Chasing Down the Eu²⁺ Ions: The Delicate Structure−Property Relationships in the Ultra‐Narrow Band Phosphor K_1.6Na_2.1Li_0.3[Li₃SiO₄]₄:Eu²⁺

Abstract Eu²⁺‐activated phosphors of the UCr₄C₄ structure type have emerged as promising new (ultra‐)narrow band phosphors. Especially the UCr₄C₄‐type alkali lithosilicates have caused a sensation in the field of phosphor research in the recent years due to their consistent (ultra)narrow‐band luminescence in the cyan and green range that can pave the way to novel backlighting LEDs with high color gamut. The compound “K₂Na₂[Li₃SiO₄]₄:Eu²⁺” stands out with a strikingly narrow emission in the cyan range (<0.2 eV or <30 nm full width at half maximum in the energy or wavelength scale, respectively) but the position of the Eu²⁺ ions within this structure has been so far controversial. High‐resolution photoluminescence spectroscopy at 10 K reveals a complex emission spectrum with seven emission bands in the violet to green range. Combination with single‐crystal X‐ray diffraction (SCXRD) demonstrates that the cation channels in the structure also contain Li⁺ ions. The presence of emission bands in the violet and blue spectral range reveals that the Eu²⁺ ions also occupy the available K sites in the structure independently confirmed by advanced ligand field calculations. This study demonstrates that a combination of SCXRD, photoluminescence spectroscopy, and theory can help elucidate advanced structural problems.