Temperature Dependent Upconversion Luminescence in Er3+/Yb3+ Codoped BaY0.67Mo0.33O3 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 BaY_0.67Mo_0.33O₃ 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 (²H_11/2/⁴S_3/2) 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 BaY_0.67Mo_0.33O₃:Er³⁺/Yb³⁺ as a robust and versatile material suitable for high-precision optical temperature sensing.