Synthesis and Photoluminescence of Eu ³⁺ ‐Activated and Eu ³⁺ /Eu ²⁺  Co‐Activated Li

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.