Development of Multifunctional Materials Based on Heavy Concentration Er3+‐Activated Lead‐free Double Perovskite Cs2NaBiCl6

Abstract Lead halide perovskites have been very promising for versatile optoelectronic applications, whereas the inherent toxicity and instability of lead halide restrict its wide application. Herein, a kind of high‐performance multifunctional phosphor is designed based on heavy concentration Er 3+ ‐activated lead‐free double perovskite Cs 2 NaBiCl 6 (CNBC), which realizes the self‐sensitization of Er 3+ ions and emits high‐brightness pure green emission upon dual near‐infrared (dual‐NIR) excitation. The luminescence properties can be further optimized by adding sensitizers (Yb 3+ /Nd 3+ ). Interestingly, the emission intensity of the green region is heightened by 212‐fold after Yb 3+ ions doping, which is 40 times higher than that of the current popular phosphors (α‐NaYF 4 :2%Er 3+ ,18%Yb 3+ ). Meanwhile, the temperature sensing properties of the thermally coupled levels (Er 3+ : 2 H 11/2 , 4 S 3/2 ) in Cs 2 NaBiCl 6 :40%Er 3+ are also investigated systematically based on the luminescence intensity ratio principle, and the maximum relative sensitivity is calculated to be as high as 1.27% K ‐1 (980 nm) and 1.57% K ‐1 (808 nm). Fortunately, the thermal stability and the performance for temperature readout of the studied phosphors are still maintaining a high level after doping sensitizers. The insights provided by this work will broaden the scope of lead‐free halide double perovskites in the fields of luminescence and thermometry.