Synthesis-Controlled Polymorphism and Anion Solubility in the Sodium-Ion Conductor Na₃InCl_6-xBr_x (0 ≤ x ≤ 2)

Motivated by the significant transport property improvement of the anion-substituted lithium metal halides, a series of anion mixed solid solutions of Na₃InCl_6-xBr_x (0 ≤ x ≤ 2) are successfully synthesized by ball milling and subsequent annealing. By milling, the Na₃InCl_6-xBr_x solid solution series crystallizes in a monoclinic P2₁/n phase, while the subsequently annealed Na₃InCl_6-xBr_x series transforms into a trigonal P3̅1c phase. Through annealing and changes of the structure type, greater anion solubility can be achieved. The halide substitution slightly improves the ionic conductivity in the Na₃InCl_6-xBr_x series, indicating that mixed halide compositions and their structural changes affect the ionic transport albeit less strongly than in the lithium analogues such as Li₃YCl_6-xBr_x and Li₃InCl_6-xBr_x.