Simultaneous Tailoring of Robust Anti‐Thermal Quenching and High Thermometric Sensitivity in Lanthanide‐Doped Cs ₂ NaYF ₆ Double Perovskites

Abstract Lanthanide‐doped double perovskites have emerged as promising candidates for remote optical thermometry owing to their pronounced thermally quenched photoluminescence. However, substantial emission attenuation at elevated temperatures poses a challenge to achieving high measurement precision. Herein, a facile solid‐state synthesis of efficient lanthanide‐doped fluoride double perovskites Cs 2 NaYF 6 , is reported presenting robust anti‐thermal quenching behavior while maintaining enhanced thermal sensitivity through partial substitution of Na + with Li + . Li + doping induces lattice contraction and increased stiffness, reducing phonon energy and suppressing electron–phonon coupling, thereby enhancing emission intensity and mitigating the thermal quenching of Er 3+ . Meanwhile, Li + ‐induced local symmetry distortion around Er 3+ leads to further Stark splitting of 2 H 11/2 , promoting the thermally assisted population of green‐emitting levels, which retain 84% of their initial intensity at 523 K. In contrast, the red emission remains unaffected due to the large energy gap between adjacent levels. Consequently, both thermally coupled and non‐thermally coupled thermometric sensitivity are enhanced, with a maximum improvement of up to 6‐fold, leading to a substantial reduction in temperature uncertainty at high temperatures. Furthermore, the practical applicability of flexible polydimethylsiloxane‐based optical fiber and thin‐film temperature sensors is demonstrated. This provides insights into simultaneously optimizing the intensity and sensitivity of luminescent thermometers.