Study Of Eu³⁺ Luminescence Thermal Quenching Behaviours In Molybdate-tungstate Materials

White light emitting diodes?WLEDs?are considered as the next generation illumination light source for their advantages of long working lifetime,robustness,high emission efficiency and environmental friendliness.Since the near-ultraviolet excitable WLEDs?NUV-WLED?have advantages of high colour rendering index,tunable colour temperature and superior colour uniformity,it can be widely applied in the field of illumination,displayer and street signs.The Y₂O₂S:Eu³⁺phosphor is currently the common commercial phosphor as the red component for NUV-LED.But this phosphor suffers from the drawbacks of poor absorbance in NUV region,toxic,chemical instability and short working lifetime.Eu³⁺doped tungstate and molybdate have advantages of efficient red emission,nontoxic and good chemical stability,thus these phosphors are promising red component.The excitation of CTS of WO₄^2-and WO₆^6-groups can only cover the range of 200-350 nm.The excitation can cover250-425 nm in Eu³⁺doped molybdate phosphors with MoO₆^6-groups in host matrix,which matches well with the emission of GaN LED chip.But the CTS of MoO₆^6-groups in pure molybdate phosphors usually suffer from concentration quenching.Thus,the molybdate-tungstate solid solutions are generally used as host matrix to reduce such quenching.But most of the researches on such phosphors are focused on the room temperature luminescence.The working temperature of LED devices is almost above 370 K,which requires good luminescent thermal stability for the phosphors.Thus,it is still necessary to study the thermal quenching performance of tungstate-molybdate solid solution phosphors.Nowadays,the problem to be solved is as follows:it seems that according to the luminescent thermal quenching mechanism,the good luminescent thermal stability and the good absorption of MoO₆^6-groups cannot be obtained simultaneously.In this work,we intend to find out the luminescent thermal quenching mechanism in tungstate-molybdate solid solution phosphors by studying the luminescent thermal quenching of Eu³⁺in lattice with different symmetry and host matrix with different structure of acid radical framework.We hope that the investigate results can offer technical reserve and theoretical direction for finding red phosphors applicable to NUV-WLEDs.This dissertation is composed of six chapters.In chapter 1,the methods of obtaining WLEDs and the phosphors applicable to WLEDs are introduced.The research progress of tungstate-molybdate solid solution phosphors has been reviewed.The possible luminescent thermal quenching mechanisms are also presented.In chapter 2,the synthesis and measurement methods are briefly introduced.In chapter 3,the distortion of Eu³⁺doped Ca₃?Mo/W?O₆ with structure of three dimension net acid radical framework are investigated by using of temperature-dependent Raman spectra.And we correlate the distortion and luminescent thermal quenching.It is also found that the luminescent thermal stability becomes worse with the introduction of Mo atom.The temperature-dependent Raman spectra for Ca₃?Mo/W?O₆ are analyzed.And the distortion is found to be more severe in solid solution host matrix at high temperature,which may be a reason for the worse luminescent thermal stability in Mo contained solid solution phosphors.In chapter 4,the Eu³⁺doped Sr₂Ca?Mo/W?O₆ with similar structure are synthesized.By investigating the temperature-dependent emission spectra and decay curves,the emissions of593 nm and 612 nm are analyzed.The contribution of Eu³⁺at A and B sites to these emission peaks are confirmed.It is also found that the luminescent thermal quenching is more severe for Eu³⁺at B site.By observing the non-radiative relaxation processes of ⁵D₀-⁵D₂ in decay curves at low temperature,we found that the coupling with phonons for B-site Eu³⁺is stronger than A-site Eu³⁺,which may by a reason for the worse luminescent thermal stability of B-site Eu³⁺ions.For the B-site Eu³⁺ions,the centrosymmetry is degenerated in solid solution host matrix,which makes the emission of 612 nm and the excitation of 465 nm more effective.The luminescent thermal stability is also enhanced by introducing of W atoms.The existence of Mo atoms in solid solution enhances the distortion of host matrix at high temperature.In chapter 5,the Eu³⁺doped Gd₃B?Mo/W?O₉ and Ca?Mo/W?O₄ solid solution phosphors are synthesized,and the acid radicals are islands-like distributed in such phosphors.When no Mo atom exists in the host matrix,the decay of emission lifetimes is less than 10%at 573 K.It is also found that the luminescent thermal stability becomes worse when the Mo atoms are introduced into host matrix.For the Gd_2.94B?Mo/W?O₉:Eu_0.06 solid solution phosphors,the luminescent thermal quenching happens at 423 K when excited by the charge transfer state?CTS?of MoO₆^6-groups.When excited by the CTS of MoO₄^2-groups,the decay of Eu³⁺ions'emission lifetimes in Mo contained Ca?Mo/W?O₄ solid solution phosphors is also faster than CaWO₄ ions(with the excitation of WO₄^2-groups).In chapter 6,the luminescent thermal quenching performance of Eu³⁺doped phosphors with different distribution of acid radicals,i.e.Ca₃?Mo/W?O₆,Sr₂Ca?Mo/W?O₆,Gd₃B?Mo/W?O₉ and Ca?Mo/W?O_4,are analyzed and compared.The favorable luminescent thermal stability are found in Eu³⁺doped CaWO₄ and Gd₃BWO₉,while luminescent thermal stability of Eu³⁺doped Sr₂CaWO₆ is worse.The decay of emission lifetimes of Eu³⁺doped CaWO₄ and Gd₃BWO₉ are less than 10%when the testing temperature rising from 323K to573 K.For Eu³⁺doped solid molybdate-tungstate solution phosphors,the best luminescent thermal performance are found in Eu³⁺doped Ca?Mo/W?O₄ phosphors.In the Gd₃B?Mo/W?O₉ solid solution host matrix,the luminescent thermal performance of Eu³⁺is not satisfying.The worst luminescent thermal stability is found in Eu³⁺doped Sr₂Ca?Mo/W?O₆ solid solution phosphors.The luminescent thermal quenching take place by absorbing the active energy and overcome the energy barrier and then vie the crossover of the CTS and ⁷F ground state back to the ground state with a non-radiative process.According to the schematic configurational coordinate diagram,the CTS energy of MoO₆^6-groups is lower than the CTS energy of WO₆^6-and MoO₄^2-groups WO₄^2-.The active energy required is lower for overcoming the energy barrier.That may be a reason for the severe luminescent thermal quenching of Mo contained solid solution phosphors.The distortion of acid radical groups can also degenerate the luminescent thermal stability of phosphors,which is confirmed by the Raman spectra and the calculation of lattice distortion.The introduction of Mo makes the distortion of?Mo/W?O₆ groups more severe at high temperature,and the luminescent thermal quenching is enhanced in such solid solution phosphors.The phosphors with double perovskite structure have three dimension net acid radical framework,and such structure makes the energy can transfer through?Mo/W?O₆-CaO₆-?Mo/W?O₆ framework with thermal stimulation,and then trapped by quenching centers.Thus the luminescent thermal performance of phosphors with such structure is unpromising.The luminescent thermal quenching of phosphors with islands-like distributed acid radicals is improved.This may provide reference and guidance for finding novel molybdate-tungstate phosphors.