Eu³⁺ Site Distribution and Local Distortion of Photoluminescent Ca₃ WO₆ :(Eu³⁺ , K⁺ ) Double Perovskites as...

In this study, Eu³⁺ and K⁺ co-doped Ca₃ WO₆ double perovskite, a high-color-purity red phosphor, is quantitatively investigated using synchrotron X-ray diffraction Rietveld refinement, energy dispersive X-ray spectroscopy, and high-resolution PL spectroscopy. The Eu³⁺ fluorescence line-narrowing (FLN) results, used to estimate the Eu³⁺ occupation at given sites (so-called A and B sites) in the host crystal (A₂ BMO₆ ; A, B = Ca; M = W), reveal that the Eu³⁺ ions have a twin distribution in both the A and B sites with high asymmetry ratios of Λ_A  = 9.7 and Λ_B  = 10.7, respectively. More interestingly, at lower Eu³⁺ doping levels, the ions are predominantly located at the B sites (≈75%), indicating that the high color purity of Ca₃ WO₆ :(Eu³⁺ , K⁺ ) is mainly caused by the high asymmetry ratios of the Eu³⁺ sites. Rietveld refinement combined with the FLN technique provides more reliable results for increasing the Eu³⁺ substitution at the A site with Eu³⁺ and K⁺ doping concentrations, which lower the lattice energy of Ca₃ WO₆ :(Eu³⁺ , K⁺ ). The structural distortions owing to K⁺ co-doping in terms of the quadratic elongation and bond-angle variance exhibit good correlations with the Ca(Eu)O₁₂ A-site cuboctahedra and Ca(Eu)O₆ B-site octahedra, partially accounting for the higher Λ values.