Multimode Luminescence Tailoring and Antithermal Quenching Effect in Cs 2 NaLuCl 6 via Sb 3+ /Ho 3+ /Yb 3+ Multidoping for Color‐Coded Encryption and Real‐Time Temperature Responses

ABSTRACT The demand for advanced luminescent perovskites with tunable emission and excellent thermal stability is rapidly growing in secure optical encryption and high‐precision temperature sensing. Herein, we develop a multifunctional Cs 2 NaLuCl 6 : Sb 3+ /Ho 3+ /Yb 3+ (CNLC: Sb‐Ho‐Yb) perovskite that achieves coordinated control of color modulation and thermal robustness. By enabling efficient Sb 3+ →Ho 3+ energy transfer in downshifting (DS) and simultaneously suppressing Ho 3+ nonradiative losses in upconversion (UC), the Sb 3+ ‐doped strategy significantly enhances both Ho 3+ DS and Ho 3+ ‐Yb 3+ UC emissions, serving as a foundation for color‐coded encryption. The CNLC: Sb‐Ho‐Yb system further enables dual‐mode thermometry by exploiting the contrasting thermal behaviors of its UC (enhanced) and DS (quenched) emissions, allowing for temperature sensing tailored to the specific thermal responses of the respective luminescence mode through two independent fluorescence intensity ratio (FIR) methodologies. Specifically, excitation‐wavelength‐controlled color switching enables high‐security and multi‐dimensional optical encryption with strong anti‐interference capability, while the high luminescence sensitivity and reversible thermal response ensure high‐precision temperature sensing with excellent signal fidelity. Overall, this work establishes a new design paradigm that bridges thermal stability and tunable luminescence in perovskite systems, paving the way toward next‐generation intelligent optical encryption and reliable thermal sensing technologies.