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

Abstract Rare-earth (RE3+) activated phosphors with temperature dependent luminescence are promising candidates for noncontact optical thermometry. Herein, the upconversion (UC) and near-infrared (NIR) luminescence properties of Er3+/Yb3+ codoped BaY0.67Mo0.33O3 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 Fm3̅m). Under 980 nm excitation, intense green and red UC emissions and strong NIR emission at 1549 nm from Er3+ transitions are observed and significantly enhanced by Yb3+ sensitization. Luminescence decay measurements confirm efficient energy transfer from Yb3+ to Er3+. 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 (2H11/2/4S3/2) and nonthermally coupled Er3+ 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 T2 dependence, consistent with thermodynamic considerations. Excellent repeatability and thermal stability are confirmed through multicycle measurements, thus establishing BaY0.67Mo0.33O3:Er3+/Yb3+ as a robust and versatile material suitable for high-precision optical temperature sensing.