Highly Efficient Mn⁴⁺‐Doped Red‐Emitting Oxyfluorides with Excellent Water Resistance Toward Flexible Composite Fluorescent Optical Fiber Sensor

Flexible thermal sensors are crucial for monitoring the important thermodynamic parameter of temperature in daily life, industrial production, and scientific research. However, significant challenges remain in simultaneously achieving reproducible, sensitive, real‐time, and in situ temperature sensing capabilities. Herein, a Mn 4+ mono‐doped CsNaNbOF 5 :Mn 4+ (CNNOFM) phosphor is designed and synthesized as a dual‐mode optical thermometric material, showing high luminescence efficiencies and excellent temperature‐dependent behaviors. Combined density functional theory calculations and experimental characterization reveal the isovalent group substitution mechanism between [MnF 6 ] 2− and [NbOF 5 ] 2− octahedrons, eliminating charge compensation defects in the CNNOFM system and thereby leading to enhanced luminescence efficiencies. Furthermore, CNNOFM exhibits remarkable water resistance, retaining 88.12% of its luminescent efficiency after 4 h of water immersion. The CNNOFM demonstrates high relative sensitivity in both fluorescence intensity ratio and lifetime modes, with S r values of 0.37% K −1 and 5.8% K −1 at 440 K, respectively. Finally, a flexible composite fluorescent fiber temperature sensor is fabricated based on the CNNOFM phosphor to monitor the temperature of an ice‐water mixture, exhibiting satisfactory performance. This work not only provides a promising thermally sensitive material for optical thermometry but also offers a new pathway for the development of flexible optical temperature sensors.