Synergistic Phase Transition and Defect Engineering Enables High‐Sensitivity Optical Thermometry in Ferroelectric Oxides

High‐sensitivity optical thermometry based on non‐thermally coupled levels (NTCLs) is vital for advanced remote sensing. However, achieving robust anti‐thermal luminescent quenching remains a significant challenge by relying solely on negative thermal expansion effect or defect engineering designs, owing to material scarcity and limited carrier compensation. For this, we propose a novel synergistic strategy that integrates ferroelectric phase transitions with defect engineering in NaNbO 3 :Nd 3+ /Yb 3+ . This approach leverages oxygen vacancies for carrier storage and release, combined with polymorphic phase transitions to modulate local crystal‐field symmetry, effectively offsetting non‐radiative energy relaxation. Consequently, our results demonstrate a 654‐fold luminescent enhancement at 673 K. This dramatic enhancement significantly enhances the fluorescence intensity ratio of Nd 3+ ions (498 nm/683 nm), yielding a high relative sensitivity of 4.3% K −1 and absolute sensitivity of 9.38 K −1 . These results not only offer a novel strategy for designing thermally enhanced upconversion luminescent materials, but also reveal the significant potential of ferroelectric oxides as promising candidates for high‐temperature optical thermometry.