Temperature Dependent Upconversion Luminescence in Er ³⁺ /Yb ³⁺ Codoped BaY _0.67 Mo _0.33 O ₃ Double Perovskite Phosphor for Optical Thermometry

Rare-earth (RE 3+ ) activated phosphors with temperature dependent luminescence are promising candidates for noncontact optical thermometry. Herein, the upconversion (UC) and near-infrared (NIR) luminescence properties of Er 3+ /Yb 3+ codoped BaY 0.67 Mo 0.33 O 3 double-perovskite phosphors are systematically investigated for temperature sensing. Phase-pure compounds synthesized via a solid-state reaction method crystallize in a cubic double-perovskite structure (space group Fm 3̅ m ). Under 980 nm excitation, intense green and red UC emissions and strong NIR emission at 1549 nm from Er 3+ transitions are observed and significantly enhanced by Yb 3+ sensitization. Luminescence decay measurements confirm efficient energy transfer from Yb 3+ to Er 3+ . Power dependent studies indicate that both green and red UC emissions predominantly originate from two photon processes. Temperature dependent measurements in the range 293–718 K reveal pronounced thermal redistribution among thermally coupled ( 2 H 11/2 / 4 S 3/2 ) and nonthermally coupled Er 3+ levels. The fluorescence intensity ratio (FIR) of the thermally coupled levels (TCLs) follows a Boltzmann behavior, yielding an energy gap of 612 cm –1 and a maximum absolute sensitivity of 4.5 × 10 –3 K –1 at 485 K. FIRs based on nonthermally coupled levels (NTCLs) are well fitted by polynomial models, offering complementary thermometric modes over extended ranges. The temperature resolution reaches 0.7 K at room temperature and follows a T 2 dependence, consistent with thermodynamic considerations. Excellent repeatability and thermal stability are confirmed through multicycle measurements, thus establishing BaY 0.67 Mo 0.33 O 3:Er 3+ /Yb 3+ as a robust and versatile material suitable for high-precision optical temperature sensing.