Luminescent Properties Of Rare-earth (Ce³⁺, Eu²⁺ Doped Strontium Phosphate Silicate Sr₅(PO₄)₂SiO₄

Rare-earth doped luminescent materials with excellent luminescent properties have been widely used in display, lighting, high-energy particle detection and recording, optoelectronic communications, agriculture and military fields. When Eu²⁺, Ce³⁺ ions are doped in a crystal, they exhibit characteristic wide 5dâ†'4f emission band. Therefore, the luminescence color can be tuned by different substrates from ultraviolet to infrared; fluorescence lifetime is short and temperature has a great impact on the emission spectra.Apatite [Ca₅(PO₄)₃X](X=F-,Cl-,OH-) has good chemical, thermal stability. In this kind of structure, there are many possible substitutions, for example, Ca cation can be replaced by other alkaline-earth metals, transition metals or rare-earth elements and PO₄ can be partly substituted by SiO₄. Thus apatite [Ca₅(PO₄)₃X] has many variations, which could present different luminescent properties or phenomena. So RE ions doped hosts with apatite structure have become important photoluminescence materials. In this paper, Eu²⁺, Ce³⁺ doped Sr5(PO₄)₂SiO₄ phosphors were synthesized and impacts of cations on structure and luminescent properties were also studied by substituting Sr²⁺ with Ca²⁺ and Ba²⁺.In chapter three, Ce³⁺-doped Sr5(PO₄)₂SiO₄ phosphors were synthesized by high temperature solid sintering method. XRD results show that the doping does not affect the structure, which remains space group P63/m. The excitation spectrum has a very broad absorption from 240-400nm, which agrees with the emission light of near UV-LED chips. The emission spectrum shows characteristic double peaks: 410nm and 438nm, corresponding to 5d1â†'2F5/2 and 5d1â†'2F7/2 transitions of Ce³⁺. The luminescence intensity reaches the maxium when the doping concentration becomes 3%. Lifetime is calculated through decay curves. From the chromaticity coordinate graph, it is found that Ce³⁺-doped Sr5(PO₄)₂SiO₄ phosphors emit lights within blue zone and chromaticity coordinates do not change much with the increasing doping concentration.In chapter four, Sr_5-5xEu_5x(PO₄)₂SiO₄(x=0.005,0.05,0.07,0.1,0.13,0.15) phosphors were synthesized by high temperature solid state method. XRD analysis shows that when the doping concentration is beyond 0.1, new phase Sr2P2O7 emerges. When the concentration is between 0.005 and 0.1, there is a wide band from 400nm to 700nm, corresponding to 4f65d1â†'4f7(8S7/2) transition of Eu²⁺. The excitation spectrum present a broad absorption from 300nm to 450nm, which can match both UV-LED and blue LED chips. Chromaticity coordinates diagram shows that when the doping concentration increases from 0.005 to 0.1, luminous color changes from yellow-green(0.387,0.577) to yellow(0.471,0.512); Sr_5-5xEu_5x(PO₄)₂SiO₄(x=0.13) and Sr_5-5xEu_5x(PO₄)₂SiO₄(x=0.15) show white (0.289,0.291) and blue (0.212,0.204) color, respectively. In addition, the study of the impact of temperature on the luminescence indicates that there is energy transition between the two cation positions.In chapter five, Eu²⁺-doped Sr4.75-5xCa5xEu0.25(PO₄)₂SiO₄ (x=0.01, 0.05, 0.1, 0.15, 0.30, 0.50, 0.75, 1) phosphors were synthesized and the impacts of substituting Sr²⁺ with Ca²⁺ on structure and luminescent properties are explored. As the concentration increases, the structure of the phosphor changes from P63/m to Pnma(62) and the color changes from yellow to blue-green light and then turn back to yellow again. When x is between 0.50 and 1.0, a new phase Ca5(PO₄)₂SiO₄ forms, and with the increasing doping concentration, long wavelength peak moves from 500nm to 520nm. The emission spectrum of Ca5Eu0.25(PO₄)₂SiO₄ indicates that there are three different cations emitting position. By studying the thermal stability of Sr4.75-5xCa5xEu0.25(PO₄)₂SiO₄ (x=0.15), it is found that when the temperature reaches 150℃, the strength drops to 86%.In the sixth chapter, Sr_4.75-5yBa_5yEu_0.25(PO₄)₂SiO₄ (y=0.01,0.05,0.1,0.15,0.30,0.50) series phosphors were synthesized by high temperature solid state method. Doped with Ba²⁺, the matrix structure did not change, but lattice parameters decreased slightly. As the doping concentration changes from 0.01 to 0.50, the peak position moves from 570nm to 504nm and color gradually changes from yellow to green. There is a wide absorption peak from 250nm to 450nm, as the concentration changed from 0.01 to 0.50, the excitation peak move gradually from 390nm to 347nm.The novelities of this dissertation are the following: it is the first time to explore Ce³⁺, Eu²⁺-doped Sr5(PO₄)₂SiO₄ with apatite structure; the results show that this phosphor can be excited by near ultraviolet light, which matches well with UV-LED chips, and it is promising in white LED phosphor applications. The study of Ca²⁺, Ba²⁺ substituting Sr²⁺ in the Eu²⁺-doped phosphors shows that the color tuneable phosphors from yellow to green can be synthesized.