Synthesis And Performance Study Of Molybdate Luminescent Materials

Thermal stability、refractive index、afterglow property and other characteristics,scheelite structured molybdate were used in solid scintillator,medicine,high energy physics and other applications.Therefore molybdate phosphors have received extensive attention in ecent years.In the present work,series of rare earth doped molybdate phosphors were successfully prepared by soild-state reaction technique at high temperature.Systematical study on optimization of preparation technology,the structure,morphology,infrared spectroscopy,luminescence property and afterglow property,First principles study was used to calculate the band structure、density of states and opticalproperty of the molybdate.The main research of this paper are as follows:1、SrMoO₄:Eu³⁺,Dy³⁺phosphors were successfully prepared via the conventinal soild-state reaction method,and the process conditions were studied in this sysytem,then the optimum condition was calcination at 850℃for 6 hour.The reaction temperature of phosphors、morpholgy and structure、photoluminescence luminescence properties are studied.The results indicate that the SrMoO4 was pure phase.It has a tetragonal crystal structure with space group of I4₁/a（88）.SrMoO₄:Eu³⁺phosphors can be excited by 394 nm and 464 nm lights.The emission spectra exhibit the most intense emission at 614 nm,especially the later which is corresponding to the ⁵D₀?⁷F₂ transition of the Eu³⁺.As regards the influence of Eu³⁺concentration on the luminescent property,the highest emission intensity on the 614 nm is exhibited when x is 15 mol%.Compared with the commercially available red phosphor Y₂O₃:Eu³⁺,the emission intensity of these phosphors is much stronger than that of Y₂O₃:Eu³⁺.Moreover,the CIE chromaticity coordinates are closer to the National Television Standard Committee stanf CIE chromaticity coordinate values for red.The temperature dependence of SrMoO₄:Eu³⁺phospors with concentrations is 15mol%under the excitation of 394 nm,and the temperature dependence in the emission of Eu³⁺was observed.Indicated that the excited spectra have intense excitation at(⁶H_15/2-⁶P_7/2)352 nm and(⁶H_15/2?⁴I_13/2)388 nm,and meanwhile the emission spectra exhibited the most intense emission at(⁴F_9/2?⁶H_15/2)485 nmand(⁴F_9/2?⁶H_13/2)573 nm,especially the later which is corresponding to the ⁴F_9/2?⁶H_15/2transition of the Dy³⁺.From the excitation spectra we can see that the f-ftransitions intensities monotonously is increasing with the increasing of the concentration,the intensity of excitation peaks monitored at 573 nm reaches the maximum when xequals to 2 mol%.The concentration quenching will occur when the concentration of Dy³⁺is beyond 2 mol%.According to the theory of Dexter,the electric dipole-dipole（d–d）interactionmechanism is dominant by concentrationquenching of⁴F_9/2-⁶H_13/2transition of Dy³⁺ions in Sr_1-xMoO₄:Dy³⁺phosphors.The temperature dependence of SrMoO4:Dy³⁺phospors with concentrations is 2 mol%under the excitation of 352 nm,and the temperature dependence in the emission of Dy³⁺was observed2、The Eu³⁺and Dy³⁺co-doped strontium molybdenum phosphors were prepared by soild-state reaction method.The emission colors of SrMoO₄:Eu³⁺,Dy³⁺could be turned from red to white through tuning the energy transfer.And their energy transfer.efficiency increases gradually with increasing the Eu³⁺doped concentration.The chromaticity coorfinate when the relative ration of Eu³⁺/Dy³⁺is much closer to the standard chromaticity coordinate for white.3、AseriesofMSrMoO₄:0.15Eu³⁺0.07R⁺(M=Ca²⁺,Ba²⁺;R⁺=Li⁺,Na⁺,K⁺)phosphors were successfully prepared via the conventinal soild-state reaction method.（a）.The effect of the Ca²⁺doped concentration in the SrCaMoO₄:0.15Eu³⁺phosphors on the relativephotoluminescent intensity is improved.（b）.For the fist time,Si atoms eplaced part of the location of Mo to achieve the pupuse of impoving luminous intehsity in SrMoO₄:0.15Eu³⁺.The results show that the addition of Si does not affect the shapes of excitation and emission spectra and the position of emission peak.It can also be seen that the optimal doping concentration of Ca²⁺is 0.25 with the strongest luminous intensity.the optimal doping concentration of SiO₂ in the Sr_0.6Ca_0.25MoO₄:0.15Eu³⁺is determined to be 5 mol%.The emission spectra intensity decreaseswhen the Si⁴⁺concentration exceeds the criticalconcentration.Thestrongestemissionintensityofthe Sr_0.6Ca_0.25(Mo_0.95Si_0.05)O₄:Eu³⁺_0.15phosphor is remarkably enhanced by a factor of 1.2with comparison to that of the Sr_0.6Ca_0.25MoO₄:0.15Eu³⁺.