Thermal Enhancement of Upconversion Luminescence in Negative-Thermal-Expansion Ho³⁺-Doped Yb_2−xW₃O₁₂ Phosphors

The fluorescence intensity of lanthanide (Ln³⁺)-doped upconversion (UC) materials generally exhibits significant thermal quenching with increasing temperature, which is a critical challenge for their application. In this work, Yb_2−xW₃O₁₂:xHo phosphors with thermally enhanced UC luminescence are fabricated by a facile solid-state sintering method. The x-ray diffraction (XRD) patterns reveal that the Yb_2−xW₃O₁₂:xHo samples have a pure phase, which indicates that the Ho³⁺ ions are successfully doped into the crystal lattice of Yb₂W₃O₁₂. In addition, the in situ XRD patterns show that when the temperature is increased, the diffraction peaks gradually shift to a higher angle, clearly illustrating the negative thermal expansion phenomenon of Yb₂W₃O₁₂. The UC luminescence shows that Yb_2−xW₃O₁₂:xHo phosphors have red and green emissions when excited by a 980 nm laser diode. By changing the doping amount of Ho, it has been found that the luminescence is strongest at 2 mol%. Further investigation of the temperature-dependent upconversion emission properties of Yb_2−xW₃O₁₂:xHo samples shows that the red emission increases by several times from 323 K to 498 K. The temperature-sensing characteristics of Yb_2−xW₃O₁₂:xHo are studied using fluorescence intensity ratio (FIR)-based technology. The maximum relative sensitivity and the maximum absolute sensitivity are calculated as 2.04%/K at 323 K and 0.066/K at 498 K, respectively. These results indicate that Yb_2−xW₃O₁₂:xHo fluorescent powder can be used for optical temperature measurement.