Fig. 1: Eu³⁺ luminescence.

a Energy level diagram of Eu³⁺. b Emission spectra of various Eu³⁺-doped host lattices acquired at room temperature. The luminescence of β-NaREF₄:Eu³⁺ (RE = La, Y, Lu) was excited at 395 nm into the ⁵L₆ level, while Eu³⁺ in the other samples was excited into the ligand-to-metal charge-transfer band at 250–310 nm. To prevent cross-relaxation low Eu³⁺ concentrations of 0.4–0.5% were used in LaBO₃, LaPO₄, NaLuF₄, NaYF₄, and NaLaF₄, while this required even lower dopant concentrations of 0.1% and 0.05% for Y₂O₂S and Y₂O₃, respectively. The vertical lines mark the wavelength range of the ⁵D₁\(\to\)⁷F_0–3 emissions of β-NaREF₄:Eu³⁺ and the ⁵D₁\(\to\)⁷F_0–2 emissions of the other materials. The emissions were magnified by a factor 10 for Y₂O₃ and by a factor of 50 for LaBO₃ and LaPO₄