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Recent progress in advanced optical materials based on gadolinium aluminate garnet (Gd 3 Al 5 O 12 )

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ABSTRACT

This review article summarizes the recent achievements in stabilization of the metastable lattice of gadolinium aluminate garnet (Gd3Al5O12, GAG) and the related developments of advanced optical materials, including down-conversion phosphors, up-conversion phosphors, transparent ceramics, and single crystals. Whenever possible, the materials are compared with their better known YAG and LuAG counterparts to demonstrate the merits of the GAG host. It is shown that novel emission features and significantly improved luminescence can be attained for a number of phosphor systems with the more covalent GAG lattice and the efficient energy transfer from Gd3+ to the activator. Ce3+ doped GAG-based single crystals and transparent ceramics are also shown to simultaneously possess the advantages of high theoretical density, fast scintillation decay, and high light yields, and hold great potential as scintillators for a wide range of applications. The unresolved issues are also pointed out.

No MeSH data available.


A scheme showing possible pathways of energy transfer (left) in the [(Gd0.8Lu0.2)0.9−xTb0.1Eux]AG phosphor and digital pictures (right) showing color-tunable emission through the energy transfer (excitation: 275 nm).
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Figure 17: A scheme showing possible pathways of energy transfer (left) in the [(Gd0.8Lu0.2)0.9−xTb0.1Eux]AG phosphor and digital pictures (right) showing color-tunable emission through the energy transfer (excitation: 275 nm).

Mentions: Energy transfer between two types of activators is widely utilized in the phosphor field to tune the emission color, to produce a specific color that cannot be attained with one single type of activator, and to enhance the desired emission. The Ce3+/Tb3+ and Tb3+/Eu3+ combinations are among the most frequently adopted activator pairs. In the former, the direction of energy transfer largely depends on the 5d1 energy level of Ce3+, which is as aforesaid readily subjected to centroid shift and crystal field splitting [55]. For example, Ce3+ → Tb3+ energy transfer is found in CePO4:Tb [75] while Tb3+ → Ce3+ in TbAG:Ce [28, 29]. Dorenbos [76] determined that crystal field splitting of the Ce3+ 5d level is affected by coordination geometry, and tends to decrease following the order: octahedral > cubic > dodecahedral > tricapped trigonal prisms and cuboctahedral. Only Tb3+ → Eu3+ transfer can be observed for the Tb3+/Eu3+ pair, since the 5D3,4 excited states of Tb3+ lie higher than the 5D0,1 emission states of Eu3+ and both the ions have relatively fixed energy levels for the 4f electrons [64–66]. The Tb3+ → Eu3+ energy transfer is of high efficiency ( can be ∼90%, for example), because of significant overlapping of the emission spectrum of Tb3+ with the excitation spectrum of Eu3+ [77, 78]. With such an energy transfer, occurring via electric multipole interactions [78], the emission color of Tb3+/Eu3+ codoped Y2O3 can be finely tuned between red and green by varying the atomic ratio of the two activators [78]. Energy transfer and emission control were recently studied for the GAG-based phosphor of [(Gd0.8Lu0.2)0.9−xTb0.1Eux]AG [53], where the Eu content was varied from x = 0 to 0.1. The excitation spectra taken for the Tb3+ green emission at ∼545 nm and the Eu3+ red emission at ∼592 nm are shown in figure 15. For Tb3+ emission (figure 15(a)), only the characteristic excitation bands of Tb3+ are resolved, with the inter-configurational 4f8 → 4f75d1 transition at ∼276 nm being dominant as found for (Gd,Lu)AG:Tb3+. Intensity of the excitation significantly decreases with increasing Eu3+ addition and finally becomes negligible at x = 0.1, primarily owing to Tb3+ → Eu3+ energy transfer and also concentration quenching at high total contents of the two activators. The excitation spectra taken for Eu3+ emission are, however, dominated by Tb3+ transitions, and only very weak CTB and intra-4f6 transitions originated from Eu3+ are found (figure 15(b)). This indicates that, in the codoped system, exciting Tb3+ is the only efficient way to produce Eu3+ luminescence through energy transfer. Intensity of the 276 nm excitation reaches its maximum at x = 0.03, followed by a steady decrease at higher Eu contents owing to concentration quenching. Figure 16(a) analyzes intensities of the 592 nm Eu3+ and 545 nm Tb3+ emissions (λex = 276 nm), where the strongest emission is normalized to 10 for both the activators. It is seen that the Tb3+ emission is monotonically weakened at a higher Eu content while the Eu3+ emission gradually gains intensity up to x = 0.03 and then decreases, following the tendency found from the excitation spectra. The I592/I545 intensity ratio steadily increases with increasing Eu3+ incorporation, which may suggest a persistent energy transfer from Tb3+ to Eu3+ or the quenching of Eu3+ emission is less than that of Tb3+. The CIE color coordinates shown in figure 16(b) indicate that the emission can be well tuned from green to orange red via yellow (figure 17). Further analysis indicated that energy transfer may have occurred via electric dipole-quadrupole interactions [53]. It should be noted that the energy process is more complicated for (Gd, Lu)AG than Y2O3 owing to the presence of optically active Gd3+. Since the 8S7/2 → 6IJ Gd3+ transition well overlaps the 4f8 → 4f75d1 Tb3+ transition at ∼276 nm, multichannel energy transfer is highly possible, including Gd3+ → Tb3+ → Eu3+, Tb3+ → Eu3+ and Gd3+ → Eu3+ (figure 17), though further studies are needed to clarify the exact routes. The excitation behavior of Eu3+ and the significantly lowered Tb3+ while improved Eu3+ emissions up to x = 0.03, however, unambiguously reveal the presence of Tb3+ → Eu3+ transfer path.


