Synthesis And Properties Of Y₂Mg₂Al₂Si₂O₁₂-Based Phosphors

White light-emitting diodes（LEDs）have many advantages over traditional light sources,such as energy saving,environmental protection and high efficiency,and have gradually become the fourth generation of lighting sources.Currently,the mainstream white LEDs are based on phosphor conversion light-emitting realization.As one of the key materials of white LED,phosphor plays an important role in improving the overall luminous efficiency of LED devices,optimizing color quality and expanding application fields.The development of new high-efficiency phosphor materials is essential to promote the development and progress of white LEDs.In this thesis,we have explored a variety of activator ion doping schemes,combined with co-doping strategy and energy transfer regulation to develop new phosphor materials,explored the physical phase and crystal structure of related materials as well as the occupation of doping ion sites,and studied the luminescence properties and application prospects of related materials,and the following research results were obtained:1.As a derivative structure of Y₃Al₅O₁₂（YAG）,the Y₂Mg₂Al₂Si₂O₁₂（YMAS）was designed by replacing Y³⁺/Al³⁺-Al³⁺units with Mg²⁺-Si⁴⁺units,and was prepared by high-temperature solid-phase synthesis at 1450℃under 6 h calcination.In the unit cell of YMAS,16 Y³⁺and 8 Mg²⁺are in a dodecahedron with 8 O^2-coordination,8Al³⁺and 8 Mg²⁺are in an octahedron with 6 O^2-coordination,and 16 Si⁴⁺and 8 Al³⁺are in a tetrahedron with 4 O^2-coordination each.The multiple substitution of Mg²⁺-Si⁴⁺to Y³⁺/Al³⁺-Al³⁺is most likely to be a symmetrically regular hierarchical spaced substitution as determined by the Density Functional Theory（DFT）calculations.Compared with the original YAG structure,the YMAS has more abundant cation sites,which lays the material foundation for the doping of multiple types of activator ions and the regulation of luminescence performance.2.The YMAS:Ce³⁺yellow phosphor and YMAS:Ce³⁺,Mn²⁺yellow-orange phosphor suitable for blue light excitation were prepared by Ce³⁺,Mn²⁺single/co-doping strategy.By Rietveld refinement and DFT calculations,it was determined that Ce³⁺tends to occupy the Y³⁺lattice of the dodecahedron and Mn²⁺tends to occupy the Mg²⁺lattice of the dodecahedron and octahedron.The emission spectra of the YMAS:x Ce³⁺（x=0.01–0.10）series phosphors can be tuned from 547to 566 nm in the luminescence center by Ce³⁺doping concentration modulation.By further introduction of Mn²⁺and modulation of the energy transfer process,the YMAS:x Ce³⁺,y Mn²⁺（y=0–0.25）series phosphors can achieve a spectral redshift from 564 to 606 nm.The YMAS:0.06Ce³⁺,0.25Mn²⁺sample achieves a spectral redshift of 76 nm compared to the YAG:0.06Ce³⁺phosphor.Compared with YAG:0.06Ce³⁺phosphors,the LED devices encapsulated by YMAS:x Ce³⁺,y Mn²⁺series phosphors have better correlated color temperature parameters and have potential applications in the field of warm white LED lighting.3.The YMAS:Eu²⁺phosphors suitable for ultraviolet excitation were prepared based on the divalent crystal lattice of the YMAS matrix.It was determined by the Rietveld refinement that Eu²⁺tends to occupy dodecahedral and octahedral Mg²⁺lattice sites in the YMAS matrix,resulting in the existence of two luminescence centers exhibiting asymmetry in the emission broadband.The intensity of the emission peaks of the YMAS:x Eu²⁺（x=0.001–0.100）series samples increased and then decreased with the increase of the doping concentration,accompanied by the displacement of the emission peaks from 436 to 491 nm,the full width at half-maximum broadened from 80 to 105 nm,and the luminescence color changed from blue to cyan and then to green.The white LEDs encapsulated by YMAS:0.04Eu²⁺phosphor and Ca Al Si N₃:Eu²⁺commercial red phosphor have good parameters,including near-standard CIE chromaticity coordinates（0.3329,0.3282）,suitable correlated color temperature（5474 K）,and good color rendering index（87.3）.The phosphor can modulate the luminescent color by changing the doping concentration,which has potential applications in white LED lighting.This work provides