Local Environments of Dilute Activator Ions in the Solid-State Lighting Phosphor Y_3–xCe_xAl₅O₁₂

Abstract

The oxide garnet Y3Al5O12 (YAG), when substituted with a few percent of the activator ion Ce3+ to replace Y3+, is a luminescent material that is nearly ideal for phosphor-converted solid-state white lighting. The local environments of the small number of substituted Ce3+ ions are known to critically influence the optical properties of the phosphor. Using a combination of powerful experimental methods, the nature of these local environments is determined and is correlated with the macroscopic luminescent properties of Ce-substituted YAG. The rigidity of the garnet structure is established and is shown to play a key role in the high quantum yield and in the resistance toward thermal quenching of luminescence. Local structural probes reveal compression of the Ce3+ local environments by the rigid YAG structure, which gives rise to the unusually large crystal-field splitting, and hence yellow emission. Effective design rules for finding new phosphor materials inferred from the results establish that efficient phosphors require rigid, highly three-dimensionally connected host structures with simple compositions that manifest a low number of phonon modes, and low activator ion concentrations to avoid quenching.

Inorganic Phosphor Materials for Lighting

“…Table 3 shows the B factors for YAG and YAG:Ce 3? as extracted from neutron diffractions [54]. A general observation can be made here that the B factors of all constituent atoms (Y/Ce, Al, and O) increase as the Ce 3? ions are incorporated in the YAG matrix.…”

“…ions of lower energy. This leads to a counteracting and predominating red-shift [54]. A comparison with the low-temperature spectrum for one selected composition, YAG:0.0033 %Ce 3? , reveals splitting of the 4f states into 2 F 5/2 and 2 F 7/2 states due to spin-orbit coupling [4], see Fig.…”

“…is related to the degree of crystallinity. Other, more recent examples include the study by Ghigna et al [53] and George et al [54], who examined the local structure of the Ce 3? ion using extended XAFS (EXAFS) and found that the nearest eight Ce-O distances are expanded by about 3 % compared to the average Y-O distances in Y 2.97 Ce 0.03 Al 5-O 12 , and Gracia et al [55] and Muñoz-García and Seijo [56], who showed that the substitution of Ce 3?…”

“…Several [54]. The lattice expansion obeys Vegard's law [54,70] previous works have focused specifically on the difference in local structure with and without Ce 3? doping.…”

“…A general observation can be made here that the B factors of all constituent atoms (Y/Ce, Al, and O) increase as the Ce 3? ions are incorporated in the YAG matrix. The increase of the B factors is further seen as a decrease of the Debye temperatures (h D ) of these atoms, which in turn gives a larger accessibility of vibrations, particularly low-frequency vibrational modes [54]. Since the lowestfrequency vibrations in YAG:Ce 3?…”