Fluorosulfide La2.7Ba6.3F8.7S6 with Double-Layer Honeycomb Structure Enabling Fluoride-Ion Conduction

Mixed-anion compounds comprise anion-ordered layered structures with fluoride ionic conducting layers, which are not found in conventional metal fluorides. Hence, they represent a new frontier in the search for fluoride-ion conductors. Previous studies investigated only mixed-anion compounds with known crystal structures, but failed to exploit a flexible structural design. In this study, we performed a materials search based on the ternary phase diagram of BaS-LaF3-BaF2 for new fluorosulfide phases and found an unreported fluorosulfide, La2.7Ba6.3F8.7S6, showing the fluoride ion conductivity of 4.23×10−7 S cm−1 at 343 K. La2.7Ba6.3F8.7S6 forms an anion-ordered two-dimensional crystal lattice with double-honeycomb (La-Ba)F2 fluoride-ion-conducting layers, which cannot be realized in single-anion compounds. In the (La-Ba)F2 layers, the fluoride ion conduction is realized through normal F1 site and interstitial F2 site via a vacancy mechanism. The presence of sulfide ions in the crystal structure contributes to the spreading of (La-Ba)F2 layers along the ab plane, resulting in a longer La-F distance. Material development using a systematic phase diagram search on fluorosulfides allows to increase the variation of the crystal structure for fluoride ion conductors and to discover the novel fluoride ion conducting layers that are inaccessible to single anion compounds.