Fluorosulfide La2+xSr1–xF4+xS2 with a Triple-Fluorite Layer Enabling Interstitial Fluoride-Ion Conduction

Abstract

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 been limited to the single-anion compounds such as fluorite-type, tysonite-type, and perovskite-type structures. These 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 an anion-ordering two-dimensional crystal lattice with triple fluorite layers, which cannot be realized for metal fluorides. Sulfide ions in the crystal structure increase the number of interstitial sites of fluoride ions, forming a fluoride-ion conduction pathway.