Studies in crystal structure and luminescence properties of Eu ³-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 A_4-3x(WO₄)₂:Eu_x ³ (A=Li, Na, K) and B_(4-3x)/2(WO₄) ₂:Eu_x³ (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 4f4f 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 EuO distance in the host lattice affected the shape of emission spectrum by splitting of emission peak at the ⁵D₀→ ⁷F₂ transition of Eu³.