| White Light-emitting diode(WLED)is widely used due to its superior properties such as small size,low heat generation,energy-saving and long service life.It is the fourth generation after incandescent lamps,fluorescent lamps and gas discharge lamps.Currently,the most widely used method to produce white light is to combine InGaN blue emitting LED chip and yellow phosphor Y3Al5O12:Ce3+(YAG:Ce3+).However,this yellow and blue combination lacks of red light component and leads to a cool white light performance with a low color rendering index and a high correlated color temperature.This limitation restricts the application of LEDs on many devices.In order to resolve this problem and obtain a warm white light output,it is necessary to compensate the red phosphors in the WLEDs devices rather than a single yellow phosphor.Currently,the transition metal ion Mn4+-activated red phosphors have attracted much interest because of their simple synthesis strategy and excellent luminescence properties.The phosphor can be effectively excited by blue light to obtain ideal narrow-band red light emission.It can be applied to WLED devices with blue LED chips as an excitation light source to obtain the warm white light with high color rendering index,which has a good application prospect.In this paper,several Mn4+activated K3TaO2F4 and BMF7(B=Ba,Na;M=Ta,Nb)red phosphors were prepared.The crystal structure,morphology,luminescent properties and electronic band structure of these phosphors were investigated in detail using X-ray single crystal diffraction,X-ray powder diffraction(XRD),scanning electron microscopy(SEM),photo-luminescent spectra and DFT calculation.The main contents of the paper are summarized as follows:1.K3TaO2F4 was prepared by solid state method,and Mn4+activated K3TaO2F4 fluoride red phosphor was prepared with HF atmosphere.The structure and luminescence properties of K3TaO2F4:Mn4+phosphors were investigated and discussed in detail.The real structure of K3TaO2F4 was carefully studied through X-ray powder diffraction data Rietveld refinement.K3TaO2F4.The real crystal structure of oxyfluoride host K3TaO2F4 shows tetragonal structure with space group I4/mmm(No.139),Z=2 and the lattice parameters are a=6.3057(4)A,c=8.9173(6)A,and V=354.57(5)A3.The observed and calculated XRD patterns of K3TaO2F4 as well as difference profile are illustrated with residual factors of RwP=6.48%,RP=4.73%,and GOF=1.28.Under ultraviolet(UV)and blue light excitation,K3TaO2F4:Mn4+phosphor emits a narrow band red emission peaking at 630 nn and the color purity is calculated to be about 95%.The results demonstrate that Mn4+-doped K3TaO2F4 would be a promising red phosphor for White LED applications.2.Red fluoride phosphor BaNbF7:Mn4+and BaTaF7:Mn4+were prepared by the traditional co-precipitation method.Based on single crystal diffraction data,BaNbF7 shows the cubic structure with the space group Pa 3(No.205)and the lattice parameters are a=9.8855(2)A,V=966.0(5)A3,Z=8.0.The structure of BaNbF7 is similar to that of BaTaF7,and the corresponding lattice parameters are a=9.9009(3)A,V=970.56 A3,Z=8.0.The morphology of the two red phosphors is a regular cube.Under ultraviolet and blue excitation,BaNbF7:Mn4+and BaTaF7:Mn4+red phosphors show strong zero phonon(ZPL)at-630 nm,attributed to the highly distorted C2v symmetry of the Mn4+octahedral coordination environment.The as-prepared red phosphors,commercial yellow phosphor YAG:Ce3+and blue GaN chip were used to fabricate White LED and the results imply that the BaNbF7:Mn4+and BaTaF7:Mn4+red phosphor have potential applications in warm White LED.3.Na2TaF7 was prepared by evaporation crystallization method.The Mn4+-activated Na2TaF7 fluoride red phosphor was prepared by HF atmosphere.The phosphor exhibited a narrow-band spectrum near 630 nm under the excitation of 460 nm.With the increase of Mn4+concentration,the luminescence intensity reached the maximum when the doping concentration reached 4.0 mol.%,and then the fluorescence intensity gradually decreased due to the concentration quenching. |
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