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. (C) 2010 Elsevier B.V. All rights reserved.