Recent progress in advanced optical materials based on gadolinium aluminate garnet (Gd 3 Al 5 O 12 )
A scheme showing possible pathways of energy transfer (left) in the [(Gd0.8Lu0.2)0.9−xTb0.1Eux]AG phosphor and digital pictures (right) showing color-tunable emission through the energy transfer (excitation: 275 nm).
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC5036492&req=5

Figure 17: A scheme showing possible pathways of energy transfer (left) in the [(Gd0.8Lu0.2)0.9−xTb0.1Eux]AG phosphor and digital pictures (right) showing color-tunable emission through the energy transfer (excitation: 275 nm).
Mentions: Energy transfer between two types of activators is widely utilized in the phosphor field to tune the emission color, to produce a specific color that cannot be attained with one single type of activator, and to enhance the desired emission. The Ce3+/Tb3+ and Tb3+/Eu3+ combinations are among the most frequently adopted activator pairs. In the former, the direction of energy transfer largely depends on the 5d1 energy level of Ce3+, which is as aforesaid readily subjected to centroid shift and crystal field splitting [55]. For example, Ce3+ → Tb3+ energy transfer is found in CePO4:Tb [75] while Tb3+ → Ce3+ in TbAG:Ce [28, 29]. Dorenbos [76] determined that crystal field splitting of the Ce3+ 5d level is affected by coordination geometry, and tends to decrease following the order: octahedral > cubic > dodecahedral > tricapped trigonal prisms and cuboctahedral. Only Tb3+ → Eu3+ transfer can be observed for the Tb3+/Eu3+ pair, since the 5D3,4 excited states of Tb3+ lie higher than the 5D0,1 emission states of Eu3+ and both the ions have relatively fixed energy levels for the 4f electrons [64–66]. The Tb3+ → Eu3+ energy transfer is of high efficiency ( can be ∼90%, for example), because of significant overlapping of the emission spectrum of Tb3+ with the excitation spectrum of Eu3+ [77, 78]. With such an energy transfer, occurring via electric multipole interactions [78], the emission color of Tb3+/Eu3+ codoped Y2O3 can be finely tuned between red and green by varying the atomic ratio of the two activators [78]. Energy transfer and emission control were recently studied for the GAG-based phosphor of [(Gd0.8Lu0.2)0.9−xTb0.1Eux]AG [53], where the Eu content was varied from x = 0 to 0.1. The excitation spectra taken for the Tb3+ green emission at ∼545 nm and the Eu3+ red emission at ∼592 nm are shown in figure 15. For Tb3+ emission (figure 15(a)), only the characteristic excitation bands of Tb3+ are resolved, with the inter-configurational 4f8 → 4f75d1 transition at ∼276 nm being dominant as found for (Gd,Lu)AG:Tb3+. Intensity of the excitation significantly decreases with increasing Eu3+ addition and finally becomes negligible at x = 0.1, primarily owing to Tb3+ → Eu3+ energy transfer and also concentration quenching at high total contents of the two activators. The excitation spectra taken for Eu3+ emission are, however, dominated by Tb3+ transitions, and only very weak CTB and intra-4f6 transitions originated from Eu3+ are found (figure 15(b)). This indicates that, in the codoped system, exciting Tb3+ is the only efficient way to produce Eu3+ luminescence through energy transfer. Intensity of the 276 nm excitation reaches its maximum at x = 0.03, followed by a steady decrease at higher Eu contents owing to concentration quenching. Figure 16(a) analyzes intensities of the 592 nm Eu3+ and 545 nm Tb3+ emissions (λex = 276 nm), where the strongest emission is normalized to 10 for both the activators. It is seen that the Tb3+ emission is monotonically weakened at a higher Eu content while the Eu3+ emission gradually gains intensity up to x = 0.03 and then decreases, following the tendency found from the excitation spectra. The I592/I545 intensity ratio steadily increases with increasing Eu3+ incorporation, which may suggest a persistent energy transfer from Tb3+ to Eu3+ or the quenching of Eu3+ emission is less than that of Tb3+. The CIE color coordinates shown in figure 16(b) indicate that the emission can be well tuned from green to orange red via yellow (figure 17). Further analysis indicated that energy transfer may have occurred via electric dipole-quadrupole interactions [53]. It should be noted that the energy process is more complicated for (Gd, Lu)AG than Y2O3 owing to the presence of optically active Gd3+. Since the 8S7/2 → 6IJ Gd3+ transition well overlaps the 4f8 → 4f75d1 Tb3+ transition at ∼276 nm, multichannel energy transfer is highly possible, including Gd3+ → Tb3+ → Eu3+, Tb3+ → Eu3+ and Gd3+ → Eu3+ (figure 17), though further studies are needed to clarify the exact routes. The excitation behavior of Eu3+ and the significantly lowered Tb3+ while improved Eu3+ emissions up to x = 0.03, however, unambiguously reveal the presence of Tb3+ → Eu3+ transfer path.

View Article: PubMed Central - PubMed

ABSTRACT

This review article summarizes the recent achievements in stabilization of the metastable lattice of gadolinium aluminate garnet (Gd3Al5O12, GAG) and the related developments of advanced optical materials, including down-conversion phosphors, up-conversion phosphors, transparent ceramics, and single crystals. Whenever possible, the materials are compared with their better known YAG and LuAG counterparts to demonstrate the merits of the GAG host. It is shown that novel emission features and significantly improved luminescence can be attained for a number of phosphor systems with the more covalent GAG lattice and the efficient energy transfer from Gd3+ to the activator. Ce3+ doped GAG-based single crystals and transparent ceramics are also shown to simultaneously possess the advantages of high theoretical density, fast scintillation decay, and high light yields, and hold great potential as scintillators for a wide range of applications. The unresolved issues are also pointed out.

No MeSH data available.