Modification of the crystal structure of Sr2-xBaxSi(O,N)4: Eu2+ phosphors to improve their luminescence properties

A series of Sr1.98-xBaxSi(O,N)4: 0.02Eu(2+) (0 <= x <= 0.5) phosphors were synthesized by a conventional solid state reaction method. The Sr-N and Si-N bonds could be observed in FT-IR spectra. The XRD refinement results indicated that N3- would substitute for O2(2-) and form Si-NO3 tetrahedrons during the process of forming the Sr1.98Si(O,N)(4): Eu2+ (SSON: Eu2+) substitutional solid solution. The lattice constants of Sr1.98-xBaxSi(O,N)(4): 0.02Eu(2+) (SBSON) were increased due to the longer bond length of Ba-O. Compared with Sr2SiO4: Eu2+, the SSON: Eu2+ showed a remarkable red-shift, which is due to the nephelauxetic effect and the strong crystal field splitting originating from the stronger covalent bonding effect of the Eu-N bond. The SSON: Eu2+ presented a beta phase structure before the introduction of Ba2+, whereas the alpha'-SBSON phase was obtained by the substitution of Ba2+ for Sr2+. Ba2+ doping led to an obvious blue-shift under 375 nm and 460 nm excitation. The 5d orbital of Ba2+ is coupled with the 5d orbital of Eu2+ on the higher energy level position in the host crystal. Under 375 nm excitation, the PL intensity gradually increased with the increase of the Ba2+ content. Under 460 nm excitation, the PL intensity gradually declined with the increase of Ba2+ content. The thermostability of alpha'-SBSON: Eu2+ was significantly improved compared with beta-SSO: Eu2+ and beta-SSON: Eu2+. On the basis of their adjustable emission wavelength, enhanced PL intensity and excellent thermostability in SBSON: Eu2+, we anticipate that these materials can be used as green or red phosphors in white light emitting diodes.