Shinobu Aoyagi

A novel phosphor for glareless white light-emitting diodes

The luminous efficiency of white light-emitting diodes, which are used as light sources for next-generation illumination, is continuously improving. Presently available white light-emitting diodes emit with extremely high luminance because their emission areas are much smaller than those of conventional light sources. Consequently, white light-emitting diodes produce a glare that is uncomfortable to the human eye. Here we report a yellow-emitting phosphor, the Eu(2+)-doped chlorometasilicate (Ca(1-x-y,)Sr(x,)Eu(y))(7)(SiO(3))(6)Cl(2), which can be used to create glareless white light-emitting diodes. The (Ca(1-x-y,)Sr(x,)Eu(y))(7)(SiO(3))(6)Cl(2) exhibits a large Stokes shift, efficiently converting violet excitation light to yellow luminescence, and phosphors based on this host material have much less blue absorption than other phosphors. We used crystal structure analysis to determine the origin of the desired luminescence, and we used (Ca(1-x-y,)Sr(x,)Eu(y))(7)(SiO(3))(6)Cl(2) and a blue-emitting phosphor in combination with a violet chip to fabricate glareless white light-emitting diodes that have large emission areas and are suitable for general illumination.

A layered ionic crystal of polar Li@C 60 superatoms

If the physical properties of C(60) fullerene molecules can be controlled in C(60) products already in use in various applications, the potential for industrial development will be significant. Encapsulation of a metal atom in the C(60) fullerene molecule is a promising way to control its physical properties. However, the isolation of C(60)-based metallofullerenes has been difficult due to their insolubility. Here, we report the complete isolation and determination of the molecular and crystal structure of polar cationic Li@C(60) metallofullerene. The physical and chemical properties of Li@C(60) cation are compared with those of pristine C(60). It is found that the lithium cation is located at off-centre positions in the C(60)-I(h) cage interior and that the [Li(+)@C(60)] salt has a unique two-dimensional structure. The present method of purification and crystallization of C(60)-based metallofullerenes provides a new C(60) fullerene material that contains a metal atom.

Composite Structure of BaTiO3 Nanoparticle Investigated by SR X-Ray Diffraction

The size effects on the crystal structure and phase transitions have been investigated for BaTiO 3 fine particles of 700, 400, 300 and 100 nm size, by means of powder diffraction using synchrotron ...

Rock‐Salt‐Type Crystal of Thermally Contracted C60 with Encapsulated Lithium Cation

Rock solid: fullerene-encapsulated Li(+) (Li(+)@C(60)) is an alkaline cation owing to the spherical shape and positive charge. Li(+)@C(60) crystallizes as a rock-salt-type crystal in the presence of PF(6)(-). The orientations of C(60) and PF(6)(-) (orange) are perfectly ordered below 370 K, and Li(+) (purple) hops within the cage. At temperatures below 100 K two Li(+) units are localized at two polar positions within each C(60) .

Structural, transport, and thermal properties of the single-crystalline type-VIII clathrate Ba8Ga16Sn30

The analysis of temperature dependence of mH suggests a crossover of a dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity s1.1 W / m K at 150 Kd. The potential of type-VIII clathrate compounds for thermoelectric application is discussed.

Kinetic Study of the Diels–Alder Reaction of Li+@C60 with Cyclohexadiene: Greatly Increased Reaction Rate by Encapsulated Li+

We studied the kinetics of the Diels-Alder reaction of Li(+)-encapsulated [60]fullerene with 1,3-cyclohexadiene and characterized the obtained product, [Li(+)@C60(C6H8)](PF6(-)). Compared with empty C60, Li(+)@C60 reacted 2400-fold faster at 303 K, a rate enhancement that corresponds to lowering the activation energy by 24.2 kJ mol(-1). The enhanced Diels-Alder reaction rate was well explained by DFT calculation at the M06-2X/6-31G(d) level of theory considering the reactant complex with dispersion corrections. The calculated activation energies for empty C60 and Li(+)@C60 (65.2 and 43.6 kJ mol(-1), respectively) agreed fairly well with the experimentally obtained values (67.4 and 44.0 kJ mol(-1), respectively). According to the calculation, the lowering of the transition state energy by Li(+) encapsulation was associated with stabilization of the reactant complex (by 14.1 kJ mol(-1)) and the [4 + 2] product (by 5.9 kJ mol(-1)) through favorable frontier molecular orbital interactions. The encapsulated Li(+) ion catalyzed the Diels-Alder reaction by lowering the LUMO of Li(+)@C60. This is the first detailed report on the kinetics of a Diels-Alder reaction catalyzed by an encapsulated Lewis acid catalyst rather than one coordinated to a heteroatom in the dienophile.

Direct Observation of Covalency between O and Disordered Pb in Cubic PbZrO3

In cubic PbZrO 3 , which undergoes an antiferroelectric phase transition, the distinct disorder of Pb at twelve sites toward the neighboring O is detected for the first time by analyzing high-energ...

Extremely High Resolution Single Crystal Diffractometory for Orbital Resolution using High Energy Synchrotron Radiation at SPring‐8

The investigation of accurate structure at charge density level will not only understand the function mechanism of physical property but also lead to design new functional materials. We have successfully installed large cylindrical camera at the BL02B1/SPring‐8. In conceptual design, the image plate (IP) was selected as the detector, because such IP will not only detect wide range in one shot but also yield reliable data. In commissioning, the performance of this camera demonstrated to be suitable for the direct observation of d‐electron system.

Distinctive Charge Density Distributions of Perovskite-Type Antiferroelectric Oxides PbZrO3 and PbHfO3 in Cubic Phase

The electron charge density distributions of simple perovskite oxides, PbBO3 (B = Ti, Zr and Hf), in their cubic phase are investigated by analyzing high-energy synchrotron powder diffraction data by the maximum entropy method (MEM)/Rietveld method. Clear structural differences between the antiferroelectric and ferroelectric perovskites are revealed. In the cubic phase of PbZrO3 and PbHfO3 that undergo antiferroelectric phase transitions, the Pb atom is disordered around the special Wyckoff position. The thermal motion of the O atom is anisotropic, and the charge density distributions around the O atom are extended in the directions perpendicular to the Zr(Hf)-O covalent bond. None of these structural characteristics are observed in the cubic phase of PbTiO3 that undergoes ferroelectric phase transition. The distinctive structural features observed in PbZrO3 and PbHfO3 should provide a clue to the mechanism of antiferroelectric phase transition.

High-Energy SR Powder Diffraction Evidence of Multisite Disorder of Pb Atom in Cubic Phase of PbZr1-xTixO3

High-energy synchrotron-radiation (SR) powder diffraction experiments have been carried out to investigate the relationship between the crystal structure of the paraelectric phase in PbZr1-xTixO3 (PZT) and phase transition. The Rietveld refinement adopting the split-atom method reveals that the Pb atom in PZT, except PbTiO3 (x=1), is settled at multisites in relevant directions from the corner site of an ideal cubic structure. Namely, the Pb atom is found to be disordered at 12 sites in the directions in PbZrO3 (x=0), and at 8 sites in the directions in the Zr-rich region (0 disorder is preferable in the Ti-rich region (0.5<x<1). The disordered characteristics of the Pb atom and the structural boundary observed at x=0.5 in the paraelectric phase are significant for understanding of the mechanism of the order-disorder phase transition and appearance of a morphotropic phase boundary (MPB) in PZT.