Chasing Down the Eu2+ Ions: The Delicate Structure-Property Relationships in the Ultra-Narrow Band Phosphor K1.6Na2.1Li0.3[Li3SiO4](4):Eu2+

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.