Bayesian‐Optimized Dual‐Mode Luminescence‐Based Optical Thermometry Enabled by Negative Thermal Expansion in Lu ₂ (MoO ₄ ) ₃ :Yb–Er–Tm Phosphor

ABSTRACT Highly sensitive thermometry based on luminescence intensity ratio (LIR) of negative thermal expansion (NTE) Lu 2 (MoO 4 ) 3 :Yb 3+– Er 3+– Tm 3+ (LuMO:Yb–Er–Tm) phosphor with dual luminescence mode of upconversion (UC) and downshifting (DS) emissions is demonstrated. High emission intensity is fundamental for superior sensing performance. Since both UC and DS emissions strongly depend on interactions between lanthanide ions, optimization of dopant concentrations is crucial. To efficiently determine the optimal compositions, Bayesian optimization approach was employed, enabling identification with significantly fewer experimental configurations. Only 24 samples over 5 generations were synthesized to successfully identify the optimized compositions. The optimized LuMO:Yb–Er–Tm phosphor exhibited intense UC and DS emissions over the wide wavelength range under 980 nm excitation. Notably, the DS emissions displayed anti‐thermal quenching behavior corresponding to the host's NTE nature, while the UC emissions showed thermal quenching over the range of 398 to 573 K. Exploiting these distinct temperature dependencies, the LIR based on DS/UC emission demonstrated high relative sensitivity. Furthermore, the material exhibited exceptional repeatability and low temperature uncertainty. Overall, this study demonstrates that LuMO:Yb–Er–Tm represents a promising phosphor material for practical high‐performance optical thermometry.