Point Defects And Thermal Stability Of Photoluminescence Of Ba₂ScAlO₅ And Ba₃EuAl₂O_7.5

Rare earth doped inorganic photoluminescence materials,as important optical functional materials,play a unique role in solid-state lighting,display,new energy,anti-counterfeiting and other fields.However,phosphors with remarkable luminous efficacy and thermal stability remain in critically short supply.Researchers have not only improved the luminescent properties of materials by some effective methods such as surface coating and ion doping,but also further developed hosts with outstanding luminescent properties.In this dissertation,using Ba₂Sc Al O₅ and Ba₃Eu Al₂O_7.5 as hosts,the effects of sintering atmosphere,doping ion types,and concentrations on point defects in materials were systematically studied by means of X-ray diffraction,neutron diffraction and electron spin resonance techniques.The relationship between luminescent intensity,thermal stability and defects of Ba₂Sc Al O₅ and Ba₃Eu Al₂O_7.5 phosphors doped with rare earth ions was discussed.Valuable and innovative achievements have been made in the analysis of point defects and their relationship with the fluorescent properties of materials,which have certain reference significance for developing luminescent materials with high luminous efficiency and thermal stability.The main contents and conclusions of this dissertation are as follows:1.The influence of reducing and oxidizing atmospheres on the type and content of oxygen defects inβ-Ba₂Sc Al O₅ compounds was analyzed by neutron diffraction.The results provide experimental evidence for the up-conversion fluorescence quenching mechanism in an oxidizing atmosphere.On the basis of first-principles calculations,our research group proposed that the interstitial oxygen was found in theβ-Ba₂Sc Al O₅:Yb,Er treated in an oxidizing atmosphere,which is the main reason for the up-conversion fluorescence quenching.However,this conclusion lacks direct experimental evidence.The results show that the samples prepared in the hydrogen atmosphere have oxygen vacancy defects,which are located in O2（0.1659,0.3318,0.6308）and O3（0,0,0.25）,with a total content of about 10%.After air annealing,the oxygen vacancies of O2 were filled,and the oxygen vacancy content of O3 decreased to 3.7%.In addition,a large amount of interstitial oxygen occurs at（0.7010,0.2990,0.7628）,and the content is 7.4%.The samples sintered in air atmosphere have oxygen vacancies and interstitial oxygen,which are located in the same position as the annealed samples,and their concentrations are 4.4%and 2.1%respectively.The appearance of interstitial oxygen in an oxidizing atmosphere and the disappearance of interstitial oxygen the in a reducing atmosphere provide an experimental basis for the related physical mechanisms.Furthermore,cation vacancy analysis shows that a small amount of Sc³⁺ions will replace Al³⁺ions.In order to promote the transformation of Ba Al O₄ formed by excessive Al³⁺ions into the target product,it is necessary to add 10-20%Ba²⁺ions and 10%Sc³⁺ions on the basis of stoichiometry to maintain a singleβ-Ba₂Sc Al O₅ phase.2.The photoluminescence property of Eu³⁺-dopedβ/α-Ba₂Sc Al O₅ was studied.The results showed thatα-Ba₂Sc Al O₅:Eu³⁺phosphor has good temperature stability.By adjusting the Sc:Al ratio and sintering temperature,single phaseβ/α-Ba₂Sc Al O₅:Eu³⁺phosphors were successfully prepared in an air atmosphere.The XRD results show that the maximum solid solubility of Eu³⁺ions inβandα-Ba₂Sc Al O₅ is 0.20 in order to maintain the single-phase structure.Moreover,Eu³⁺ions preferentially occupy the Sc1position inα-Ba₂Sc Al O₅.The luminous intensity and thermal stability of the optimal sampleα-Ba₂Sc Al O₅:0.20Eu³⁺are better than that ofβ-Ba₂Sc Al O₅:0.15Eu³⁺.Under the excitation of 393 nm,the integrated emission intensity ofα-Ba₂Sc Al O₅:0.20Eu³⁺is about twice as much as that ofβ-Ba₂Sc Al O₅:0.15Eu³⁺.The integrated emission intensity ofα-Ba₂Sc Al O₅:0.20Eu³⁺at 150℃maintained 95%of room temperature,while theβ-Ba₂Sc Al O₅:0.15Eu³⁺decreased to 12%at room temperature.At the same time,based on the thermal disturbance model,it is revealed that the near-zero thermal quenching mechanism ofα-Ba₂Sc Al O₅:Eu³⁺excited at 390 nm is that the Eu-O CTB as the intermediate energy level blocks the non-radiation relaxation path from high excited energy levels ⁵H₆,⁵D₄,⁵L₆ to ⁵D_J（J=2,1,0）of Eu³⁺,thus enhancing the thermal stability of the material.3.The anti-thermal quenching effect was found in Ba₃Eu Al₂O_7.5 compound under excitation at 270 and 392 nm,which is due to the release of electrons captured by defect levels.The results of XRD refinement show that Ba₃Eu Al₂O_7.5 has a monoclinic Ba₃Er Ga₂O_7.5 type structure,with the space group of P2/c,and the oxygen vacancy concentration of 4.8%,which is located in O2（4 g）and O7（4 g）.Under excitation at 270and 392 nm,the magnetic dipole transition ⁵D₀→⁷F₁（591 nm）is dominant,and the lattice occupied by Eu³⁺ions is highly symmetric.Temperature-dependent emission spectra show that when the excitation wavelength is 270 and 392 nm,the integrated emission intensity of Ba₃Eu Al₂O_7.5 reaches the maximum at 443 and 483 K,which is 145%and 156%of the integrated emission intensity at room temperature,respectively.The thermoluminescence spectrum and electron spin resonance spectra reveal the mechanism:oxygen vacancy defects form the defect energy levels in the band gap,and the trapped electrons are gradually released from the trap and transferred to Eu³⁺ions during heating,thus enhancing the emission of Eu³⁺ions.In addition,reducing the concentration of Eu³⁺ions can lead to a decrease in the number of oxygen vacancies,resulting in a weakening of the thermal enhancement effect.4.The luminescence intensity and thermal stability of Ba₃Eu Al₂O_7.5 phosphor were improved by doping with Y³⁺and Ga³⁺ions,respectively.By replacing Eu³⁺ions with Y³⁺ions,the concentration quenching effect of Eu³⁺ions is weakened.Under the excitation at 393 nm,the emission intensity of Ba₃Eu_0.40Y_0.60Al₂O_7.5 is three times that of Ba₃Eu Al₂O_7.5.Moreover,Ga³⁺ions were introduced into Ba₃Eu Al₂O_7.5,the absorption intensity of the material in the range of 330-650 nm is reduced,the intensity of hypersensitive transition is reduced,and the symmetry of the surrounding environment of Eu³⁺is enhanced,thereby improving the luminescence intensity and thermal stability of Ba₃Eu Al₂O_7.5.Under the excitation of 393 nm,the integrated emission intensity of Ba₃Eu Al_1.985Ga_0.015O_7.5 is 13 times that of Ba₃Eu Al₂O_7.5,and the thermal enhancement amplitude is increased by about 5%.