Exploring the Effect of Anion Substitution on the Solid Ionic Conductor NaTaCl₆

Isovalent anion substitution has been shown to have a tremendous effect on the transport properties in lithium halide solid ionic conductors. Although sodium-ion solid state batteries based on chloride ionic conductors have recently gathered significant attention, investigations of anion substitution in sodium containing chlorides remain scarce. Here, we investigate the role of Br^- isoelectronic anion substitution in a perovskite-related compound with nominal composition of NaTaCl₆. The presence of a large amount of ordered cation vacancies in the A^- and B^- sites is suggested to favor Br^- accommodation, allowing the complete solid solution in a sodium halide ionic conductor. Rietveld refinements against X-ray diffraction data show increasing unit cell volume, while nuclear magnetic resonance reveals gradual changes in the Na⁺-coordination with increasing Br^- substitution. Impedance spectroscopy analysis shows that Br^- substitution leads to decreasing room-temperature ionic conductivities, in contrast to observations in lithium chloride solid ionic conductors. Structural factors are proposed to be behind the different Na⁺ transport evolution as a function of Br^- content suggesting that special attention should be paid when well-explored principles from the Li⁺ chemistry are transferred to the design of Na⁺ halide solid electrolytes for Na⁺ solid-state batteries.