Tunable photoluminescence properties and energy transfer in oxyapatite-based Ca₂Tb₈(SiO₄)₆:Eu³⁺ phosphors for UV-LEDs

New phosphors Ca₂Tb_8–xEu_x(SiO₄)₆O₂ with color-tunability were synthesized in this work. The Rietveld refinement indicated that the prepared phosphors were crystallized to a hexagonal structure with space group P6₃/m (no. 176). When Eu³⁺ ions were introduced into Ca₂Tb₈(SiO₄)₆O₂, characteristic red–orange emissions of Eu³⁺ ions and the green emission of Tb³⁺ ions were obtained. Eu³⁺-doped phosphors emitted with enhanced intensity and had broader excitation spectra than the undoped Ca₂Tb₈(SiO₄)₆O₂ phosphors. As Eu³⁺ content increased, a decline in Tb³⁺ emission was observed owing to resonant energy transfer from the ⁵D₃ level of Tb³⁺ ions to the ⁵D₀ level of Eu³⁺ ions. Moreover, the transfer of energy from Tb³⁺ ions to Eu³⁺ ions proceeded via a dipole–dipole interaction, and the critical distance for this energy transfer was calculated to be 7.2 Å. The efficiency of energy transfer in Ca₂Tb_8−xEu_x(SiO₄)₆O₂ increased from 72% to 87% with an increase in the Eu³⁺ concentration, x, from 0.02 to 0.08. The emitted color of Ca₂Tb_8−xEu_x(SiO₄)₆O₂ was tuned from green to red via controlling the concentration of Eu³⁺ ions. Meanwhile, the optimum phosphor system Ca₂Tb_7.94Eu_0.6(SiO₄)₆O₂ exhibited adequate thermal stability even at 200 °C and the corresponding activation energy was calculated to be 0.027 eV. These results suggest that Ca₂Tb₈(SiO₄)₆O₂:Eu³⁺ may be potential phosphors for the application in UV LEDs. - Highlights: • New phosphors Ca₂Tb_8−xEu_x(SiO₄)₆O₂ were synthesized in this work. • The structure of Ca₂Tb_8−xEu_x(SiO₄)₆O₂ was refined to be P6₃/m (no. 176). • As Eu³⁺ was doped into the host, efficient energy transfer from Tb³⁺ to Eu³⁺ was observed. • The efficiency of energy transfer increased with the concentration of Eu³⁺. • The emitted color of the present phosphors was tuned from green to red.