Luminescence Properties and Energy Transfer of Site-Sensitive Ca6−x−y Mg x−z (PO4)4:Eu y 2+,Mn z 2+ Phosphors and Their Application to Near-UV LED-Based White LEDs

On the basis of the structural information that the host material has excellent charge stabilization, blue-emitting Ca6−x−y Mg x (PO4)4:Eu y 2+ (CMP:Eu2+) phosphors were synthesized and systematically optimized, and their photoluminescence (PL) properties were evaluated. Depending upon the amount of Mg added, the emission efficiency of the phosphors could be enhanced. The substitution of Eu2+ affected their maximum wavelength (λmax) and thermal stability because the substitution site of Eu2+ could be varied. To obtain single-phase two-color-emitting phosphors, we incorporated Mn2+ into CMP:Eu2+ phosphors. Weak red emission resulting from the forbidden transition of Mn2+ could be enhanced by the energy transfer from Eu2+ to Mn2+ that occurs because of the spectral overlap between the photoluminescence excitation (PLE) spectrum of Mn2+ and the PL spectrum of Eu2+. The energy transfer process was confirmed by the luminescence spectra, energy transfer efficiency, and decay curve of the phosphors. Finally, the optimized Ca6−x−y Mg x−z (PO4)4:Eu y 2+,Mn z 2+ (CMP:Eu2+,Mn2+) phosphors were applied with green emitting Ca2MgSi2O7:Eu2+ (CMS:Eu2+) phosphors to ultraviolet (UV) light emitting diode (LED)-pumped white LEDs. The CMS:Eu2+-mixed CMP:Eu2+, Mn2+-based white LEDs showed an excellent color rendering index (CRI) of 98 because of the broader emission band and more stable color coordinates than those of commercial Y3Al5O12:Ce3+ (YAG:Ce3+)-based white LEDs under a forward bias current of 20 mA. The fabricated white LEDs showed very bright natural white light that had the color coordinate of (0.3288, 0.3401), and thus CMP:Eu2+,Mn2+ could be regarded as a good candidate for UV LED-based white LEDs.