Regulating A‐Site Alloying of Te⁴⁺‐Doped Hafnium‐Halide Perovskite for Fluorescence Thermometry Achieving Breakthrough Sensitivity at High Temperatures

Abstract Fluorescence thermometry has garnered significant attention for its noninvasive, remote, highly sensitive, and rapid temperature measurement capabilities. However, achieving superior sensitivity at high temperatures remains a persistent challenge. Herein, a series of Rb 2− x Cs x HfCl 6 : y%Te 4+ perovskites are synthesized to investigate the impact exerted by A‐site regulating regarding the luminescence efficiency and temperature‐responsive capabilities. Te 4+ doping significantly enhances luminescence intensity, with the broadband yellow light resulting from electron–phonon coupling within the soft lattice, driven by self‐trapped exciton emission. Furthermore, A‐site regulation demonstrates that increasing Cs⁺ content induces enhanced Jahn–Teller distortion, thereby boosting luminescence efficiency. However, the pure Rb sample exhibits the lowest thermal activation energy, as the temperature rises, increased phonon‐exciton coupling and the defect activity accelerate the nonradiative recombination, drastically decreasing fluorescence lifetime by two orders of magnitude from 300 to 390 K. Utilizing this exceptional thermal sensitivity, a lifetime‐based fluorescence thermometer is developed, attaining a breakthrough relative sensitivity reaching 20.69% K −1 at 390 K, with high repeatability across multiple heating and cooling cycles. Additionally, a high‐temperature warning system for lubricating oil pipelines is designed, demonstrating robust monitoring capabilities. This work offers a new perspective on developing highly sensitive thermometers that utilize halide perovskites for high‐temperature applications.