Improving the luminescent properties of Fe3+ in CaAl4O7 by co-doping with Bi3+ ions

In the framework of luminescent transition metal ions-doped phosphors for near-infrared (NIR) lighting, Fe³⁺-activated phosphors have been recently demonstrated to be a potential alternative to the most common Cr³⁺ and Ni²⁺-based NIR materials. However, this family of phosphors still suffer from low absorption efficiency and severe thermal quenching. This study investigates the effect of Bi³⁺ ion concentration on the spectroscopic features of Fe³⁺ ions in CaAl₄O₇:Fe³⁺, Bi³⁺ system. The presence of the ¹S₀→¹P₁ transition band in Fe³⁺ PLE spectra indicates the Bi³⁺→Fe³⁺ energy transfer leading to a corresponding increase in luminescence intensity of Fe³⁺ ions by over 30-fold compared to Fe³⁺-singly doped sample. High Bi³⁺ concentrations also quench Bi³⁺ ion luminescence, improving NIR emission purity. Additionally, the presence of Bi³⁺ ions enhances Fe³⁺ ion luminescence stability by delaying the thermal depopulation, as evidenced by a T₅₀ shift from 323 K to 393 K. Overall, co-doping CaAl₄O₇:Fe³⁺ with Bi³⁺ ions expands excitation spectra, boosts luminescence intensity, and enhances the thermal stability.