The influence of Pb-doping on the crystal structure and luminescence properties of LiEuMo2O8 red-emitting phosphors

High efficiency red phosphors, showing spectral range of 610-620 nm and a narrow-line emission, are helpful for achieving high color rendering index (CRI) and low correlated color temperature (CCT) phosphor-converted white-light diodes. Red phosphors activated by trivalent europium (Eu3+) thus have attracted a renewed interest for pc-WLEDs. In our case, we prepare a series of Pb2+-doped LiEuMoO8 red-emitting phosphors by solid-state reactions. The introduction of Pb2+ ions expands and distorts slightly the unit cell volume of LiEuMo2O8 matrix, and red-shifts the charge transfer absorption (CTA) band of LiEuMo2O8, which are demonstrated by X-ray powder diffraction, absorption spectra and micro-Raman spectra. Photoluminescence and photoluminescence excitation spectra results show that the excitation efficiency of Eu3+ is improved significantly in the near ultraviolet region when Pb2+ ions are doped into the host lattice. At x=0.015, the red-emitting intensity of LiEuMo2O8 is enhanced by 50%, and the internal quantum yields is 53% under excitation at 395 nm. The enhanced red emission is discussed in term of the relaxation of parity-forbidden selection rules for trivalent europium luminescent center, and the sensitization effect of Pb2+ as a sensitizer for Eu3+ ions is negligible in LiEuMo2O8 under near ultraviolet excitation.