Real structure and diffuse scattering of Sr0.5Ba0.5Si2O2N2:Eu2+ - A highly efficient yellow phosphor for pc-LEDs

Sr0.5Ba0.5Si2O2N2:Eu2+ is a promising new phosphor for white light phosphor converted (pc)—LEDs. The material shows broad-band emission due to parity allowed 4f6(7F)5d → 4f7(8S7/2) transition in the yellow spectral range (λem ≈ 560 nm) while excited with UV to blue radiation. The X-ray powder diffraction pattern shows noticeable intensity maxima indicative for diffuse scattering from planar defects and also “missing” reflections compared to SrSi2O2N2:Eu2+. Rietveld refinement reveals an average structure of Sr0.5Ba0.5Si2O2N2:Eu2+ which is isotypic to that of SrSi2O2N2:Eu2+, with the latter representing a twinned layered oxonitridosilicate with disordered metal atoms. The average structure of Sr0.5Ba0.5Si2O2N2:Eu2+ was refined in space group P1 (no. 1) resulting in lattice parameters a = 7.2059(2), b = 7.3887(3), c = 7.3340(2) A, α = 88.524(4), β = 84.454(3), γ = 75.980(4)° and V = 377.07(2) A3. Based on the crystallographic results and considering lattice relaxation behavior as a consequence of lattice expansion, the observed unexpectedly large Stokes shift (as compared to SrSi2O2N2:Eu2+; 3573 vs. 3285 cm−1) can be explained using a least square fit of the emission spectra. With almost identical chromaticity coordinates with respect to the most frequently used commercial LED phosphor YAG:Ce3+ but significantly higher luminous efficacy (LE = 495 lm/W), Sr0.5Ba0.5Si2O2N2:Eu2+ is a promising material for outdoor lighting, e.g. in cool-white pc-LEDs. To elucidate the real structure, powder XRD simulations have been recorded compiling a disorder model taking into account all permutations of metal ion sets and silicate layer orientations. Experimental diffraction data were well reproduced including the diffuse intensities observed in powder XRD and also in SAED patterns. These simulations show that crystallites of Sr0.5Ba0.5Si2O2N2:Eu2+ are built up of small anti-phase domains within larger twin domains.