Study On Luminescent Properties Of Eu～(3+) And Tb～(3+) Ions Doped Three Borates In VUV-Vis Range

The luminescent materials with borates as the host are considered as very potential in practice, and the prefened materials for the high definition television (HDTV) and projective televisions, due to their low synthesizing temperature, simple preparing conditions and high luminescent brightness. The investigation on the luminescent properties of borates activated by Eu³⁺ or Tb³⁺, which is significative both on the scientific research and in practice.In this article, the single phase of (Gd,Ln)Al₃(BO₃)₄:Eu³⁺(Ln=La³⁺, Y³⁺, Bi³⁺, Sc³⁺), (Gd,Ln)Al₃(BO₃)₄:Tb³⁺(Ln=Y³⁺, Bi³⁺), Gd_1-xEu_xBaB₉O₁₆(0.059O₁₆:Tb³⁺,Ce³⁺,Zn²⁺, GdBa₃B₉O_<sub>18:Eu³⁺ and GdBa₃B₉O₁₈:Tb³⁺powder samples was synthesized by the thermal decomposition of the corresponding nitrates and the solid-state reaction respectively. Their luminescent properties were investigated under ultraviolet (UV), vaccum ultraviolet (VUV) and cathode ray (CR) excitation. The results indicate that:VUV light excites majorly the host lattice of GdAl₃(BO₃)₄:Eu³⁺. On the one hand, the excitation energy can be absorbed by borates and AlO₆ groups, the charge transfer transition of O^2-→Gd³⁺ and 4f→5d transitions of Gd³⁺, the absorbed energy is transferred to Eu³⁺ by Gd³⁺. Eu³⁺ is relaxed to the ⁵D₀ level and transits to its ⁷F_J levels emitting a photon in the red visible spectral region. On the other hand, Eu³⁺ centers can be excited direct and then show red emission.The dominating emission peak of Gd_1-xEu_xAl-3(BO₃)₄(0.05≤x≤1) is all at about 613nm, and the quenching concentration is x=0.4,0.1 and 0.05 respectively under UV, VUV and CR excitation. Namely, the quenching concentration of phosphors increases with the increasing of the excitation energy. This result could be assigned to: on the one hand, the different energy transferring process under the three excitation energiges. On the other hand, it also could be caused by defects in the lattice acting as killers in the energy transfer process, which can be created by higher energy excitation. Under 254 and 147nm excitation, the emission intensity of Eu³⁺ in Gd_0.9-xLa_xEu_0.1Al₃(BO₃)4(0≤x≤0.9) keeps decreasing with the decreasing of Gd³⁺ concentration, which indicates that Gd³⁺ plays an important role. Y³⁺, Bi³⁺ and Sc³⁺ replace partly Gd³⁺ of Gd_0.9Eu_0.1Al₃(BO₃)₄ respctively, the luminescence of Eu³⁺ is sensitized under 254nm excitation. Howerver, under 147nm excitation, only Y³⁺ sensitizes the emission of Eu³⁺; and compared with the red commercial phosphor (Y,Gd)BO₃:Eu³⁺, the optimum phosphor Gd_0.3Y_0.6Eu_0.1Al₃(BO₃)₄ exhibits 92% the integrated emission intensity with chromaticity coordinates of (0.66, 0.33). This indicates that Gd_0.3Y_0.6Eu_0.1Al₃(BO₃)₄ is a very potential red VUV phosphor in practice.VUV light excites majorly the host lattice of GdAl₃(BO₃)₄:Tb³⁺. On the one hand, the excitation energy can be absorbed by borates and AlO₆ groups, the charge transfer transition of O^2-→Gd³⁺ and 4f→5d transitions of Gd³⁺, the absorbed energy is transferred to 4f5d levels of Tb³⁺ by Gd³⁺; then Tb³⁺ is relaxed to the ⁵D₄ level and transits to ⁷F_J levels emitting a photon in the green visible spectral region. On the other hand, Tb³⁺ centers can be excited and then give green emission.When excited by 254 and 147nm emission, the primary emission peak of Tb³⁺ in Gd_1-xTb_xAl₃(BO₃)₄(0.055D₄→⁷F₅ transition of Tb³⁺ decreases with the increasing of Tb³⁺ concentration under 147nm excitation. This can be interpreted as that: the increasing defect impurities are likely to produce a decrease of the Tb³⁺ decay time at Tb³⁺ higher concentration. Substituting partly Y³⁺ and Bi³⁺ for Gd³⁺ in Gd_0.5Tb_0.5Al₃(BO₃)₄ respctively, the luminescence of Tb³⁺ can be sensitized under 254nm excitation. Howerver, only Y³⁺ sensitizes the emission of Tb³⁺ under 147nm excitation, and compared with the green commercial phosphor Zn₂SiO₄:Mn²⁺, the optimum phosphor Gd_0.4Y_0.1Tb_0.5Al₃(BO₃)₄ exhibites 56% the integrated emission intensity with chromaticity coordinates of (0.33, 0.58) and the decay time 1.328ms. This indicates that Gd_0.3Y_0.6Eu_0.1Al₃(BO₃)₄ is a potential green VUV phosphor.Under 254 and 147nm excitation, the main emission peak of Eu³⁺ in Gd_1-xEu_xBaB₉O₁₆(0.05≤x≤0.3) is all located at around 613nm, the quenching concentration increases with the increasing of the excitation energy and is x=0.2 and 0.15 respectively. Compared with the red commercial phosphor (Y,Gd)BO₃:Eu³⁺, Gd_0.85Eu_0.15BaB₉O₁₆ exhibites 40% the integrated emission intensity, and owns excellent color with chromaticity coordinates of (0.66, 0.33) under 147nm excitation.Under 254 and 147nm excitation, the major emission peak of Tb³⁺ in Gd_1-xTb_xBaB₉O₁₆(0.05≤x≤0.35) is all at about 541nm; the quenching concentration is x=0.3, 0.25 respectively, which follows the rule that the quenching concentration of phosphors increases with the increasing of the excitation energy. The decay time of ⁵D₄→⁷F₅ transition of Tb³⁺ decreases with the increase of Tb³⁺ concentration under 147nm excitation. When partly substituting Ce³⁺ for Gd³⁺, Ce³⁺ sensitizes the emission of Tb³⁺ under 254nm excitation, but Ce³⁺ impairs the luminescence of Tb³⁺ under 147nm excitation. When Zn²⁺ partly replacing Ba²⁺, Zn²⁺ impairs the emission of Tb³⁺ under 254nm excitation, but sensitized the luminescence of Tb³⁺ excited by 147nm. Under 147nm excitation, compared with the green commercial phosphor Zn₂SiO₄:Mn²⁺, the optimum phosphor Gd_0.75Tb_0.25Ba_0.925Zn_0.075B₉O₁₆ exhibites 51% the integrated emission intensity with chromaticity coordinates of (0.33, 0.55) and the decay time 2.822ms. This indicates that Gd_0.75Tb_0.25Ba_0.925Zn_0.075B₉O₁₆ is a potential green VUV phosphor.The major emission peak of Gd_0.95Eu_0.05Ba₃B₉O₁₈ is all at about 592nm, and its emission intensity is very low both under 254 and 147nm excitation. Therefore, GdBa₃B₉O₁₈:Eu³⁺ is not a good red phosphor.When excited by 254 and 147nm excitation, the dominating emission peak of Gd_0.95Tb_0.05Ba₃B₉O₁₈ is all at about 541nm, but its emission intensity is very low. Therefore, GdBa₃B₉O₁₈:Tb³⁺ is not a good green phosphor.Under VUV excitation, in view of Eu³⁺, Tb³⁺-activated GdBaB₉O₁₆ and GdBa₃B₉O₁₈, their luminescent mechanism is similar to that of Eu³⁺, Tb³⁺-activated GdAl₃(BO₃)₄ respectively. The difference between them is that: the absorption band of the host contains also the absorption of Ba-O groups but not AlO₆ groups, in Eu³⁺, Tb³⁺-activated GdBaB₉O₁₆ and GdBa₃B₉O₁₈.