Studies in crystal structure and luminescence properties of Eu3+-doped metal tungstate phosphors for white LEDs

The correlation between the crystal structure and luminescent properties of Eu3+-doped metal tungstate phosphors for white LEDs was investigated. Red-emitting A4−3x(WO4)2:Eux3+ (A=Li, Na, K) and B(4−3x)/2(WO4)2:Eux3+ (B=Mg, Ca, Sr) phosphors were synthesized by solid-state reactions. The findings confirmed that these phosphors exhibited a strong absorption in the near UV to green range, due to the intra-configurational 4f–4f electron transition of Eu3+ ions. The high doping concentration of Eu3+ enhanced the absorption of near UV light and red emission without any detectable concentration quenching. Based on the results of a Rietveld refinement, it was attributed to the unique crystal structure. In the crystal structure of the Eu3+-doped metal tungstate phosphor, the critical energy transfer distance is larger than 5 Å so that exchange interactions between Eu3+ ions would occur with difficulty, even at a high doping concentration. The energy transfer between Eu3+ ions, which causes a decrease in red emission with increasing concentration of Eu3+, appears to be due to electric multi-polar interactions. In addition, the Eu–O distance in the host lattice affected the shape of emission spectrum by splitting of emission peak at the 5D0→7F2 transition of Eu3+.