Efficient Bi³⁺ to Eu³⁺ energy transfer and color tunable emissions in K₇CaY₂(B₅O₁₀)₃-based phosphors

Rare-earth borates are well known good photoluminescent materials due to the easy manipulation of activator concentration. K₇CaY₂(B₅O₁₀)₃ belongs to a recently discovered borate family with outstanding nonlinear optical performances. A systematic study on Bi³⁺ and Eu³⁺ doped phosphors was performed to explore their potential in photoluminescence. K₇Ca(Y_1-xBi_x)₂(B₅O₁₀)₃ (0.01 ≤x≤ 0.06), K₇Ca(Y_1-yEu_y)₂(B₅O₁₀)₃ (0.10 ≤y≤ 1) and K₇Ca(Y_0.99-zBi_0.01Eu_z)₂(B₅O₁₀)₃ (0.05 ≤z≤ 0.90) were prepared by high temperature solid state reactions. Rietveld refinements reveal an 8% anti-site occupancy of Ca²⁺ and Eu³⁺ in K₇CaEu₂(B₅O₁₀)₃, and two sets of Bi³⁺ emission and excitation spectra are also observed once Bi³⁺ is introduced. For instance, the two strongest ¹S₀→³P₁ excitations at 270 nm and 281 nm correspond to two ³P₁→¹S₀ emissions at 383 and 334 nm, respectively. The Eu³⁺ emission shows a maximal intensity at y = 0.50 under charge transfer excitation, while there is no concentration quenching effect under f-f excitation. The Bi³⁺-to-Eu³⁺ energy transfer is firmly supported by the steady photoluminescence spectra and the decreased lifetime of Bi³⁺ upon increasing the Eu³⁺ content in K₇Ca(Y_0.99-zBi_0.01Eu_z)₂(B₅O₁₀)₃. This energy transfer mechanism occurs through the electric dipole-dipole interaction. Under excitation at 281 nm, the emission is tunable from deep blue (Bi³⁺) to pink and finally to red (Eu³⁺) all with high quantum yields (>80%).