Luminescence properties of Ce3+-activated alkaline earth silicon nitride M2Si5N8 (M=Ca, Sr, Ba) materials

The luminescence properties of Ce3+, Li+ or Na+ co-doped alkaline-earth silicon nitride M2Si5N8 (M=Ca, Sr, Ba) are reported. The solubility of Ce3+ and optical properties of M2-2xCexLixSi5N8 (x=<0.1) materials have been investigated as function of the cerium concentration by X-ray powder diffraction analysis and fluorescence spectroscopy. X-ray diffraction results show that the maximum solubility of Ce3+ in M2Si5N8 is about 2.5mol% (x∼0.05) for both Ca2Si5N8 and Sr2Si5N8, and at 1.0mol% (x=<0.02) for Ba2Si5N8. The Ce3+-activated M2Si5N8 phosphors exhibit broad emission bands with maxima at 470, 553 and 451nm for M=Ca, Sr, Ba, respectively, due to the 5d->4f transition of Ce3+. In addition, M2Si5N8:Ce3+, Li+ (M=Sr, Ba) obviously shows two Ce3+ emission centers due to the fact that the Ce3+ ions occupy two M sites. With increasing Ce3+ concentration, both absorption and emission intensity increase and the position of the emission bands show a slight red-shift (<10nm). The influence of using Na+ instead of Li+ ion as charge compensator on emission and excitation properties is small but Na+ enhances the emission intensity because of larger solubility of Ce3+ in M2Si5N8 (M=Ca, Sr). With increasing the ionic radius going from Ca to Sr and Ba, the ratio of the emission intensity to the absorption intensity of Ce3+ decreases related to a decreasing Ce3+ solubility. An intense absorption and excitation band in the blue range (370-450nm) in M2Si5N8:Ce, Li(Na) (M=Ca, Sr) points out that these materials are promising conversion phosphors for white-light LEDs