Synthesis and spectroscopic properties of Er3+/Yb3+-codoped NaLa(WO4)2 polycrystalline phosphor for optical thermometry

Abstract Er3+/Yb3+-codoped NaLa(WO4)2 phosphor samples with different Er3+ and Yb3+ doping levels were synthesized by solid-state chemical reaction for optical thermometry and fully characterized. The results show that each phosphor sample mainly consists of NaLa(WO4)2 crystalline phase and has a nominally stoichiometric chemical composition. The phosphor grains have massive polyhedron morphology, good size homogeneity with a mean value of ∼5 μm and a cutoff phonon energy of 927 cm−1. The 530 and 550 nm upconversion fluorescence of Er3+ in the phosphor samples and their temperature characteristics were investigated under 980 nm excitation. The results show that the phosphor exhibits strong upconversion fluorescence and the maximum fluorescence intensity is observed at the optimal Er3+ and Yb3+ doping levels of 5 and 20 mol%, respectively. The two-photon process is responsible for the upconversion fluorescence. The spectral structure of either 530 or 550 nm fluorescence band is thermally stable. The peak absolute thermal sensitivity is observed at ∼490 K and has a peak value of ∼9 × 10-3 K-1. As expected, Er3+ or Yb3+ doping level has little effect on the thermal sensitivity. The phosphor displays a wide dynamical response range of thermal sensitivity from 293 to 800 K, where the sensitivity varies within (6.5–9.0) × 10-3 K-1. A comparison study shows that the temperature characteristics obtained from the cooling procedure are almost the same as those obtained from the heating procedure. Besides, the effects of Er3+/Yb3+ doping level and heating/cooling on fluorescence color chromaticity were also studied. Based on the results, the Er3+/Yb3+-codoped NaLa(WO4)2 phosphor shows strong fluorescence, good thermal stability, high thermal sensitivity and broad response range of thermal sensitivity. Thus, it is a promising material for thermal detection over a wide temperature range of 293−800 K.