Crystal and electronic structures, luminescence properties of Eu2+-doped Si6-zAlzOzN8-z and MySi6-zAlz-yOz+yN8-z-y (M=2Li, Mg, Ca, Sr, Ba)

The crystal structure, electronic structure, and photoluminescence properties of EuxSi6-zAlz-xOz+xN8-z-x (x = 0-0.1, 0 < z < 1) and EuxMySi6-zAlz-x-yOz+x+yN8-z-x-y (M = 2Li, Mg, Ca, Sr, Ba) have been studied. Single-phase EuxSi6-zAlz-xOz+xN8-z-x can be obtained in very narrow ranges of x <= 0.06 (z = 0.15) and z < 0.5 (x = 0.3), indicating that limited Eu2+ ions can be incorporated into nitrogen-rich Si6-zAlzOzN8-z. The Eu2+ ion is found to occupy the 2b site in a hexagonal unit cell (P6(3)/M) and directly connected by six adjacent nitrogen/oxygen atorns ranging 2.4850-2.5089 angstrom. The calculated host band gaps by the relativistic DV-X alpha method are about 5.55 and 5.45 eV (without Eu2+ 4f5d levels) for x = 0 and 0.013 in EuxSi6-zAlz-xOz+xN8-z-x (z = 0.15), in which the top of the 5d orbitals overlap with the Si-3s3p and N-2p orbitals within the bottom of the conduction band of the host. EuxSi6-zAlz-xOz+xN8-z-x shows a strong green emission with a broad Eu2+ band centered at about 530 nm under UV to near-UV excitation range. The excitation and emission spectra are hardly modified by Eu concentration and dual-doping ions of Li and other alkaline-earth ions with Eu. Higher Eu concentrations can significantly quench the luminescence of Eu2+ and decrease the thermal quenching temperature. In addition, the emission spectrum can only be slightly tuned to the longer wavelengths (similar to 529-545 nm) by increasing z within the solid solution range of z < 0.5. Furthermore, the luminescence intensity of EuxSi6-zAlz-xOz+xN8-z-x can be improved by increasing z and the dual-doping of Li and Ba. (C) 2008 Elsevier Inc. All rights reserved.