Preparation And Photoluminescence Properties Of Tungstate Based Red Luminescent Materials

Red phosphors can reduce the color temperature and improve the color rendering index.New red-emitting phosphors SrZn(WO4)2:Eu3+and Ba2ZnMO6:RE3+(M=W,Mo;RE=Eu3+,Sm3+)were prepared by co-precipitation and high-temperature solid-state method,respective-ly.The crystal structure and micrographs of the simples were investigated using X-ray diffraction(XRD),infrared spectroscopy(IR),scanning electron microscope(SEM)and fluorescence spectroscopy.In addition,the photoluminescence properties,the mechanism of energy transfers,the mechanism of concentration quenching,CIE coordinates and the thermal stability of the phosphors were discussed in detail.The red luminescent material SrZn(WO4)2:Eu3+,which can be effectively excited by the near ultraviolet light and the blue light were prepared by the co-precipitation method.The X-ray diffraction peaks of the obtained samples are exactly the same as the diffraction peaks of the standard cards JCPDS 08-0490 and JCPDS 15-0774,indicating that the doping of rare earth ions does not change the lattice structure of the matrix.The main excitation peak of the sample is located at 394 nm and 465 nm,belonging to the f-f transition of Eu3+ ions.Excited by near-ultraviolet light(394 nm)or blue light(465 nm),the main emission peak of the sample is located at 616 nm,which belongs to the 5D0→F2 transition of Eu3+ ions and it emits intense red light.The doping concentration of Eu3+ has a significant effect on the luminescence intensity of the sample.When the content of Eu3+ ions is 7mol%,the red emission intensity of the samples is the strongest.Using Eu3+ ions as activator,Li+ ions as charge compensator,the novel double perovskite structure red phosphors Ba2ZnWO6:Eu3+,Li+were synthesized by high-temperature solid state method.The resulting sample has a cubic crystal structure and the sample particles are octahedron-shaped with a size of 0.2-3 μm.The sample can be effectively excited by near ultraviolet light(250-400 nm),and Li+ions doping can effectively increase the red emission intensity of the sample.There are many excitation peaks appear over the wide range of excitation spectra from 250-550 nm,including a broad excitation peak from 250-350 nm centered at 316 nm that corresponds to the charge transfer transition(O2-→Eu3+ and O2-→W6+),as well as the characteristic excitation peaks of Eu3+ ions.At the excitation wavelength of 315 nm,the samples emit intense red light,the main emission peak of the sample at 598nm,which belongs to the 5D0→7F1 transition of the Eu3+ ions.When the content of Eu3+ ions is 10 mol%,the red emission intensity of the samples is the strongest and its CIE coordinates is(0.6330,0.3660).The double perovskite structure orange-red phosphors Ba2ZnWO6:Sm3+ were prepared by high-temperature solid state method.The resulting sample has a cubic crystal structure and the sample particles are irregular polyhedral-shaped with a size of 1-3μm.The phosphors can be excited by near-ultraviolet light,and the characteristic emissions Sm3+ ions are observed at 564 nm,598 nm and 645 nm,corresponding to 4G5/2→HJ(J=5/2,7/2 and 9/2)transitions.The optimal doping concentration of Sm3+ ions in Ba2ZnWO6 is about 5 mol%and its CIE coordinates is(0.5570,0.4340).According to the Dexter theory it is mainly due to dipole-dipole interactions.Based on the Arrhenius equation,the thermal activation energy of the sample is 0.238 eV,indicating that it has good thermal stability.A series of red phosphors Ba2ZnW1-xMoxO6:Eu3+,Li+ were prepared by high-temperature solid state method.The resulting sample has a cubic crystal structure with a size of 3-5 μm.With the W6+ ions is substituted by Mo6+ ions,the fluorescence excitation wavelength has red-shifted,and the largest wavelength is changed from 316 to 373 nm,which made the phosphors can be excited more efficiently by near-ultraviolet light(350-420 nm).Under the excitation wavelength of 373 nm,Ba2ZnW0.6Mo0.4O6:Eu3+,Li+ have the strongest emission intensity.Although the emission intensity varies with the Mo content,the 5D0→7F1 emission is dominated in all the spectra.The CIE chromaticity coordinates of Ba2ZnW0.6Mo0.4O6:Eu3+,Li+(0.6385,0.3611),is closer to the standard coordinates of red phosphor.Based on the Arrhenius equation,the thermal activation energy of the sample is 0.253e V,indicating that it has good thermal stability.