Controlled Structural Transformation in Sb‐Doped Indium Halides A₃InCl₆ and A₂InCl₅∙H₂O Yields Reversible Green‐to‐Yellow Emission Switch

Abstract Lead halide perovskites have demonstrated promising emission tunability achieved by composition engineering, which makes them viable in several potential applications. Determining how to effectively control the crystalline structural transformation and composition in lead‐free halide perovskites is of great importance. Herein, a controllable synthetic method is reported to obtain the 0D metal halide perovskite derivatives (Cs_1−xRb_x)₂InCl₅∙H₂O and (Cs_1−xRb_x)₃InCl₆, through synergistic regulation of the Cs/Rb feed ratios and the reaction solvent. When hydrochloric acid (HCl) is used as the reaction solvent, (Cs_1−xRb_x)₂InCl₅∙H₂O is obtained at a high Cs/Rb feed ratio greater than 2:1, while (Cs_1−xRb_x)₃InCl₆ is obtained at a low Cs/Rb feed ratio of less than 2:5. However, when anhydrous methanol (MeOH) is used as the reaction solvent, only the (Cs_1−xRb_x)₃InCl₆ structure is obtained at all Cs/Rb feed ratios. In addition, a reversible crystalline structural transformation is demonstrated between (Cs_0.67Rb_0.33)₂InCl₅∙H₂O and (Cs_0.67Rb_0.33)₃InCl₆ by immersing the as‐prepared products into MeOH and HCl sequentially, which generates a novel green/yellow reversible emission switch. The Sb³⁺ ion self‐trapped exciton emission and stability of the synthesized (Cs_1−xRb_x)₂InCl₅∙H₂O and (Cs_1−xRb_x)₃InCl₆ are systematically investigated. The results are helpful for promoting the diverse photonics and optoelectronics applications of these environmentally stable perovskite derivatives.