Energy of the Eu2+ 5d state relative to the conduction band in compounds

Trends in the energy of the lowest Eu2+ 4f6 5d level relative to the energy at the bottom of the conduction band with type of compound are identified. By selecting 12 compounds with Eu on an Y3+ site, six compounds with Eu on a Ca2+ site and Eu on monovalent alkaline sites in KCl and RbCl the effect of the charge (3+, 2+, or 1+) of the site occupied and the strength of binding of oxygen valence electrons is studied. Very clear trends are observed. A model is presented to show that the energy associated with an electron in the 5d state of a divalent lanthanide ion on a trivalent rare earth site is always higher or close to the energy associated with the bottom of the conduction band. The implications for the performance of lanthanide based phosphors are discussed.