Ultra-High-Efficiency and Reversible Hydrochromism in a CsCaCl ₃ :Cd ²⁺ /Mn ²⁺ Perovskite for Advanced Information Encryption

Lead-free metal halide perovskites have attracted considerable interest due to their excellent optical properties and environmental friendliness. However, their practical application is often hindered by a low photoluminescence quantum yield (PLQY). Herein, we report a novel Cd²⁺/Mn²⁺ codoped CsCaCl₃ perovskite crystal that achieves a record-breaking PLQY of 93%, which is realized through a novel energy transfer pathway. By varying the Mn²⁺ doping concentration in CsCaCl₃:4%Cd²⁺, the crystal's emission color can be tuned from cyan to yellow across the 400-700 nm wavelength range. Interestingly, the material also exhibits a distinct hydrochromic behavior. Upon humid air exposure, it undergoes a structural transformation from CsCaCl₃:Cd²⁺/Mn²⁺ to Cs₂CaCl₄·2H₂O:Cd²⁺/Mn²⁺, accompanied by a dramatic emission color shift from yellow to orange. This process exhibits excellent reversibility over multiple wetting-drying cycles. The combination of ultrahigh PLQY, tunable luminescence, and stimuli-responsive hydrochromism makes this material highly promising for advanced applications in dynamic information encryption and anticounterfeiting. Moreover, the CsCaCl₃:4%Cd²⁺/3%Mn²⁺ crystal exhibits tunable energy-transfer emission under different temperatures, achieving a high relative thermal sensitivity of 5.48%·K^-1, demonstrating its potential for optical thermometry.