Fluorosulfide La2+x Sr1-x F4+x S2 with a Triple-Fluorite Layer Enabling Interstitial Fluoride-Ion Conduction

Fluoride-ion conductingsolid materials are applicable as solidelectrolytes for sensing devices and next-generation rechargeablebatteries. Most of the previously reported materials have been limitedto the single-anion compounds such as fluorite-type, tysonite-type,and perovskite-type structures. These suffered from further improvementsby crystal structure modification which derives a paradigm shift inthe material tailoring. Fluoride and sulfide ions prefer respectivecoordination environments because of the different ionic radii andelectronegativity. This feature implies that fluorosulfide mixed-anioncompounds have potential to form anion-ordering crystal structureswith new fluoride-ion conducting layers. Herein, we have found thatthe fluorosulfide La2+x Sr1-x F4+x S2 exhibitsfluoride-ion conduction. The presence of multiple anions results inthe formation of an anion-ordering two-dimensional crystal latticewith triple fluorite layers, which cannot be realized for metal fluorides.Sulfide ions in the crystal structure increase the number of interstitialsites of fluoride ions, forming a fluoride-ion conduction pathway.