Quenching mechanism and luminescence enhancement strategy of Eu²⁺-activator in fluoroborate host Ba₅B₄O₁₀F₂

Fluoroborates have garnered significant research interest due to their intricate structure and unique nonlinear optical properties. In this work, pure and Eu-activated Ba5B4O10F2 were prepared via the high-temperature solid-state ceramic method. Ba5B4O10F2 is a wide band semiconductor (E-g = 4.51 eV) with an indirect transition characteristic. The optical absorption, photoluminescence, and dynamic spectra of Eu2+/3+ activators were investigated. Luminescence intensities of Eu2+ and Eu3+ had distinctly different responses to doping content. The residual Eu3+ centers in Ba5-5xEu5xB4O10F2 (x > 0.03) cannot be avoided even it was sintered in a reducing atmosphere. Time-resolved spectra and decay curves also confirm the presence of Eu3+ with Eu2+. Under near-UV light excitation, a strong emission band from the 5d -> 4f transition of Eu2+ was detected overlapping with sharp peaks from (D-5(0)-> F-7(0,1,2,3,4)) of Eu3+ in Ba5-5xEu5xB4O10F2 (x > 0.03). As the Eu-doping level increases, the intensity ratio of Eu2+ and Eu3+ changes regularly, and the luminescent intensity of Eu3+ obviously and systematically becomes stronger. As the temperature changes from 25 degrees C to 150 degrees C, the luminescent intensity of Eu3+ increases, while that of Eu2+ gradually decreases. The results confirm the energy transfer (ET) from Eu2+ to Eu3+ in Ba5B4O10F2. Eu3+ can act as killers, leading to the luminescence quenching of Eu2+ activators. The heterovalent substitution of Eu and NaF in Ba5B4O10F2 not only reduces the content of Eu3+ but also significantly enhances the efficiency of Eu2+. The experiments demonstrate that to improve the luminescence efficiency of Eu2+, it is important to focus on reducing the residual Eu3+ in the lattice through enhancing the reduction from Eu3+ to Eu2+.