Judd-Ofelt spectroscopic properties of Er³⁺-doped NaLa(WO₄)₂ polycrystalline powder

Er³⁺-doped NaLa(WO₄)₂ is a promising phosphor material for applications in many fields including the ratiometric thermometry based on thermal effect of fluorescence intensity ratio (FIR) of green fluorescence of Er³⁺, which is directly correlated with Judd-Ofelt parameters Ω_i (i = 2, 4, 6). Present paper reports synthesis and Judd-Ofelt spectroscopic properties of Er³⁺-doped NaLa(WO₄)₂ µm-sized phosphor. The phosphor was synthesized by solid-state chemical reaction and characterized by X-ray diffraction and Raman scattering techniques. The results show that the phosphor is dominated by NaLa(WO₄)₂ crystalline phase and has a maximum phonon energy 927 cm⁻¹. Judd-Ofelt analysis was done for the phosphor using a safe, reliable method based on diffuse reflection spectrum and Er³⁺ 1.5 μm fluorescence lifetime. First, diffuse reflection spectrum of the phosphor was measured and relative absorption spectrum was calibrated from it using Kubelka-Munk theory. Second, Er³⁺ 1.5 μm fluorescence lifetime of the phosphor was measured and absorption cross-section spectrum was obtained based on the assumption that the 1.5 μm emission has 100% quantum efficiency. Finally, based on the absorption cross-section spectrum, standard Judd-Ofelt analysis was carried out to extract the Ω_i. Radiative rate, fluorescent branch ratio and radiative lifetime of some transitions have been obtained from the known Ω_i values. In addition, FIR proportional factor was evaluated in terms of Ω_i and compared with those values of other materials. The result shows that the phosphor has a better prospect for the application in ratiometric thermometry.