Long wavelength Ce 3+ emission in Y–Si–O–N materials

Abstract

An investigation of new luminescent materials, Y 5(SiO 4) 3N:Ce, Y 4Si 2O 7N 2:Ce, YSiO 2N:Ce and Y 2Si 3O 3N 4:Ce is presented. In a series of these oxynitride materials, long wavelength emission of Ce 3+ 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 N 3− versus O 2− coordinates to Ce 3+. This is ascribed to the higher formal charge of N 3− compared to O 2−. Furthermore, several parameters, like increased nitrogen versus oxygen coordination, more non-bridging versus bridging nitrogen coordinated to Ce 3+, 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 N 3− 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 Tb 3+ and that of Ce 3+ amounts to 12–15∗10 3 cm −1, which is in agreement with literature.