Synthesis-Controlled Polymorphism and Anion Solubility in the Sodium-Ion Conductor Na 3 InCl 6- x Br 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 Na 3 InCl 6- x Br x (0 ≤ x ≤ 2) are successfully synthesized by ball milling and subsequent annealing. By milling, the Na 3 InCl 6- x Br x solid solution series crystallizes in a monoclinic P 2 1 / n phase, while the subsequently annealed Na 3 InCl 6- x Br x series transforms into a trigonal P 3̅1 c 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 Na 3 InCl 6- x Br 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 Li 3 YCl 6- x Br x and Li 3 InCl 6- x Br x .