Stoichiometry‐Controlled Phase Engineering of Cesium Bismuth Halides and Reversible Structure Switch

Abstract Perovskite metal halides have attracted extensive attention because of their excellent photoelectric properties and structural adjustability. Herein, the authors report a facile method for the controlled synthesis of cesium‐bismuth halides by controlling the stoichiometry of Cs/Bi precursors at room temperature. The 0D Cs₃BiCl₆ and 1D Cs₃Bi₂Cl₉ can be obtained at the Cs/Bi feed ratio of 3:1 and 3:2, respectively. Though both metal halides exhibit a very low photoluminescence quantum yield (PLQY) due to their intrinsic nature of indirect band gaps, they exhibited bright orange broadband emission after Mn²⁺ doping. Combining density functional theory (DFT) calculations, it is found that an appropriate amount of Mn²⁺ doping does not change the crystal structures of cesium‐bismuth halides, but Mn 3d orbitals produce impurity states in the forbidden energy gaps of the host structures, resulting in efficient energy transfer from the conduction band of the host to d‐state of Mn ions. Importantly, the reversible structural switch of Mn‐doped Cs₃BiCl₆ and Cs₃Bi₂Cl₉ metal halides can be flexibly adjusted through the post addition of the CsCl/BiCl₃ precursors. These results tremendously enrich the synthetic chemistry of low‐dimensional metal halides and provide a beneficial reference for the regulation of crystal structure and optical properties of metal halides.