feasible ideas and practical examples for the application of Eu²⁺in garnet structural materials,and lays the foundation for the subsequent development of multiactivator doping and multicolor luminescent phosphors.4.The YMAS:Eu²⁺,Ce³⁺phosphors suitable for ultraviolet excitation were prepared by Eu²⁺and Ce³⁺co-doping strategy.The strongest emission peak of YMAS:0.01Eu²⁺,x Ce³⁺（x=0–0.05）series samples can be tuned from 464 to 558 nm,the full width at half maximum can be broadened from 98 to 245 nm,and the luminescence color can be tuned from blue to cyan to green and finally to yellow-green by the Ce³⁺doping concentration control.The energy transfer phenomenon from Eu²⁺to Ce³⁺was confirmed by the emission spectra of samples and the fluorescence decay curve of Eu²⁺,and the transfer efficiency could reach more than 48%.The series phosphors have excellent thermal stability and meet the application requirements of LED devices.Compared with YMAS:0.01Eu²⁺phosphors,LED devices packaged with YMAS:0.01Eu²⁺,0.01Ce³⁺phosphors have the same level of correlated color temperature（5841 K）,substantially higher color rendering index（87.8）,and CIE chromaticity coordinates（0.3258,0.3214）that are closer to standard white light.With the help of co-doping strategy and energy transfer regulation,the performance of phosphor was significantly improved by introducing Ce³⁺in the YMAS:Eu²⁺system.This work provides an effective guide for design and development of highly efficient color-tunable phosphors involving energy transfer from Eu²⁺to Ce³⁺in some specific materials,such as garnet structures.5.The YMAS:Eu²⁺,Mn²⁺phosphors with single-phase full-visible-spectra suitable for ultraviolet excitation were prepared by the Eu²⁺and Mn²⁺co-doping strategy.Eu²⁺and Mn²⁺tend to occupy the dodecahedral and octahedral Mg²⁺lattice sites.The energy transfer phenomenon from Eu²⁺to Mn²⁺was confirmed by the emission spectra and fluorescence decay curve.The transfer efficiency could reach more than 60%.Based on the energy transfer,the red light of Mn²⁺was substantially enhanced and the full-visible-spectra emission was achieved by the YMAS:0.03Eu²⁺,x Mn²⁺（x=0–0.40）series phosphors based on the doping concentration modulation.The stable photochromic performance under the excitation from 365 to 395 nm indicated that it would meet the application requirements of different ultraviolet chips.The YMAS:0.03Eu²⁺,0.30Mn²⁺phosphor-converted white LED achieves the ultra-high color rendering index（93.3）,near-standard CIE chromaticity coordinates（0.3343,0.3388）and suitable correlated color temperature（5417 K）.This work provides an effective guide for design and development of Eu²⁺and Mn²⁺co-doped single-phase full-visible-spectra YMAS:Eu²⁺,Mn²⁺phosphors,which have potential applications in the field of white light LED lighting.6.The YMAS:Eu³⁺red phosphors were designed and synthesized using the dual function of Eu³⁺as a red light activator and a spectroscopic probe.The qualitative and quantitative studies on the relationship between the local crystal structure and the luminescence properties of YMAS:Eu³⁺were performed experimentally and computationally,using the YAG:Eu³⁺as contrast.The local symmetry of Eu³⁺in YAG and YMAS both correspond to the D_4dcoordination geometry,which determines that both YAG:Eu³⁺and YMAS:Eu³⁺show abnormally strong ⁵D₀→⁷F₄transition.However,it is the D₂symmetry in YAG and C₂symmetry in YMAS,which leads to some significant differences in spectral properties.One is that the relative transition intensities of ⁵D₀→⁷F_1,2are markedly discrepant.The other is the ⁵D₀→⁷F₀transition which is absent in YAG:Eu³⁺but present in YMAS:Eu³⁺.The third is the difference in the center position and the full width at half maximum of the charge transfer band between the two.In addition,the optimal doping concentration,CIE chromaticity coordinates,quantum yields,thermal stability and application performance in LEDs were also comparatively studied.This work provides not only a new case for the study of local crystal structure and luminescence properties,but also a new possibility for the application of red optical materials in solid-state lighting.