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

• Published in: Chemistry of materials Vol. 25; no. 9; pp. 1852 - 1857
• Main Authors: Seibald, Markus, Rosenthal, Tobias, Oeckler, Oliver, Maak, Christian, Tücks, Andreas, Schmidt, Peter J, Wiechert, Detlef, Schnick, Wolfgang
• Format: Journal Article
• Language: English; Japanese
• Published: American Chemical Society 14.05.2013
• Subjects: luminescence; structure determination; oxonitridosilicate; TEM
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm400461v

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 (P21, Z = 8, a = 7.1036(14), b = 14.078(3), c = 7.2833(15) Å, β = 95.23(3)°, V = 725.3(3) Å3). 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 ∼2600 cm–1) as determined by single-crystal luminescence measurements of the monoclinic phase exhibits a shift to smaller wavelengths by ∼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.