（c）.The results show that adding appropriate amount of Ba²⁺in the SrMoO₄:0.15Eu³⁺can not only increase the luminescence intensity of phosphor but also improve the color purity.The optimum doping concentration of Ba²⁺was found to be 25 mol%.4、With alkali carbonates as charge compensators,the improvement of luminescent properties was studied and a good result had got.The results are as follow:Ca_0.55Sr_0.25MoO₄:0.15Eu³⁺and Ba_0.55Sr_0.25MoO₄:0.15Eu³⁺the charge compensating model is 2Sr²⁺/Ca²⁺→Eu³⁺+R⁺（R⁺=Li⁺,Na⁺,K⁺）,2Sr²⁺/Ba²⁺→Eu³⁺+R⁺（R⁺=Li,Na,K）with Li₂CO₃、Na₂CO₃ and K₂CO₃ as charge compensators.When the molar concentrations of the charge compensators are 0.07,respectively,the luminescent intensity（614 nm）of the Ca_0.55Sr_0.25MoO₄:0.15Eu³⁺Na_x⁺prepared at 900℃for 5h is the best.The optimized charge compensators is Na⁺.When the molar concentrations of the charge compensators are 7 mol%Li⁺,the Ba_0.55Sr_0.25MoO₄:0.15Eu³⁺Li⁺luminescent intensity.5、Eu³⁺,Tb³⁺doped and Eu³⁺/Tb³⁺co-doped double molybdates NaLa（MoO₄）₂phosphors with tunable color have beenprepared via soild-state reaction method.Concrete research content including following several aspects:（a）.The optimum doped concentration of Eu³⁺in NaLa（MoO₄）₂ is 30 mol%luminescent intensity is best.The critical distance of energy transfer among Eu³⁺ions was calculated to be R_c 10?according to the distance formula of critical non-adiative energy transfer.When the Na La（Mo O₄）₂ phosphors with Eu³⁺doping concentration are excited by a mercury lamp（254 nm）,red afterglow phenomenon can be observed.Its luminous intensity was enhanced through the introduction of tungstate radical,the best doping amount of WO₄^2-was 1,namely molar ratio of MoO₄^2-/WO₄^2-was 1:1.The series phosphor can be excited by near-UV（393 nm）and blue（462nm）,the strongest emission peak is located at 615 nm corresoonding to the ⁵D₀?⁷F₂ transition of Eu³⁺ion.When the Na La（Mo O₄）（WO₄）phosphors with Eu³⁺doping concentration are excited by a mercury lamp（254 nm）,red afterglow phenomenon can be observed.（b）.The optimum doped concentration of Tb³⁺in NaLa（Mo O₄）₂ is 30 mol%show strong green emission.Based no Dexter’s formula of multipolar interactin,the dominant concentration quenching mechanisms for Tb³⁺in NaLa（MoO₄）₂ host is dipole-dipole interaction.When the NaLa（MoO₄）₂ phosphors with Tb³⁺doping concentration are excited by a mercury lamp（254 nm）,green afterglow phenomenon can be observed.（c）.The Eu³⁺/Tb³⁺co-doped sample shows the characteristic emissions of Eu³⁺andTb³⁺under288nmexcitation.Theemissioncolorsof Na La（Mo O₄）₂:xEu³⁺,0.3Tb³⁺could be tuned from green to red through tuning the energy transfer.And their energy transfer efficiency increase gradually with increasing the Eu³⁺doped concentration.When Eu³⁺concentration is30 mol%with an energy transfer efficiency high as 94%.The energy transfer medchanism is calculated to be dipole-dipole interaction according to Dexter’s theory and Reisfeld’s approximation.The emitting color was tuned by excitation wavelength,the sample present multicolor emission,the luminescence color can change,the sample emits red orange light with the excitation wavelength in the 288 nm,emits white light the excitation wavelength in the 375 nm,emits red light with the excitation wavelength in the 394 nm,emits yellow green light with the excitation wavelength in the 486 nm.The results shows that the prepared series of color tunable phosophors,which is suitable for the needs of different special situations of white LED.6、Yellow emitting phosphors SrLaMoO₄:Dy³⁺were successfully synthesizedthrough soild-state reaction method.The optical properties and luminescence mechanism of SrLaMoO₄:Dy³⁺yellow phosphors were systematically studied.The results included the following:（a）.we had synthesized the Sr LaMoO₄:Dy³⁺phosphors and investigated the effects of La³⁺doping concentration and luminescence properties.the luminescence intensity is decreasingwhenLa³⁺concentration is higher than 3 mol%,indicating that an opportune amount of La³⁺can enhance the emission intensity.The CIE chromaticity coordinates for Sr LaMoO₄:0.3Dy³⁺phosphor were located in the yellow region,the CCT value 4192 K.Applied to investigate the temperature dependence of Sr LaMoO₄:0.3Dy³⁺phosphors under the excitation of 352 nm,and the temperature quenching in the emission was observed.The temperature quenching was well explained using the Arrhennius model.Calculated the temperature quenching activation energy is?E=0.152eV。（b）.Then we theorticatically calculated the band structure,densiy of states and optical property by the first principle method.The computed band-gap width of SrMoO₄is 4.389 eV,the calculated static optical dielectric constant of SrMoO₄ is 3.22 and refractive index is1.79;The computed results shows that the band-gap is 0.856 eV and the SrMoO₄ has larger band gap than the SrMo O₄:Dy³⁺,the SrMoO₄:Dy³⁺sample 4f levels of Dy³⁺are-1.01eV-2.33 eV,the calculated static optical dielectric constant of SrMoO₄:Dy³⁺is 3.76 and refractive index is 1.93;the static optical dielectric constantε₂（ω）is proportional to extinction coefficient K（ω）when the calculated static optical dielectric constant of SrLaMoO₄:Dy³⁺unequal to zero.