Configuration coordinate energy level diagrams of intervalence and metal-to-metal charge transfer states of dopant pairs in solids

Configuration coordinate diagrams, which are normally used in a qualitative manner for the energy levels of active centers in phosphors, are quantitatively obtained here for intervalence charge transfer (IVCT) states of mixed valence pairs and metal-to-metal charge transfer (MMCT) states of heteronuclear pairs, in solid hosts. The procedure relies on vibrational frequencies and excitation energies of single-ion active centers, and on differences between ion-ligand distances of the donor and the acceptor, which are attainable empirically or through ab initio calculations. The configuration coordinate diagrams of the Yb2+/Yb3+ mixed-valence pair in Yb-doped YAG and the Ce3+/Yb3+ heteronuclear pair in Ce, Yb-codoped YAG, are obtained and described. They are drawn from empirical data of the single-ions and their usefulness is discussed. The first diagram suggests that IVCT states of Yb2+/Yb3+ pairs may play an important role in the quenching of the Yb3+ emission and it provides the details of the quenching mechanism. The second diagram supports the interpretation recently given for the energy transfer from Ce3+ to Yb3+ in Ce, Yb-codoped YAG via a MMCT Ce4+-Yb2+ state and it provides the details. The analyses of the two diagrams suggest the formation of Yb2+/Yb3+ pairs after the Ce3+-to-Yb3+ MMCT, which is responsible for the temperature quenching of the Yb3+ emission excited via Ce3+ (4f -> 5d) absorption in Ce, Yb-codoped YAG.