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Amin, Muhammad Ruhul; Strobel, Philipp; Qamar, Amir; Giftthaler, Tobias; Schnick, Wolfgang; Moewes, Alexander (2020): Understanding of Luminescence Properties Using Direct Measurements on Eu2+-Doped Wide Bandgap Phosphors. In: Advanced Optical Materials, Vol. 8, No. 16, 2000504
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Abstract

Recent developments in solid-state lighting lead to a demand for new phosphors with excellent thermal stability, exhibiting large bandgaps for highly efficient emission. A detailed characterization of the luminescent material AELi(2)Be(4)O(6):Eu2+ (AE = Ba, Sr) is presented using soft X-ray absorption spectroscopy, X-ray emission spectroscopy, X-ray excited optical luminescence (XEOL) spectroscopy, and density functional theory (DFT) calculations. The experimental indirect bandgap for both BaLi2Be4O6:Eu2+ (BLBO:Eu) and SrLi2Be4O6:Eu2+ (SLBO:Eu) is found to be 6.5 +/- 0.3 eV, which agrees well with the DFT calculations (6.8 eV for BLBO:Eu and 7.4 eV for SLBO:Eu). The crucial Eu2+ 5d to conduction band energy separation is determined to be 0.21 +/- 0.10 and 0.25 +/- 0.10 eV for BLBO:Eu and SLBO:Eu, respectively, using resonant inelastic X-ray scattering. These measured values are in good agreement with the thermal quenching measurements (0.20 +/- 0.03 eV for BLBO:Eu and 0.26 +/- 0.03 eV for SLBO:Eu). Finally, the XEOL measurement confirms the Eu2+ 5d(1)4f(6) -> 4f(7) transition, which is responsible for the ultranarrow band (full width at half-maximum: 25 nm) blue optical luminescence at 455-457 nm for both compounds. With these measurements and calculations, deep insights are gained into the key properties of those two phosphors.