Explanation for the variance of the Ce3+ emission energy in LnI3 [Ln = Lu3+, Y3+, Gd3+]

The experimental result that the energy of the emitted photon increases with decreasing ionic radii of the host lattice is a remarkable feature of the Ce3+ luminescence in the isostructural LnI3 [Ln3+ = Lu3+, Y3+, Gd3+] family of materials. To understand this variation, the optical properties of YI3:Ce3+ is measured and compared with those reported for Ce3+ activated LuI3 and GdI3. The results indicate that the crystal field splitting and the centroid shift of the Ce3+ 5d1 electronic configuration in these iodides are of the same order of magnitude. The lowest energy Ce3+ 4f1 → 5d1 excitation transition in LnI3 [Ln3+ = Lu3+, Y3+, Gd3+] is practically uninfluenced by the ionic radii of the host lattice cation, which the Ce3+ ion substitutionally replaces. The increase in energy of the emitted photon with decreasing ionic radii of the host lattice is a result of the increasing Stokes shift of the Ce3+ emission in the sequence LuI3–YI3–GdI3. The quenching temperature of the Ce3+ emission in YI3 is high.