Synthesis and tunable luminescence properties of Eu2+ and Tb3+-activated Na2Ca4(PO4)3F phosphors based on energy transfer

A series of color-tunable blue–green emitting Na2Ca4(PO4)3F:Eu2+,Tb3+ phosphors were prepared by a high temperature solid-state reaction. Their luminescence properties reveal that there is an efficient energy transfer from Eu2+ to Tb3+ ions via a dipole–quadrupole mechanism where Eu2+ ions exhibit a strong excitation band in near ultraviolet (UV) region, matching well with the dominant emission band of near UV (350–420 nm) LED chips, and Eu2+ and Tb3+ ions can give characteristic blue and green emission light. The varied color of the phosphors from blue to green can be achieved by properly tuning the relative ratio of Eu2+ to Tb3+ dopant through the energy transfer from Eu2+ to Tb3+ ions. Thermal quenching luminescence results reveal that Na2Ca4(PO4)3F:Eu2+,Tb3+ exhibits good thermal stability. These results demonstrate that Tb3+ ion with low 4f–4f absorption efficiency in near UV region can play the role of an activator in narrow green-emitting phosphor through efficient energy feeding by allowing 4f–5d absorption of Eu2+ with high oscillator strength. The present Eu2+–Tb3+ codoped Na2Ca4(PO4)3F phosphor will have potential application for the near UV white LEDs.