Structural modulation and Judd–Ofelt-guided luminescence enhancement in Eu3+-doped LiCa4O(BO3)3 via dual-alkali co-doping

Abstract

Eu3+-doped borates stand out as promising red-emitting phosphors owing to their stable crystal structures and sharp luminescence features. In this study, we investigate LiCa4O(BO3)3:Eu3+ and explore how additional doping with K+ and Na + ions influences the symmetry of Eu3+ sites, photoluminescence efficiency, and thermal stability. All synthesized samples exhibit a single-phase orthorhombic structure. Photoluminescence spectra revealed prominent Eu3+ emissions, dominated by the 5 D0→7 F2 transition at 613 nm. Remarkably, co-doping with alkali metals enhances the long-wavelength emission (~705 nm), contributing to deeper red color output. Judd–Ofelt analysis showed that Na+-doped and Eu-only samples maintained Ω4-dominant profiles, indicating a relatively symmetric local environment. In contrast, K+ doping significantly increased the Ω2/Ω4 ratio, suggesting a more asymmetric crystal field and stronger electric-dipole transitions. Lifetime and nonradiative decay measurements supported these findings, showing that K+ improved radiative efficiency, while Na+ preserved structural rigidity with moderate trade-offs in emission strength. Thermal quenching studies revealed activation energies of ~0.36 eV for Eu-only samples, decreasing to ~0.15 eV with alkali co-doping, confirming enhanced thermal stability. Chromaticity data indicated tunable red emission, where K+ contributed to higher color purity and Na+ offered better thermal color retention. These insights underline how alkali ions fine-tune site symmetry and emission behavior, providing useful guidance for designing advanced Eu3+- based borate phosphors for LED and optical sensing applications.