Realizing Near‐Unity Photoluminescence Quantum Yield in Metal Halide RbCdCl₃:Mn²⁺ Crystals via Phase Transformation Engineering

Abstract All‐inorganic metal halides have merged as auspicious materials for optoelectronic applications due to their predominant tunable and controlled photoluminescence (PL) properties. Despite substantial efforts and advances in the development of these all‐inorganic metal halides, considerable long‐term challenges remain to be solved to realize cutting‐edge material performances. Here, an all‐inorganic metal halide RbCdCl 3 , featuring a reversible structural phase transformation from non‐perovskite (orthorhombic‐phase) to perovskite (tetragonal‐phase) structure is reported. Intriguingly, via phase transformation regulation engineering, a near‐unity photoluminescence quantum yield (PLQY) along with large Stokes shift (275 nm) and long decay lifetime (14.69 ms) is achieved in RbCdCl 3 :Mn 2+ thermochromic fluorescent materials, comparing with the pristine non‐pervoskite structure with an initial PLQY of 3.1%. Moreover, the underlying PL switching mechanisms are systematically elucidated by the in situ optical characterizations and the first‐principles calculations. This work demonstrates a thermochromic fluorescent anti‐counterfeiting material based on the tunable and reversible photoluminescence switching and also provides a phase structure engineering in metal halides to broaden their manifold applications in optoelectronic fields.