Preparation And Characterization Of Luminescent Properties Of Rare Earth Doped Molybdate And Antimonate Phosphors

As the global energy and environmental problems continue to develop,people’s interest in the development of new energy and energy saving technology is also deepening.In the field of lighting,white LED due to its low energy consumption,high energy conversion rate,long service life and enviro nmental friendly,such as excellent performance and replace the traditional incandescent lamp status,become the first target of the global lighting devices,thus got the attention of many researchers,and as an essential ingredient of the preparation of white LED phosphors,the study of its never stop scripts.The luminescent properties of inorganic luminescent materials are restricted by many factors,including crystal field environment,energy transfer efficiency of substrate and concentration of luminescent center.Molybdate and antimonate has stable chemical stability and structure stability,abundant raw materials,cheap,and centered on Mo⁶⁺ions formed（Mo O4）^2-tetrahedron structure stability,and has strong absorption in the uv region,can effectively transfer the energy to luminescent centers,suggesting that contain（Mo O₄）^2-structure of molybdate is good for phosphor matrix material;Antimony ion has a variety of valence states and can combine with multiple oxygen atoms to form different structures,providing a diverse crystal field environment,which is also suitable for phosphor matrix.Among other structures,ABO₃,a universal perovskite structure,has strong structural and chemical stability that makes it outstanding in the field of luminescence.In this paper,Li KGd₂（Mo O₄）₄:Dy³⁺/Sm³⁺phosphors and Na Sr Gd（Mo O₄）₃:Er³⁺/Er³⁺,Yb³⁺phosphors with（Mo O₄）^2-structure,as well as Sr₂Gd Sb O₆:Eu³⁺and Sr₂Zn Mo O₆:Eu³⁺phosphors with double perovskite structure were prepared by high temperature solid phase method,and their XRD,diffuse reflectance spectra,fluorescence spectra and heat quenching spectra were characterized.The conclusions are as follows:（1）Fluorescence powders with Dy³⁺and Sm³⁺ions doped by Li KGd₂（Mo O₄）₄ as the matrix were prepared by high temperature solid phase method.Li KGd₂（Mo O₄）₄:Dy³⁺phosphors glow yellow under 353 nm excitation light,and the optimal doping concentration is 12%mol.Li KGd₂（Mo O₄）₄:Sm³⁺phosphors glow orange-red under 405 nm excitation light,and the optimal doping concentration is 6%mol.According to Dexter’s energy transfer theory,the energy transfer mode between rare earth ions in the matrix is dipole-dipole interaction.As doping concentration increased,Li KGd₂（Mo O₄）₄:Dy³⁺phosphor fluorescent lifetime from200.99μs to 113.03μs,Li KGd₂（Mo O₄）₄:Sm³⁺phosphors from 607.58μs to 401.68μs life,which attributed to the increase of the luminescence center reduce the distance of the electron transition.The temperature characteristics of Li KGd₂（Mo O₄）₄:Dy³⁺/Sm³⁺phosphors were characterized,and the thermal quenching effect was observed.The activation energies of Li KGd₂（Mo O₄）₄:Dy³⁺/Sm³⁺phosphors were calculated as 0.302e V and 0.236e V.（2）The Sr₂Gd Sb O₆:Eu³⁺red phosphor with double perovskite structure was successfully prepared by high temperature solid phase method and its XRD,photoluminescence spectrum and fluorescence decay lifetime were characterized.The XRD of the Sr₂Gd_0.8Sb O₆:0.2Eu³⁺phosphor is basically consistent with the peak position of the standard card,indicating that the doping of Eu³⁺did not change the crystal structure of the matrix.The crystal parameters after Rietveld refinement also verify the result.The photoemission spectrum shows that the sample has a wide excitation area centered at 340 nm in the ultraviolet region;under the excitation of a 340 nm light source,the strongest emission of the sample is an orange-red emission band with a center wavelength of 593 nm.Using Dexter theory to analyze the concentration quenching mechanism of the sample is the dipole-dipole interaction.The fluorescence decay life of Sr₂Gd_0.8Sb O₆:0.2Eu³⁺phosphors at room temperature is 4.202ms,and the external quantum efficiency is 24.8%.When the temperature is raised to 150℃,the external quantum efficiency remains 68.8%of that at room temperature.The calculated color purity of Sr₂Gd Sb O₆:Eu³⁺phosphor can reach 99.5%.Sr₂Gd_0.8Sb O₆:0.2Eu³⁺phosphor,blue phosphor,green phosphor and 365 nm UV chip package were made into LED devices which can emit bright white light.These results indicate that Sr₂Gd Sb O₆:Eu³⁺phosphor is a potential red phosphor for white LED.（3）The Eu³⁺doped Sr₂Zn Mo O₆ phosphors were prepared by high temperature solid phase method.In the matrix,the radius of Sr²⁺and Zn²⁺ions is similar to that of Eu³⁺ions,so it is necessary to further analyze the site of which ion in the matrix is replaced by Eu³⁺ions.The excitation and emission spectra of Sr₂Zn Mo O₆:Eu³⁺phosphors were characterized.It was found that the emission spectra of materials under different excitation were different,indicating that Eu³⁺ions occupied two different sites in the crystal lattice.Further analysis showed that Eu³⁺ions mainly occupied the asymmetric sites of Sr²⁺ions in the crystal lattice.The emission intensity of the material can be increased by doping different charge compensators,but the enhancement effect is different with the location,and the enhancement of the two emission points by Na⁺ion is more balanced.Therefore,the influence of doping concentration on the emission spectrum of the material is studied by using Na⁺ion as charge compensator,and the energy transfer mechanism of the material is analyzed.（4）The phosphors with Na Sr Gd（Mo O₄）₃ as the substrate were mono-doped with Er³⁺and co-doped with Er³⁺and Yb³⁺.In the matrix,the charge and ion radius of Gd³⁺ions are the closest to those of doped ions.Therefore,it can be judged that Er³⁺and Yb³⁺ions mainly replace the positions of Gd³⁺ions in the crystal lattice in the process of substitution.The XRD standard PDF card and crystal structure of Na Sr Gd（Mo O₄）₃ were not included in the database,so the XRD comparison lines were calculated by using Vesta software based on the crystal structure of Na Sr La（Mo O₄）₃,and the results showed that the material was successfully prepared.The down-conversion emission shows that the optimal doping concentration of Er³⁺ion is 15%mol,and the doping of Yb³⁺ion has little effect on down-conversion emission.The up-conversion emission spectrum shows that the doping of Yb³⁺can greatly improve the up-conversion emission of the material.The thermal stability of Na Sr Gd_0.65（Mo O₄）₃:0.15Er³⁺,0.20Yb³⁺phosphors was characterized.It was found that the thermal quenching behavior of Na Sr Gd_0.65（Mo O₄）₃:0.15Er³⁺,0.20Yb³⁺phosphors did not conform to the conventional thermal quenching behavior.It was speculated that the phase transition occurred during the heating process.