Phase formation and spectral evolution of (Sr,Ba)2Si(O,N)4:Eu2+ phosphors

Sr2-xBaxSi(O,N)(4):Eu2+ (SBxSON:Eu2+) oxynitridosilicate phosphors were prepared via incorporation of N3-, Eu2+, and Ba2+ ions into Sr2SiO4 (SSO) lattices. X-ray diffraction patterns of the prepared powders revealed that SBxSON:Eu2+ was a solid-solution form of SSO. An increase in x values caused a phase transition and an expansion of the unit cell. The photoluminescence excitation (PLE) spectra of SBxSON:Eu2+ were broad, covering the ultraviolet range to the visible range. Corresponding PL emission spectra strongly depended on the excitation wavelengths and consisted of two emission bands, one in the green-blue region (A-band) and the other in the red region (B-band), which were assigned to Eu(I) and Eu(II), respectively. The B-band resulted from a dramatic red-shift of the green emission band assigned to Eu(II) of SSO:Eu2+, revealing that the nitridation process preferentially affected the Eu(II) sites. This behavior was explained by crystal field splitting, the fluorescence decay time, and thermal quenching. The Ba2+ substitution caused evolution of the PL spectra, and its effects on the spectra were discussed under consideration of ionic size and covalence.