Tetsuo Honma

Photoluminescence and Crystal Structure of Green-Emitting Ca3Sc2Si3O12 : Ce3 + Phosphor for White Light Emitting Diodes

We have developed a new oxide-based phosphor Ca 3 Sc 2 Si 3 O 12 :Ce 3+ , which is applicable as a green-emitting color converter for white light emitting diodes (LEDs). This phosphor absorbs blue light around 450 nm and emits green luminescence, with a peak wavelength around 505 nm. It is a promising candidate for application in LEDs as quenching of the phosphor at 150°C was smaller than that of Y 3 Al 5 O 12 :Ce yellow phosphor. A white LED with high color rendering was fabricated by combining this phosphor with a blue GaN LED and a red phosphor. The luminescence of this phosphor is derived from the 5d-4f transition of the Ce ion and the luminescence decay curve fit a single exponential function. This phosphor has a garnet-type host crystal structure. X-ray absorption fine structure analysis showed that Ce ions replaced the Ca position of the host crystal as Ce 3+ .

Ba-Substitution Effect on Luminescent Properties and Thermal Degradation of Sr3MgSi2O8 : Eu2 + Blue Phosphor Under Vacuum-UV-Light Excitation

Luminescence properties and mechanisms of thermal degradation in (Sr,Ba) 3 MgSi 2 O 8 :Eu 2+ under vacuum-UV-light excitation have been investigated. Sr 2.98 MgSi 2 O 8 :Eu 2 0.02 and a Ba-substituted composition, Sr 2.48 Ba 0.5 MgSi 2 O 8 :Eu 2 0.02 , are over 90 and 75%, respectively, as high as commercially used BaMgAl 10 O 17 :Eu 2+ in photoluminescence peak intensity under 146 nm light excitation. At the same Eu content, Ba-substituted Sr 3 MgSi 2 O 8 :Eu 2+ shows much smaller thermal degradation of luminescence than Sr 3 MgSi 2 O 8 °Eu 2+ . X-ray fine structure measurements show that the thermal degradation is closely correlated to the oxidation of Eu 2+ ion into Eu 3+ ion, which is suppressed by Ba substitution.

Oxidation of doped europium in BaMgAl10O17 by annealing studied by x-ray-absorption fine-structure measurements

We studied the influence of annealing in air on doped europium in BaMgAl 10 O 17 by performing x-ray absorption fine-structure measurements. We determined the oxidation of doped divalent europium by annealing in air at over 500°C. The interatomic distance between the europium and the surrounding oxygen atoms was compressed by oxidation. It also appears that the oxidation process of europium is determined by the diffusion of oxygen into BaMgAl 10 O 17 .

Site of Doped Eu in BaMgAl 10 O 17 Studied by X-Ray Absorption Fine Structure

The sites that doped divalent Eu ions occupy in BaMgAl 10 O 17 were studied by X-ray absorption fine structure (XAFS) measurement. The radial structural function and the Fourier-filtered EX-AFS wiggle derived from the observed XAFS spectrum suggested that Eu ions occupy the Beevers-Ross and/or anti-Beevers-Ross sites. Observed XANES spectrum also could be explained by Beevers-Ross site occupation.

Influence of x-ray irradiation on doped europium in BaMgAl10O17 studied by x-ray absorption fine structure and x-ray diffraction

— We studied the influence of x-ray irradiation on the doped europium ion in BaMgAl10O17 (BAM) by x-ray absorption fine structure measurement and x-ray diffraction. We found that x-ray irradiation promoted oxidation of doped europium and fading emissions. However, no difference was found in x-ray diffraction patterns before and after x-ray irradiation. We concluded that the fading of BAM after x-ray irradiation was mainly caused by oxidation of the doped europium in the BAM.

XAFS Analysis of Local Structure around Ce in Ca3Sc2Si3O12:Ce Phosphor for White LEDs

We have studied the local structure around Ce atom in Ca3Sc2Si3O12 host crystal, which has been developed as a new green phosphor for white light emitting diodes (LEDs). As the local structure and chemical environment of the dopant atom are very important to improve the performance of the phosphor, we have used XAFS to get chemical and structural information around the Ce dopant. The XANES spectrum of the Ce LIII‐edge reveals that the Ce atom is trivalent in Ca3Sc2Si3O12. There are two kinds of possible Ce substitution sites, Ca site and Sc site, in garnet type Ca3Sc2Si3O12 crystal structure. The Ce atom is found to be at the Ca site in the host crystal by the comparison of the Fourier transform of Ce K‐edge EXAFS spectrum with those of Ca and Sc K‐edge EXAFS spectra. The theoretical analysis with FEFF also clarified the Ce substitution at the Ca site. Furthermore, the result of the analysis indicates the structural disorder around Ca and Si atoms at 3.75 A. It is possible that there are some defects arou...

Analysis of the Local Structure around Eu and Mn Ions in Alkaline‐Earth Silicate Phosphors for White Light Illumination

M2SiO4‐based phosphors (M: alkaline‐earth metal) that emit red to blue light are expected to offer high color rendering to white light‐emitting diodes (LEDs) in combination with blue or near‐UV excitation sources. It is very important for the complete control of the emission color to understand the crystal field around the active elements (rare‐earth and transition metals). XAFS spectroscopy is applied to a (Ba,Ca)2SiO4:Eu,Mn phosphor at Eu L3‐ and Ba, Ca, Eu, Mn K‐edges to elucidate the local environments of Eu and Mn. Eu L3‐ and Mn K‐edge XANES spectra showed that Eu and Mn are both divalent, like Ba and Ca. K‐edge EXAFS spectra indicated that the local structures of Eu and Mn are similar to those of Ba and Ca, respectively. However, the curve‐fitting analysis showed that the first coordination shell of Eu has two Eu–O bonds that are both shorter than the Ba–O bond. FEFF calculations were also performed based on a BaCaSiO4 model constructed from the crystal structure of KNaSO4. They suggested that Eu su...

Site Determination of Doped Ga Ions in SrTiO3:Pr3+ Phosphor for Field Emission Displays by XAFS

The enormous enhancement of the emission intensity from SrTiO3:Pr3+ by addition of Al or Ga ions was found. In case of Al, it was reported that the emission enhancement is caused by charge compensation and reduction of planar faults in the host lattices. In order to clarify the mechanism of the enhancement of the emission from SrTiO3:Pr3+ by addition of Ga ions, we investigated the local structure of doped Ga ions in SrTiO3:Pr3+ (a red phosphor for Field Emission Display) by XAFS analysis. The XAFS analysis indicates that doped Ga ions substitute for Ti site in the crystal.

Effect on deoxidation of doped europium in oxidized BaMgAl10O17

— We studied the effect of the deoxidation of oxidized blue-light-emitting europium-doped BaMgAl10O17 (BAM) phosphor in a plasma-display panel by making photoluminescence and powder X-ray diffraction measurements and by X-ray absorption fine structure analysis. Photoluminescence spectra, powder X-ray diffraction, and X-ray absorption near-edge structure, and extended X-ray absorption fine structure spectra of BAM at the europium L3-edge suggest that the oxidized BAM annealed in air is revived by heating in a H2/N2 (5% H2) reducing atmosphere above 500°C.