The role of sodium compound fluxes used to synthesize Gd₂O₂S:Tb³⁺ by sulfide fusion method

Different sodium compound fluxes were chosen to synthesize Gd₂O₂S:Tb³⁺ phosphor by the sulfide fusion method. X-ray diffraction, fluorescent spectrophotometer, field emission scanning electron microscope (FE-SEM), thermal gravimetric analyses and differential scanning calorimetry were used to analyze the influences of sodium compound fluxes on the phase formation, luminescence properties, morphology and reaction process of Gd₂O₂S:Tb³⁺. It was shown that the existence of Na₂S₂O₃ and Na₂S_x (x = 2~6) ensured the formation of Gd₂O₂S phase and improved the Tb³⁺ doping into substrate materials. The particle growth of phosphors was attributed to the effective flux agent of Na₂S, and larger particles could be formed by adding Na₂S. The grown particles presented a hexagonal shape, and larger particles were constructed by packing smaller ones along the c-axis direction. Furthermore, Gd₂O₂S:Tb³⁺ exhibited 544 nm emission peak due to the ⁵D₄ → ⁷F₅ transition of Tb³⁺ when excited at 254 nm, and the luminescence intensity of it with the addition of Na₂S₂O₃ was stronger than that prepared from other sodium compound fluxes including Na₂S_x, Na₂CO₃, Na₂SO₃ and Na₂S.