5d Levels of rare-earth ions in oxynitride/nitride phosphors: To what extent is the idea covalency reliable?

It is often said that promising phosphors for white-LEDs are Ce3+/Eu2+-doped multinary oxynitrides/nitrides, mainly because they are more covalent than conventional oxide phosphors so as to be effectively excited by blue or near-ultra-violet light. In this paper, detailed discussions are given to clarify factors affecting 5d levels of the rare-earth ions in nitride/oxynitride phosphors by examining some examples: LaSi3N5:Ce (blue phosphor)/La3Si6N11:Ce (yellow phosphor) and BaSi2O2N2:Eu (bluish green phosphor)/Ba3Si6O9N4:Eu (bluish green phosphor)/Ba3Si6O12N2:Eu (green phosphor). It is stressed that dielectric constant/refractive index (as an indicator of polarizability of ligand) along with coordinate bond length should be examined rather than covalency to grasp the trend of 4f–5d transitions. For this purpose, some useful rules-of-thumb are addressed.