Eu3+ Site Distribution and Local Distortion of Photoluminescent Ca3WO6:(Eu3+, K+) Double Perovskites as High-Color-Purity Red Phosphors

In this study, Eu3+ and K+ co-doped Ca3WO6 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 (A(2)BMO(6); 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 Lambda(A) = 9.7 and Lambda(B) = 10.7, respectively. More interestingly, at lower Eu3+ doping levels, the ions are predominantly located at the B sites (approximate to 75%), indicating that the high color purity of Ca3WO6:(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 Ca3WO6:(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)O-12 A-site cuboctahedra and Ca(Eu)O-6 B-site octahedra, partially accounting for the higher Lambda values.