Structural consideration on Eu2+ distribution inside glaserite-type Na(Ba, Sr)PO4: Eu2+ phosphor: High-intensity and broadband emission consisting of three overlapping Eu2+ emission bands

Glaserite-type NaBaPO4: Eu2+ phosphor has attracted considerable attention because of the intense blue emission. Although this emission was discussed under the assumption that Eu2+ ions were distributed only in A- and B-sites of the crystal structure, this assumption has proved inconsistent with previous results and hindered the quantitative evaluation of the emission. This study aims to quantitatively interpret the emission characteristics in Eu2+-activated Na(Ba, Sr)PO4 solid solutions by tracking spectroscopic changes resulting from the modifications in the crystal structure when Ba2+ in NaBaPO4 is replaced by Sr2+. The broadband emission observed in Na(Ba, Sr)PO4: Eu2+ consists of three partially overlapping bands. Electron spin resonance (ESR) spectroscopy reveals that the three bands correspond to 4f65 d1-4f7 interconfigurational electron transitions in crystallographically distinct three Eu2+. The distribution in three sites has not been observed in M3MgSi2O8 or A2MMgP2O8 (A: Li, Na, M: Ba, Ca, Sr), and specific to NaBaPO4 host. Because the emission wavelengths of the three Eu2+ ions are close, NaBaPO4: Eu2+ exhibits a high resultant emission intensity. Based on the Eu2+'s preference for sites with high Sr2+ occupancy, Eu2+ distribution and the associated Na(Ba, Sr)PO4: Eu2+ emission color can be tailored through Ba2+/Sr2+ replacement. The structure-emission relationship revealed in this study provides valuable insights for designing high-luminous-efficiency in glaserite-type phosphors.