfide La2+xSr1−xF4+xS2 with Triple-fluorite Layer Enabling Interstitial Fluoride-ion Conduction

Fluoride-ion conducting solid materials are applicable as solid electrolytes for sensing devices and next generation rechargeable batteries. Most of the previously reported materials have limited to the single-anion compounds such as fluorite-type, tysonite-type, and perovskite-type structures. These are suffered from further improvements by crystal structure modification which derives a paradigm shift in the material tailoring. Fluoride and sulfide ions prefer respective coordination environments because of the different ionic radii and electronegativity. This feature implies that fluorosulfide mixed-anion compounds have potential to form anion-ordering crystal structures with new fluoride-ion conducting layers. Herein, we have found that the fluorosulfide La2+xSr1−xF4+xS2 exhibits fluoride ion conduction. The presence of multiple anions results in the formation of anion-ordering two-dimensional crystal lattice with triple fluorite layers, which cannot be realized for metal fluorides. Sulfide ions in the crystal structure increases the number of interstitial sites of fluoride ions, forming fluoride ion conduction pathway.