Ryo Yoshimatsu

Optical transitions near the fundamental absorption edge and electronic structures of YAl3(BO3)4:Gd3+

Optical properties of Gd3+ ions doped in YAl3(BO3)4 are investigated as a new UV phosphor under excitation of VUV radiation. A strong emission line at 3.96 eV (313 nm) corresponding to the transitions from the 6P7/2 to 8S7/2 states of Gd3+ ions is observed. We performed the Rietveld refinement using an X-ray diffraction result to obtain the structural parameters of a synthesized YAl3(BO3)4:Gd3+ phosphor. From the result, it is shown that the refined crystal structure of YAl3(BO3)4:Gd3+ phosphors is slightly distorted. The electronic structures have been calculated using the relativistic discrete variational Xα method. The calculations indicate that the optical properties of a non-doped YAl3(BO3)4 crystal near the fundamental absorption edge are characterized by the electronic transitions from O 2 p valence states to B 2 p conduction states. On the other hand, it is found that the origin of the absorption edge of YAl3(BO3)4:Gd3+ phosphor is due to the transitions from O 2 p states to mixed states of mainly B 2 p and Y 4d orbitals.

Blue-Emitting Eu2+-Doped CaAl2O4 Phosphor Thin Films Prepared Using Pulsed Laser Deposition Technique with Post Annealing

Blue-emitting Eu2+-doped calcium aluminate phosphor thin films were obtained using the pulsed laser deposition technique with post annealing. As-deposited films were amorphous and showed weak red Eu3+ photoluminescence (PL). By annealing in reducing atmosphere (N2/H2:2% mixed gas) at 950°C for 3 h, the film was crystallized and showed a PL emission band peaking at about 447 nm, which originated from the 4f65d to 4f7 transition of Eu2+ ion. It is considered that the deposited film consists mainly of CaAl2O4 and partly of other binary compounds of the CaO–Al2O3 system. It was determined that the PL intensity of Eu2+ in CaAl2O4 can be controlled by the laser fluence, target-substrate distance and injection gas.

Development of a new blue CaMgSi2O6:Eu2+ phosphor for PDPs

— A blue-light-emitting Eu2+-doped CaMgSi2O6 phosphor having a long lifetime for a plasma-display panel (PDP) was developed. The CaMgSi2O6:Eu2+(CMS:Eu) phosphors show no luminance degradation during the baking process, and an equivalent photoluminescence peak intensity compared to that of the conventional blue-phosphor BaMgAl10O17:Eu2+ (BAM) after baking. CMS: Eu shows a poor luminescent characteristic for the Xe excimer band excitation due to the lack of absorption. To introduce the absorption center for the Xe excimer band, we performed Gd-codoping of CMS: Eu as a sensitizer and found a new excitation band around 172 nm, which originated from Gd3+. The test PDPs panels using synthesized CMS: Eu phosphor and CMS: Eu, Gd phosphor were examined to investigate the luminescent and aging characteristics of a Xe-discharge excitation source. The CMS: Eu panel shows an emission peak intensity comparable to that of the BAM panel (i.e., a comparable stimuli L/CIEy, 93% of BAM), while the CMS: Eu, Gd panel shows poorer blue emission intensity compared to the BAM panel (up to 53% of total stimuli of BAM). The CMS: Eu panel and the CMS: Eu, Gd panel show less luminance degradation than the BAM panel under the aging test, and the panel retains 90% of its luminance after 300 hours of driving. It was found that CMS: Eu appears to be a candidate for a new blue PDP phosphor because of its longevity in a Xe-discharge plasma environment.

Correlational analysis of Eu3+ charge transfer state using La effective charge in La-based mixed-anion host compounds

A prediction of the Eu3+ charge transfer state (E CT) was attempted in La-based mixed-anion host compounds. We paid attention to La3OF3S2:Eu, since it is expected to have a more covalent La site than La2O2S. The La effective charge (La EC) was proposed as the index factor of covalency and/or ionicity. The correlation between the experimental E CT and the calculated La EC was systematically analyzed for La2S3, LaFS, La2O2S, La2O3, LaOF, and LaF3 host materials, and good approximation was obtained using the single exponential function with a variable number of La ECs. According to the fitting curve, the E CT of La3OF3S2:Eu was predicted to be 5.8 and 2.1 eV for Eu3+ centers activated at ionic and covalent sites, respectively. To confirm the prediction accuracy, La3OF3S2:Eu phosphor powder samples were synthesized by solid-state reaction. From the photoluminescence excitation and absorption measurements, the E CT values of about 4.7 eV (ionic La site) and 2.4 eV (covalent La site) were obtained. Even though the energy difference between the predicted and experimental values is large for the higher E CT, La EC is the useful index factor for estimating E CT. In addition, it indicates that the estimation can be applied to phosphor materials having multication sites.

P‐92: Luminescence Properties of YAl3(BO3)4: Tb3+ Phosphors Substituted with Sc3+ Ions under VUV Excitation

We have investigated the luminescence and excitation spectra of (Y, Gd)Al3(BO3)4: Tb3+ phosphors substituted with Sc3+ ions. The luminescence of green, which has two emission peaks at 542 and 549 nm, is observed for (Y, Gd, Tb, Sc)Al3(BO3)4 and (Y, Tb, Sc)(Al, Sc)3(BO3)4 phosphors. It originates from 5D4) 7F5 transitions of 4f-4f inner shell of Tb3+ ions. These phosphors have an additional absorption band in the range of about 170–200 nm. As the results under the excitation light at 172 nm generated by xenon-discharge excimer lamp, we found out an enhancement of luminescence intensity of green due to the substitutions with Sc3+ ions.