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ABSTRACT: A comparative study of the crystal and electronic structure and magnetism of divalent europium perovskite oxides EuMO(3) (M = Ti, Zr, and Hf) has been performed on the basis of both experimental and theoretical approaches playing complementary roles. The compounds were synthesized via solid-state reactions. EuZrO(3) and EuHfO(3) have an orthorhombic structure with a space group Pbnm at room temperature contrary to EuTiO(3), which is cubic at room temperature. The optical band gaps of EuZrO(3) and EuHfO(3) are found to be about 2.4 and 2.7 eV, respectively, much larger than that of EuTiO(3) (0.8 eV). On the other hand, the present compounds exhibit similar magnetic properties characterized by paramagnetic-antiferromagnetic transitions at around 5 K, spin flop at moderate magnetic fields lower than 1 T, and the antiferromagnetic nearest-neighbor and ferromagnetic next-nearest-neighbor exchange interactions. First-principles calculations based on a hybrid Hartree-Fock density functional approach yield lattice constants, band gaps, and magnetic interactions in good agreement with those obtained experimentally. The band gap excitations are assigned to electronic transitions from the Eu 4f to Mnd states for EuMO(3) (M = Ti, Zr, and Hf and n = 3, 4, and 5, respectively).
Inorganic Chemistry 04/2012; 51(8):4560-7. · 4.60 Impact Factor
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ABSTRACT: We have synthesized hierarchically porous Y3Al5O12 (YAG) ceramics doped with trivalent cerium (Ce3+) ions (YAG:Ce) by the sol–gel process and examined their optical properties including scattering strength. Starting from the ionic precursors, i.e., AlCl3·6H2O, YCl3·6H2O, CeCl3·7H2O, and poly(ethylene oxide) (PEO) dissolved in a mixture of water and ethanol, monolithic wet gels are synthesized using propylene oxide as a geletion initiator. PEO induces phase separation parallel to the gelation in the sol–gel system to form bicontinuous morphologies consisting of gel- and fluid-phases. The wet gels thus obtained are subjected to either evaporative or supercritical drying, and then heat-treated to crystallize the gel skeletons to form YAG phase. The drying process critically affects the nanostructures of YAG skeletons after heat treatment; the supercritical drying results in skeletons consisting of fine grains (<50 nm), while evaporative drying yields skeletons with larger grains. The scattering strength of the heat-treated samples is evaluated by using coherent backscattering measurements and scattering-based hole burning, both of which clarify that the samples prepared via evaporative drying scatter visible light much more strongly than that prepared via supercritical drying.
08/2011;
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ABSTRACT: Photoreactive random media containing silver nanoparticles (Ag NPs) have been fabricated, and hole burning effects based on the interference of multiply scattered light have been investigated. Through the analysis of hole profiles, transport mean free path ℓ is estimated for the light with the wavelength of 585 nm. We find that ℓ in the samples embedded with Ag NPs is much shorter compared with that in a medium containing the same amount of titania (TiO2) NPs, indicating that Ag NPs scatter 585 nm lightwaves much stronger than TiO2 NPs because of their larger scattering cross sections due to the localized surface plasmon resonance.
Applied Physics Letters 03/2011; 98(12):121917-121917-3. · 3.84 Impact Factor
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ABSTRACT: We have prepared macroporous Y3Al5O12 (YAG) monoliths doped with rare earth (RE) ions (RE = Ce, Eu, Sm, and Pr) via the ionic precursor-derived sol–gel reaction accompanied by phase separation and investigated their fluorescence and scattering properties. YAG monoliths with well-defined bicontinuous macropores and skeletons were synthesized from the mixture of water and ethanol containing yttrium(III), aluminum(III), and RE(III) salts using propylene oxide as a gelation initiator and poly(ethylene oxide) as a phase separation inducer. X-ray diffraction measurements reveal that the sample as-dried and that heat-treated at 700 °C are amorphous, while a single crystalline phase of YAG is precipitated in the samples heat-treated at temperatures between 800 and 1100 °C. The macroporous YAG:RE ceramics heat-treated at 800 °C and higher temperatures exhibit fluorescence due to 4f–5d (Ce3+) and 4f–4f (Eu3+, Sm3+, Pr3+) electronic transitions characteristic of the RE ions occupying the eight-coordinated dodecahedral sites in YAG lattice. Coherent backscattering experiments indicate that the scattering strength is enhanced by the densification of skeletons in macroporous YAG monoliths, depending on the heat treatment temperature.
Optical Materials. 33(2):123-127.
