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

Overcoming thermal quenching remains a pivotal challenge for the practical application of upconversion luminescence (UCL) materials. Herein, a novel UCL material based on Er³⁺-doped layered perovskite oxyhalide Bi₄Ti_0.5W_0.5O₈Cl 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 (⁴I_9/2 → ⁴I_15/2) in Bi₄Ti_0.5W_0.5O₈Cl:10%Er³⁺ 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 Er³⁺ emissions, an ultrahigh absolute sensitivity of 72.26% K^-1 was achieved at 493 K based on the fluorescence intensity ratio of the NIR (⁴I_9/2 → ⁴I_15/2) to green (⁴S_3/2 → ⁴I_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.