Understanding of Luminescence Properties Using Direct Measurements on Eu²⁺‐Doped Wide Bandgap Phosphors

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 AELi₂Be₄O₆: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⁺ 5d¹4f⁶ → 4f⁷ 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.