Tunable Optical Properties and Increased Thermal Quenching in the Blue-Emitting Phosphor Series: Ba₂(Y_1–<i>x</i>Lu_<i>x</i>)₅B₅O₁₇:Ce³⁺ (<i>x</i> = 0–1)

The preparation of cerium-substituted barium lutetium borate, Ba₂Lu₅B₅O₁₇:Ce³⁺, is achieved using high temperature solid state synthesis. This compound crystallizes in the Ba₂Y₅B₅O₁₇-type structure and shows an efficient blue emission (λ_max = 447 nm) when excited by UV-light (λ_ex = 340 nm) with a photoluminescent quantum yield near 90%, a fast luminescence decay time (<40 ns), and a thermal quenching temperature of 452 K. Further, preparing a solid solution following Ba₂(Y_1–<i>x</i>Lu_<i>x</i>)₅B₅O₁₇:Ce³⁺ (<i>x</i> = 0, 0.25, 0.50, 0.75, 1) confirms that all compounds are isostructural and follow Vegard’s law. Substituting Y³⁺ for Lu³⁺ yields a nearly constant emission spectrum that blue-shifts by only 9 nm and has a consistent luminescence lifetime across the range prepared. The photoluminescent quantum yield (PLQY) and thermal quenching (<i>T</i>₅₀) of the solid solution, however, are dramatically impacted by the composition, with the PLQY decreasing to ≈70% and the <i>T</i>₅₀ dropping 49 K going from <i>x</i> = 1 to <i>x</i> = 0. These significant changes in the optical properties likely stem from enhanced structural rigidity as the larger, more polarizable Y³⁺ is substituted for the smaller, harder Lu³⁺ cation. These results highlight the importance of optimizing chemical bonding to improve a phosphor’s optical properties