Upconversion enhancement through engineering the local crystal field in Yb³⁺ and Er³⁺ codoped BaWO₄ along with excellent temperature sensing performance

High-efficiency upconversion (UC) luminescence has been a hot topic in the development of optical materials. In this study, an extremely strong green UC is successfully achieved in the BaWO4:Yb3+/Er3+ phosphor through engineering the local environment around the luminescent centers, realized by Ca2+ doping. The green UC intensity of Ca2+ doped BaWO4:Yb3+/Er3+ is 6.5 and 2.3 times stronger than that of BaWO4:Yb3+/Er3+ and CaWO4:Yb3+/Er3+, respectively. Eu3+ ions are employed as the structure probe to detect the evolution of the crystal field symmetry with the increase of Ca2+ doping concentration, the results of which match well with the corresponding variation of the UC spectra. Meanwhile, the BaWO4:Yb3+/Er3+/Ca2+ exhibits excellent temperature sensing performance based on the fluorescence intensity ratio (FIR) between the thermally coupled H-2(11/2) and S-4(3/2) states of Er3+ ions. Its absolute sensitivity and relative sensitivity for optical thermometry can reach 1.21% K-1 and 1.31% K-1, outperforming the majority of the same type optical thermometers. The temperature resolution of the present thermometer remains lower than 0.1 K within the temperature range of 298 K to 573 K. The intense green UC luminescence along with the outstanding thermal properties makes the BaWO4:Yb3+/Er3+ phosphor a promising candidate for optical thermometry.