Ratiometric Optical Thermometer with High Sensitivity Based on Site-Selective Occupancy of Mn²⁺ Ions in Li₅Zn₈Al₅Ge₉O₃₆ under Controllable Synthesis Atmosphere

A fluorescence intensity ratio (FIR) thermometer based on inorganic luminescence materials is a new-type temperature sensor with potential and significance. However, research based on single transition metal ions dual-emission fluorescence ratio thermometer is very rare. Especially, a FIR thermometer based on Mn 2+ ions has never been reported. Here, we successfully control the site-selective occupancy of Mn 2+ ions in Li 5 Zn 8 Al 5 Ge 9 O 36 by controlling the synthesis atmosphere conditions. Mn 2+ ions form [MnO 4 ] tetrahedrons with a weak crystal field and [MnO 6 ] octahedrons with a strong crystal field corresponding to synthesis atmospheres of N 2 and O 2, respectively. In addition, a novel FIR thermometer Mn 2+ -doped material Li 5 Zn 8 Al 5 Ge 9 O 36:Mn 2+ based on different dependences of Mn 2+ ions in different crystal fields on temperature is prepared in 50%N 2 +50%O 2 conditions. The thermometer has high sensitivity in the range of 303–383 K including absolute sensitivity ( S a ) and relative sensitivity ( S r ) (the S a maximum is 0.476 K –1 at 373 K and the S r maximum is 8.489% K –1 at 323 K) with low uncertainty (lower 0.1 K). Our researches have added a new and excellent member into optical thermometer materials and provided a valuable reference for exploring a new ratiometric optical thermometer.