Phase‐Transition‐Tunable CsMnBr ₃ ‐Cs ₃ MnBr ₅ Perovskite Nanocrystal‐Embedded Glass and Its Application to High‐S _R Fiber‐Optic Temperature Sensing

ABSTRACT Luminescent thermometry is critically important for the development of intelligent health monitoring devices. However, it remains a significant challenge to construct stable fiber‐optic temperature sensors by identifying luminescent materials with ideal dual‐wavelength emission characteristics that enable highly sensitive real‐time temperature monitoring. The distinct thermal quenching behaviors are achieved through precise coordination engineering, where octahedral [MnBr 6 ] 4 − configurations confer exceptional thermal stability while tetrahedral [MnBr 4 ] 2 − structures enable pronounced temperature sensitivity. Collectively, the complementary properties of these two materials establish a multifunctional foundation for developing programmable thermo‐optic systems. In this work, guided by physical mechanisms and mathematical modeling, a dual‐emissive temperature‐sensing material based on CsMnBr 3 ‐Cs 3 MnBr 5 perovskite nanocrystal‐embedded glass is developed by integrating the thermal inertia of CsMnBr 3 with the thermoresponsive behavior of Cs 3 MnBr 5 . Systematic investigation using the fluorescence intensity ratio (FIR) technique yields a high temperature sensitivity (S R = 4.0724% K − 1 ), demonstrating excellent thermometric performance. The material is further fabricated into a fiber probe for physiological temperature monitoring, exhibiting good accuracy and stability. This study provides a new design strategy for flexible fiber‐optic temperature sensors and highlights their potential for health‐monitoring applications.