Syntheses,Characterization And Energy Transfer Of Novel Rare Earth Borate Luminescent Materials

Compared with current incandescent and fluorescent lamps,white light-emitting diode（W-LED）have the advantages of high efficiency,no pollution,low power consumption and durability.However,disadvantages of high cost and low efficiency when excited by UV/near-UV LED chips have limited the development of W-LEDs.At the same time,the urgent demand for luminescent color-tunable phosphors has stimulated extensive research on phosphors.Tb³⁺is well-known as an activator for blue-green emission,Bi³⁺is representative non-rare-earth activator ion of blue emission,while Eu³⁺plays a crucial role in commercial red phosphors.When Tb³⁺and Eu³⁺(Bi³⁺and Eu³⁺)are co-doped in a matrix,efficient energy transfer from Tb³⁺to Eu³⁺(Bi³⁺to Eu³⁺)can occur,which is of great significance for the study of high-efficiency phosphors.In this thesis,a variety of novel rare earth borates with single crystal morphology and their phosphors were synthesized,and their fluorescence properties were explored in detail.The phase purity and crystal structure type of the samples were determined by PXRD and single crystal XRD.In addition,the types of anionic groups in the crystals were determined by laser confocal Raman spectroscopy,Fourier transform infrared spectroscopy,and their optical energy gaps were estimated by ultraviolet-visible diffuse reflectance absorption spectroscopy.First,single crystals of a new europium-containing borate Na_3-3δEu_2+δ（BO₃）₃（δ=0.16）were prepared.The compound crystallizes in the orthorhombic space group Amm2 with a=5.08902（11）?,b=10.9754（2）?,c=7.01996（14）?,V=392.094（14）?³,and Z=2.It features a three-dimensional framework constructed by interconnecting Na O₆,Eu O₉,（Na/Eu）O₈ polyhedra and BO₃ planar triangles.Furthermore,IR,Raman,XPS,and UV-vis absorption spectra as well as photoluminescence properties of this compound were studied.Under near-UV excitation（396 nm）,Na_3-3δEu2+δ（BO₃）₃（δ=0.16）exhibits an intense reddish-orange emission centered at around 614 nm with the CIE coordinates of（0.637,0.363）,which can be assigned to the electric dipole ⁵D₀→⁷F₂transition of Eu³⁺ions.The phosphor shows the stable luminescence at elevated temperature.Secondly,two new rare-earth borates Ba₂Cd Ln₂（BO₃）₄（Ln=Tb,Eu）were prepared by the high temperature melt solution method.The single-crystal XRD analyses show that they crystallized in the monoclinic space group P2₁/c and the crystal structures consisted of 3D networks constructed by Cd O₅ square pyramids（Cd O₆mono-capped square pyramids）,Tb（Eu）O_n（n=7,8）polyhedra and BO₃ planar triangles,with open channels accommodating Ba²⁺cations.The IR and Raman spectra demonstrate the presence of BO₃ groups in the structure,and UV-vis absorption spectra show characteristic absorption bands generated by unpaired 4f electrons for Tb³⁺and Eu³⁺ions.In addition,solid solutions Ba₂Cd Tb_2-2xEu_2x（BO₃）₄（0≤x≤0.2）were prepared via the high temperature solid-state reaction method.By doping Eu³⁺into the Ba2Cd Tb2（BO3）4 matrix and varying the doping concentration of Eu³⁺,tunable colors from green to orange and eventually to red were obtained in the Ba2Cd Tb2-2x Eu2x（BO3）4phosphors under the excitation at 377 nm.The energy transfer process from Tb³⁺to Eu³⁺was verified through luminescence spectra and fluorescence decay curves.Finally,a new rare-earth oxyborate Ba₃Bi Pb Eu O（BO₃）₄ was prepared and its crystal structure data:P63/mmc,a=5.4305（1）?,c=26.3501（3）?,V=672.96（3）?³and Z=2.It is a layer material containing two types of layers:one formed by corner-sharing Eu O₆ octahedra and BO₃ triangles and another composed of corner-or edge-sharing（Bi/Pb）O₇ hexagonal pyramids and BO₃ triangles.Furthermore,the phase composition,morphology,IR,Raman,XPS,and UV–vis absorption spectra of this compound were studied.In addition,solid solutions of Ba₃Bi Pb Y_1-xEu_xO（BO₃）₄（0≤x≤1）were prepared by solid-state reaction method and the luminescent properties were investigated.Ba₃Bi Pb YO（BO₃）₄ and Ba₃Bi Pb Eu O（BO₃）₄ show blue and reddish-orange color of luminescence when excited at 242 and 394 nm,respectively.By doping Eu³⁺into the Ba3Bi Pb YO（BO3）4 host and changing the Eu³⁺doping concentration,tunable emission colors from blue to orange-red were obtained in Ba3Bi Pb Y1-xEux O（BO3）4 phosphors under the 331 nm excitation.