Dual‐Response Photoluminescence of Halide Double Perovskite for Novel Self‐Calibrating Thermometers, Multicolor Anti‐Counterfeiting and White LED

Lead‐free halide double perovskites (LHDPs) as potentially luminescent materials are gaining recognition owing to their multi‐stimuli responsive and tunable spectra. However, the current single functionality and luminescence mode limit further application and development. Herein, Bi 3+ /Er 3+ co‐doped Cs 2 Na 0.8 Ag 0.2 LuCl 6 are successfully synthesized with excellent temperature sensing response, multi‐excitation‐capable and high photoluminescence quantum yield (PLQY = 87.7%). Bi 3+ doping generates electronic domains by forming a new localized valence band maximum and breaks the symmetry of the Lu 3+ sites, facilitating the formation of more self‐trapped excitons (STEs) and establishing an efficient energy transfer channel from STEs to Er 3+ ions. By exploiting the thermally coupled energy levels ( 2 H 11/2 and 4 S 3/2 ) of Er 3+ ions and their distinct responses to ultraviolet (UV) and near‐infrared (NIR) excitation, self‐calibrating optical thermometers for dual‐mode temperature sensing were developed, exhibiting a maximum relative sensitivity of 1.45% K −1 . Furthermore, the white‐light‐emitting diode (wLED) based on Cs 2 Na 0.8 Ag 0.2 LuCl 6 : 0.04Bi 3+ ,0.06Er 3+ phosphors exhibit excellent performance (CIE color coordinates of (0.32, 0.33), Ra = 86.5, CCT = 5776 K). The multi‐stimulus‐responsive phosphors can simultaneously respond to ultraviolet and near‐infrared excitation, demonstrating great potential in anticounterfeiting application. These findings establish Cs 2 Na 0.8 Ag 0.2 LuCl 6 : Bi 3+ , Er 3+ LHDPs as a flexible basis for multifunctional lattice‐engineered photonics.