No abstract

SrAl2O4nanosized particles (NPs) undoped as well as doped with Eu2+and Dy3+were prepared by combustion synthesis for the discussion of their intensively debated spectroscopic properties. Emission spectra of SrAl2O4:Eu2+(,Dy3+) NPs are composed by a green band at 19 230 cm−1(520 nm) at room temperature, assigned to anomalous luminescence originated by Eu2+in this host lattice. At low temperatures, a blue emission band at 22 520 cm−1(444 nm) is observed. Contrary to most of the interpretations provided in the literature, we assign this blue emission band very reliably to a normal 4f6(7FJ)5d(t2g)→4f7(8S7/2) transition of Eu2+substituting the Sr2+sites. This can be justified by the presence of a fine structure in the excitation spectra due to the different7FJlevels (J=0⋯6) of the 4f6core. Moreover, Fano antiresonances with the6IJ(J=9/2,7/2) levels could be observed. In addition, the Stokes shifts (ΔES=1980 cm−1and 5 270 cm−1for the blue and green emission, resp.), the Huang-Rhys parameters ofS=2.5and 6, and the average phonon energies ofħω=480 cm-1and 470 cm−1coupled with the electronic states could be reliably determined.

Luminescence, decay, and thermal quenching under photonic excitation of CaGa2S4:Eu2+ mono and polycrystals have been measured. For excitation into the lower 5d states, thermal quenching starts above 350 K, which makes this material appropriate for use as a yellow phosphor under excitation by blue-emitting diodes. Thermoluminescence after 337.1 nm irradiation is explained by photoionization of Eu2+ ions. © 2002 The Electrochemical Society. All rights reserved.

Raman investigation of isostructural polycrystalline MII Ga 2 (S, Se)4 compounds with MII = Pb, Sr, Eu, Yb and Ca was performed. We observed that the effect of MII atomic mass on the phonon frequencies is negligible and that the vibrations are essentially influenced by the sizes of the MII cations. As the effects of cation substitution are small we concluded that the orthorhombic thiogallates MII Ga 2 S4 exhibit nearly the same phonon energies: about 280 cm−1 (∼35 meV) and 360 cm−1 (∼45 meV) for the most intense vibration modes. The substitution of S atoms by heavier Se atoms causes the shift of the Raman spectrum to lower frequencies. The orthorhombic MII Ga 2 Se4 compounds present phonon energy at about 185 cm−1 (∼23 meV) for the most intense vibration mode. Results confirm that a molecular model is more adequate to describe the vibrations of these compounds than the factor-group analysis.