Structural Transformation‐Engineered Yttrium‐Based Lead‐Free Metal Halides with Smart Tunable Luminescence via Self‐Trapped Exciton and Lanthanide Ion Intrinsic Combined Emissions

Abstract Lead‐free metal halides with structural transformation‐induced tunable luminescence have made great progress in the field of smart materials. However, their potential applications in particular scenarios are constrained by the limitations of individual photoluminescence peak regulation. Here, Sb/Ln (Lanthanide = Ce, Ho, and Tb) co‐doped Cs‐Y‐Cl metal halides with structural transformation‐driven smart tunable luminescence is reported. The reversible structural transformation between Cs 3 YCl 6 and Cs 4 YCl 7 is achieved through the “CsCl inserting/stripping” process. With the doping of Sb, efficient self‐trapped exciton emission is realized in Cs‐Y‐Cl: Sb system, leading to the reversible color‐tuning between Cs 3 YCl 6 : Sb (green emission) and Cs 4 YCl 7 : Sb (yellow emission). By doping Sb/Ln into the Cs‐Y‐Cl lattice, the combined emissions, originating from self‐trapped excitons and the characteristic transitions of lanthanide ions, resulting in more diverse emission variations after following the structural transformation. Additionally, the lattice self‐purification effect induced by the structural transformation leads to changes in the Sb/Ln molar ratio in the ethanol‐treated Cs 3 YCl 6 . This enables the modulation of the intensity ratio between Sb‐ and Ln‐related emissions. Furthermore, anticounterfeiting and information encryption patterns are successfully implemented. The results open up new avenues for the design of structural transformation‐driven multi‐mode luminescent materials, offering innovative solutions for advanced anticounterfeiting and information encryption applications.