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

Abstract

A series of Sr_1.98−xBa_xSi(O,N)₄: 0.02Eu²⁺ (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 O2²⁻ and form Si–NO₃ tetrahedrons during the process of forming the Sr_1.98Si(O,N)₄: Eu²⁺ (SSON: Eu²⁺) substitutional solid solution. The lattice constants of Sr_1.98−xBa_xSi(O,N)₄: 0.02Eu²⁺ (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 β phase structure before the introduction of Ba²⁺, whereas the α′-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 α′-SBSON: Eu²⁺ was significantly improved compared with β-SSO: Eu²⁺ and β-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.