The reported anomalous emission intensity of the ⁵D₀ → ⁷F₄ transition of Eu³⁺ in a molybdate double perovskite

The assigned anomalous upconversion emission intensity of the ⁵D₀ → ⁷F₄ transition of Eu³⁺ in the double perovskite Sr₂Ca(W_1-xMo_x)O₆ co-doped with Yb³⁺ is shown to be due to a transition metal impurity present in the MoO₃ starting material. Relevant literature shows that the highest energy ⁵D₀ → ⁷F₄ transition is at a longer wavelength than 685 nm in molybdates. Cases of anomalous intensity of this transition are reviewed. Our 360 nm excited emission spectra of Sr₂Ca_0.88-Eu_0.02Yb_0.04Li_0.06MoO₆ show a very weak ⁵D₀ → ⁷F₄ transition, but on co-doping with Mn a structured band between 640 and 710 nm is observed due to Mn⁴⁺.