New Polymorph of the Highly Efficient LED-Phosphor SrSi2O2N2:Eu2+ - Polytypism of a Layered Oxonitridosilicate

SrSi2O2N2:Eu2+ is an outstanding yellow emitting phosphor material with practical relevance for application in high power phosphor-converted light-emitting diodes. The triclinic compound exhibits high thermal and chemical stability and quantum efficiency above 90% and can be excited by GaN-based UV to blue LEDs efficiently. We have now discovered a hitherto unknown monoclinic polymorph of SrSi2O2N2, synthesized by solid-state reaction, which is characterized by an alternating stacking sequence of silicate layers made up of condensed SiON3 tetrahedra and metal ion layers. As proven by single crystal X-ray diffraction, the arrangement of the silicate layers is significantly different from the triclinic polymorph. The translation period along the stacking direction is doubled in the monoclinic modification (P2(1), Z = 8, a = 7.1036(14), b = 14.078(3), c = 7.2833(15) angstrom, beta = 95.23(3)degrees, V = 725.3(3) angstrom(3)). TEM investigations in combination with HRTEM-image simulations confirm the structure model The powder X-ray diffraction pattern shows that the volume fractions of the monoclinic and triclinic modifications are approximately equal in the corresponding powder sample. The emission wavelength of 532 nm (fwhm similar to 2600 cm(-1)) as determined by single crystal luminescence measurements of the Monoclinic phase exhibits a shift to smaller wavelengths by similar to 5 nm compared to the triclinic polymorph. Differences of the luminescence properties between the monoclinic and triclinic phase are interpreted with respect to the differing coordination of Eu2+ in both phases. The new monoclinic SrSi2O2N2:Eu2+ polymorph is a very attractive phosphor material for the enhancement of color rendition of white-light pc-LEDs.