Direct Evidence of Intervalence Charge-Transfer States of Eu-Doped Luminescent Materials.

Direct evidence is given for the existence of intervalence charge transfer (IVCT) states of Eu/Eu pairs in Eu-doped CaF, SrF, and BaF. They are detected in diffuse reflectance spectra. In doped materials, IVCT states, in which an electron transfer occurs between two metal sites differing only in oxidation state, are rather difficult to observe because the absorption bands are extremely broad and flat, their intensity is low, and no emission follows the IVCT absorptions. Their assignment as IVCT states is provided by state-of-the-art multiconfigurational ab initio calculations. Although IVCT states of lanthanide-doped materials have largely been overlooked so far, they can cause luminescence quenching and even complete luminescence excitation loss. Their direct observation and independent assignment in classical dopant (Eu) and hosts (CaF, SrF, BaF) are very significant: They suggest that the occurrence of IVCT states in other lanthanide-activated materials is very likely overlooked and their impact is ignored.