Broad‐Temperature Optical Thermometry via Dual Sensitivity of Self‐Trapped Excitons Lifetime and Higher‐Order Phonon Anharmonicity in Lead‐Free Perovskites

Broad‐temperature optical thermometry necessitates materials with exceptional sensitivity and stability across varied thermal conditions, presenting challenges for conventional systems. Here, we report a lead‐free, vacancy‐ordered perovskite Cs2TeCl6, that achieves precise temperature sensing through a novel combination of self‐trapped excitons (STEs) photoluminescence (PL) lifetime modulation and unprecedented fifth‐order phonon anharmonicity. The STEs PL lifetime demonstrates a highly temperature‐sensitive response from 200 to 300 K, ideal for low‐to‐intermediate thermal sensing. In contrast, the Eg phonon mode undergoes significant linewidth broadening due to five‐phonon scattering processes, with a distinct nonlinear temperature dependence up to 500 K. This fifth‐order anharmonic effect enhances Raman‐based temperature sensitivity, yielding a specific sensitivity (Sr) of 0.577% K−1 at 330 K and remaining above 0.5% K−1 at elevated temperatures. This study presents the first evidence of fifth‐order anharmonic effects enhancing Raman‐based temperature sensitivity, establishing Cs2TeCl6 as a versatile candidate for broad‐temperature optical thermometry and opening new avenues for precise non‐contact temperature sensing in advanced technological applications.