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.

中文翻译：

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氟硫化物La2.7Ba6.3F8.7S6，具有双层蜂窝结构，可实现氟离子传导

混合阴离子化合物包含具有氟离子传导层的阴离子有序层状结构，这在传统金属氟化物中是不存在的。因此，它们代表了寻找氟离子导体的新领域。先前的研究仅研究了具有已知晶体结构的混合阴离子化合物，但未能利用灵活的结构设计。在本研究中，我们根据BaS–LaF ₃ –BaF ₂三元相图进行了材料搜索，寻找新的氟硫化物相，发现了一种未报道的氟硫化物La _2.7 Ba _6.3 F _8.7 S ₆，其氟离子电导率为4.23 × 10 ^-7 S cm ^-1 at 343 K。 La _2.7 Ba _6.3 F _8.7 S _{6形成具有双蜂窝状 (La–Ba)F 2}氟离子传导层的阴离子有序二维晶格，不能形成可以在单阴离子化合物中实现。在(La–Ba)F ₂层中，氟离子传导是通过空位机制通过正常F1位点和间隙F2位点实现的。晶体结构中硫化物离子的存在有助于 (La–Ba)F ₂层沿ab平面扩散，从而导致更长的 La–F 距离。使用氟硫化物的系统相图搜索进行材料开发可以增加氟离子导体晶体结构的变化，并发现单一阴离子化合物无法达到的新型氟离子导电层。