Eu<sup>3+</sup> Site Distribution and Local Distortion of Photoluminescent Ca<sub>3</sub>WO<sub>6</sub>:(Eu<sup>3+</sup>, K<sup>+</sup>) Double Perovskites as High‐Color‐Purity Red Phosphors
Takahito Otsuka, Ryohei Oka, Tomokatsu Hayakawa
Abstract
Abstract In this study, Eu 3+ and K + co‐doped Ca 3 WO 6 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 Eu 3+ fluorescence line‐narrowing (FLN) results, used to estimate the Eu 3+ 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 Eu 3+ 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 Eu 3+ doping levels, the ions are predominantly located at the B sites (≈75%), indicating that the high color purity of Ca 3 WO 6 :(Eu 3+ , K + ) is mainly caused by the high asymmetry ratios of the Eu 3+ sites. Rietveld refinement combined with the FLN technique provides more reliable results for increasing the Eu 3+ substitution at the A site with Eu 3+ and K + doping concentrations, which lower the lattice energy of Ca 3 WO 6 :(Eu 3+ , 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 Λ values.