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

Abstract

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.