Synthesis and photoluminescence of Bi³⁺,Eu³⁺ doped CdWO₄ phosphors: Application of energy level rules of Bi³⁺ ions

Bi³⁺,Eu³⁺ doped CdWO₄ phosphors have been synthesized using a co-precipitation method and regular micro-rods were obtained in Bi³⁺ or Eu³⁺ single-doped samples. X-ray diffraction, scanning electron microscopy, UV-vis spectrophotometry, and photoluminescence and decay time measurements were used to characterize the as-prepared samples. CdWO₄:Bi³⁺ and CdWO₄:Bi³⁺,Eu³⁺ can be excited ranging from 250 to 400 nm. The excitation at 350 nm was assigned to be the Bi^{3+ 1}S₀ → ³P₁ transition according to the energy level rules of Bi³⁺ ions. The origin of O-W charge transfer transition has been analyzed using the calculated band structure and density of states of CdWO₄ based on density functional theory. The approach to charge compensation was two impurity ions substituting for three Cd²⁺ sites. Energy transfer properties from the WO₆ group to Bi³⁺ as well as from Bi³⁺ to Eu³⁺ were discussed. The mechanism of energy transfer from Bi³⁺ to Eu³⁺ was determined to be the quadrupole-quadrupole interaction and the critical distance of energy transfer from Bi³⁺ to Eu³⁺ was calculated to be 15.31 Å. The quantum efficiency, CIE chromaticity and thermal quenching properties have also been investigated.