Long wavelength Ce3+ emission in Y-Si-O-N materials

An investigation of new luminescent materials, Y5(SiO4)3N:Ce, Y4Si2O7N2:Ce, YSiO2N:Ce and Y2Si3O3N4:Ce is presented. In a series of these oxynitride materials, long wavelength emission of Ce3+ is observed, the exact position determined by the nephelauxetic effect, the crystal-field splitting and the Stokes shift. It was found that the crystal-field splitting becomes larger when more N3− versus O2− coordinates to Ce3+. This is ascribed to the higher formal charge of N3− compared to O2−. Furthermore, several parameters, like increased nitrogen versus oxygen coordination, more non-bridging versus bridging nitrogen coordinated to Ce3+, as well as its coordination by free oxygen ions, contribute to the increase of the covalency, which shifts the centre of gravity of the 5d state to lower energy (nephelauxetic effect). The Stokes shift is observed to become smaller for lattices with a more extended silicon network formation when more N3− is incorporated. The smaller Stokes shift is ascribed to the increasing rigidity of the lattice. The energy difference between the lowest 5d excitation band of Tb3+ and that of Ce3+ amounts to 12–15∗103 cm−1, which is in agreement with literature.