Dual‐Mode Thermal Response of Visible/NIR Emission in Er 3+ ‐Doped Layered Perovskite via Modulating Thermally Induced Oxygen Vacancies and Cross Relaxation

Abstract Overcoming thermal quenching remains a pivotal challenge for the practical application of upconversion luminescence (UCL) materials. Herein, a novel UCL material based on Er 3+ ‐doped layered perovskite oxyhalide Bi 4 Ti 0.5 W 0.5 O 8 Cl is demonstrated to exhibit a unique dual thermal‐response behavior: conventional thermal quenching of green/red emissions alongside remarkable thermally enhanced near‐infrared (NIR) UCL within the temperature range of 293–493 K. Under 980 nm excitation, the NIR UCL intensity ( 4 I 9/2 → 4 I 15/2 ) in Bi 4 Ti 0.5 W 0.5 O 8 Cl:10%Er 3+ at 493 K reached 133‐fold that at 293 K. The opposite thermal response is attributed to the synergistic effects of tunable oxygen vacancy concentrations and optimized cross‐relaxation processes at elevated temperatures. By leveraging the opposing thermal responses of Er 3+ emissions, an ultrahigh absolute sensitivity of 72.26% K −1 was achieved at 493 K based on the fluorescence intensity ratio of the NIR ( 4 I 9/2 → 4 I 15/2 ) to green ( 4 S 3/2 → 4 I 15/2 ) transitions, demonstrating exceptional performance for highly sensitive optical thermometry. This work not only provides a new strategy for designing thermally enhanced UCL materials but also underscores the great potential of layered perovskite oxyhalides as advanced platforms for optical temperature sensing.