Fluorosulfide La2.7Ba6.3F8.7S6 with a double-layer honeycomb structure enabling fluoride-ion conduction

Abstract

Mixed-anion compounds comprise anion-ordered layered structures with fluoride-ion 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, La_2.7Ba_6.3F_8.7S₆, showing a fluoride-ion conductivity of 4.23 × 10⁻⁷ S cm⁻¹ at 343 K. La_2.7Ba_6.3F_8.7S₆ 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.