Multi‐Ion Doping in Low‐Phonon‐Energy Nd ³⁺ Based Double Perovskite Unlocks Multimodal Anti‐Counterfeiting and High‐Performance Optical Thermometry

ABSTRACT The development of multifunctional luminescent materials with tunable upconversion (UC), downshifting (DS), and thermally responsive emissions in a single host is essential for next‐generation photonic and optoelectronic technologies. Here, we report a Nd ³⁺ ‐based low‐phonon‐energy lead‐free halide double perovskite, Cs ₂ NaNdCl ₆ , which overcomes key limitations of conventional halide double perovskites, including doping inflexibility, concentration quenching, and lack of integration of diverse optical functionalities. Leveraging the large unit cell and the dual role of Nd ³⁺ as both emitter and sensitizer, we achieve efficient high‐concentration doping of Tm ³⁺ , Er ³⁺ , and Yb ³⁺ without compromising emission intensity. Tm ³⁺ doping induces energy back transfer to Nd ³⁺ , leading to an 8.2‐fold enhancement in the orange emission of Nd ³⁺ ions. Co‐doping with Er ³⁺ /Yb ³⁺ enables excitation‐dependent, color‐tunable UC (green, orange, red) under 980 nm excitation, stable green UC emission under 808 nm excitation, and red DS emission under 365 nm excitation, useful for advanced anti‐counterfeiting applications. Furthermore, the synergistic incorporation of multiple lanthanides facilitates multi‐level fluorescence intensity ratio (FIR) thermometry via thermally coupled levels (TCLs), non‐thermally coupled levels (non‐TCLs), and Stark‐split transitions, achieving a record relative sensitivity (S _r ) of 11.4% K ⁻¹ in the 80–340 K temperature range. These findings establish Cs ₂ NaNdCl ₆ as a versatile platform for multifunctional lattice‐engineered photonics.