On the Ce3+ luminescence in the melilite-type oxide nitride compound Y2Si3-xAlxO3+xN4-x

Ce3+ luminescence was studied in the system Y1.98Ce0.02Si3-xAlxO3+xN4-x wit h a weighed-out x of 0, 0.25, 0.5, 0.6, and I. The lattice parameters show a nearly linear increase between x = 0 and x = 0.6, indicating an increasin g substitution of Si-N by the larger Al-O in the melilite-type lattice. The : solubility limit is near x = 0.6; for higher values of x, the lattice par ameters remain constant. The luminescence spectra show typical Ce3+ lumines cence (excitation maxima at 310 and 390 nm and emission maximum near 475 nm ). No shifts in the excitation spectrum and only slight shifts in the emiss ion for increasing x were observed, indicating that a change in the overall composition does not affect the local coordination of the Ce3+ ion. This i s explained by the preferential occupation of the large Ce3+ ion on "roomie r" O-rich sites, as compared with the average coordination around Y3+ in Y2 Si3O3N4. As a result of the preferential Ce3+ coordination, extra O2-, intr oduced with the incorporation of Al-O in Y2Si3-xAlxO3+xN4-x, will substitut e on N-richer sites, which preferentially coordinate with the smaller Y3+ i on. The minimal shift of the emission spectrum results in a slightly larger Stokes shift (from about 4200 to 4300 cm(-1)), which suggests a decreasing rigidity of the host-lattice for increasing substitution of Si-N by Al-O. This is explained by the preferential substitution of Al on Si sites near C e3+, which counterbalances the deficit in negative charge due to extra oxyg en versus nitrogen in the Ce3+ coordination. (C) 2000 Elsevier Science Ltd, All rights reserved.