Mixed Microscopic Eu~(2+) Occupancies in the Next-Generation Red LED Phosphor SrLi_2Al_2O_2N_2:Eu~(2+) (SALON:Eu~(2+))

摘要

Red-emitting narrow-band phosphors are of utmost importance fornext-generation white-light phosphor-converted light-emitting diodes(pc-wLEDs) for improved efficacy and optimized correlated color temperatures.A promising representative crystallizing in an ordered variant of theUCr_4C_4 structure type is Sr[Li_2Al_2O_2N_2]:Eu~(2+) (SALON:Eu~(2+)) emitting at adesirable wavelength of 614 nm. Despite an expected eightfold coordinationof the Eu~(2+) ions by four N~(3?) and O~(2?) ions, respectively, the exact localcoordination symmetry and a 1:1 ratio between the two types of ligands isnot straightforwardly proven by X-ray diffraction. Low-temperature luminescencespectroscopy in conjunction with ligand field theory are powerfulalternatives to resolve local features of Eu~(2+) as its excited 4f~65d~1 configurationreacts sensitively to the polarity of the ligands. The dominant emissionat 614 nm shows pronounced vibronic fine structure at 10 K. In addition,weak emission bands can be resolved at 570 and 650 nm even at a lowdoping concentration of 0.5 mol% and are assigned to Eu~(2+) ions being eightfoldcoordinated by N~(3?) and O~(2?) ions in a ratio different from 1:1. Due to thefeature of vierer ring-type channels in SALON:Eu~(2+), those Eu~(2+) centers aresufficiently close for mutual energy transfer, which is characterized by timeresolvedluminescence at 10 K.