Bi ³⁺ ‐Mediated Energy Redistribution Assisted by Negative Thermal Expansion for Transparent Flexible Film‐Based Optical Thermometry

ABSTRACT Real‐time, self‐calibrated, and in situ optical temperature sensing plays a crucial role in industrial, aerospace, and biomedical applications. However, the severe thermal quenching and low sensitivities limit its sensing range and accuracy. Herein, the synergistic effect of ionic doping and negative thermal expansion (NTE) is explored to achieve 31 times thermal‐enhanced upconversion luminescence (UCL) at 563 K, and high relative sensitivity of 2.55% K −1 based on non‐thermally coupled levels. We demonstrate that as the temperature increases, the shortened distance between Er/Tm in the NTE material weakens the modulation effect of Bi 3+ doping, resulting in a variation in Er/Tm ratio, which consequently increases the sensing sensitivity. It is also confirmed that Bi 3+ doping and NTE effect have little impact on the multiphonon relaxation process, instead, altering the local crystal‐field environment and the energy transfer processes by mechanism analysis and DFT calculations. Our work provides new insights into advanced optical thermometry at high‐temperature region, and offers guidance for contriving high‐sensitivity and high‐precision flexible optical thermometers.