Unlocking Thermally Enhanced NIR‐II Luminescence in Er ³⁺ via Integer Phonon‐Matching for High‐Sensitivity Ratiometric Thermometry

Luminescent thermometry in the second near‐infrared (NIR‐II) window holds significant promise for non‐invasive. However, its practical application is severely hindered by thermal quenching at elevated temperatures, which limits the development of highly sensitive NIR‐II thermometers capable of thermally enhanced emission. Here, we present a rationally designed integer phonon‐matching strategy that enables NIR thermal enhancement specifically in singly Er 3+ ‐doped Ca 2 YNbO 6 double perovskite hosts. Through combined experimental investigations and rate equation modeling, we reveal that efficient multi‐phonon relaxation (MPR) between the Er 3+ : 4 I 11/2 and 4 I 13/2 levels is activated when the electronic energy gap corresponds to an integer multiple ( n ≤ 5) of the dominant phonon energy. This resonant phonon‐matching condition facilitates highly efficient MPR with increasing temperature, leading to a pronounced 1.66‐fold enhancement in the Er 3+ NIR‐II emission. Furthermore, we demonstrate the simultaneous occurrence of NIR thermal enhancement and ratiometric temperature sensing in Ca 2 YNbO 6 : Yb 3+ /Er 3+ /Ho 3+ /Pr 3+ crystals, achieving an exceptional relative sensitivity of 0.87% K −1 at 398 K. These findings not only circumvent the thermal quenching limitations of NIR‐II phosphors but also establish phonon engineering as a versatile strategy for designing thermally enhanced luminescence, opening new avenues for flexible monitoring of pipeline integrity and organic compound leakage based on differential NIR‐II luminescence absorption.