Synthesis and Photoluminescence of Eu ³⁺ ‐Activated and Eu ³⁺ /Eu ²⁺  Co‐Activated Li ₄ Al ₄ B ₆ O ₁₇  Phosphors

ABSTRACT Eu 3+ ‐doped and Eu 2+/3+ co‐doped Li 4 Al 4 B 6 O 17 ​ were synthesized by a high‐temperature solid‐state reaction under air and reducing atmospheres, respectively. The phase purity, crystal structure, and elemental composition were investigated. Under ultraviolet light excitation, the Eu 3+ ‐only sample exhibits intense orange‒red luminescence dominated by the 5 D 0 → 7 F 1 magnetic dipole transition ​ at 592 nm, suggesting that Eu 3+ centers may occupy high‐symmetry sites, possibly with an inversion center, although the precise crystal structure of the host remains unknown. Li 4 Al 4 B 6 O 17 :Eu prepared under a reducing atmosphere shows luminescence from both Eu 2+ and Eu 3+ centers, which was confirmed by XPS and decay kinetics. At a low doping concentration ( x = 0.005), complete reduction to Eu 2+ was achieved, giving a broad blue emission band centered at 470 nm. With increasing Eu concentration ( x ≥ 0.01), residual Eu 3+ appears, and the energy transfer from Eu 2+ to Eu 3+ was evidenced by excitation spectra and decay dynamics. Temperature‐dependent luminescence reveals good thermal stability with an activation energy of approximately 0.40 eV for both Eu 3+ and Eu 2+ emissions. The internal quantum efficiency reaches 37.5% for the Eu 3+ ‐only sample, while the co‐doped sample exhibits a lower efficiency of 17.5%, which is attributed to defect‐related non‐radiative losses arising from heterovalent ion substitution. This work provides a comprehensive understanding of the luminescence mechanisms in Eu‐doped Li 4 Al 4 B 6 O 17 , which features a rigid aluminum borate framework with Li + as the only cation site. This study serves as a reference for rare‐earth luminescence in such Li‐only hosts.