Material Properties and Structural Characterization of M3Si6O12N2:Eu2+ (M = Ba, Sr)-A Comprehensive Study on a Promising Green Phosphor for pc-LEDs

The efficient green phosphor Ba3Si6O12N2:Eu2+ and its solid-solution series Ba3-xSrxSi6O12N2 (with x approximate to 0.4 and 1) were synthesized in a radio-frequency furnace under nitrogen atmosphere at temperatures up to 1425 degrees C. The crystal structure (Ba3Si6O12N2, space group P (3) over bar (no. 147), a = 7.5218(1), c=6.4684(1) angstrom, wR2 = 0.048, Z = 1) has been solved and refined on the basis of both single-crystal and powder X-ray diffraction data. Ba3Si6O12N2:Eu2+ is a layer-like oxonitridosilicate and consists of vertex-sharing SiO3N-tetrahedra forming 6er- and 4er-rings as fundamental building units (FBU). The nitrogen atoms are connected to three silicon atoms (N-[3]), while the oxygen atoms are either terminally bound (O-[1]) or bridge two silicon atoms (O-[2]) (numbers in superscripted square brackets after atoms indicate the coordination number of the atom in question). Two crystallographically independent Ba2+ sites are situated between the silicate layers. Luminescence investigations have shown that Ba3Si6O12N2:Eu2+ exhibits excellent luminescence properties (emission maximum at approximate to 527 nm, full width at half maximum (FWHM) of approximate to 65 nm, low thermal quenching), which provides potential for industrial application in phosphor-converted light-emitting diodes (pc-LEDs). In-situ high-pressure and high-temperature investigations with synchrotron X-ray diffraction indicate decomposition of Ba3Si6O12N2 under these conditions. The band gap of Ba3Si6O12N2:Eu2+ was measured to be 7.05 +/- 0.25 eV by means of X-ray emission spectroscopy (XES) and X-ray absorption near edge spectroscopy (XANES). This agrees well with calculated band gap of 6.93 eV using the mBJ-GGA potential. Bonding to the Ba atoms is highly ionic with only the 4p(3/2) orbitals participating in covalent bonds. The valence band consists primarily of N and O p states and the conduction band contains primarily Ba d and f states with a small contribution from the N and O p states.