Anomalous luminescence of Eu²⁺ and Yb²⁺ in inorganic compounds

In many compounds the broadband emission of Eu²⁺ and Yb²⁺ is subject to a very large (0.6–1.2 eV) Stokes shift and it behaves peculiarly with temperature change. Conduction band states of the host compound are involved in this 'anomalous' emission. Cases of anomalous emission are identified and the conditions for it to occur studied. Clear trends with the size of the lanthanide ion, the size of the site occupied, the size of anions in the compound, and the binding strength of oxygen ligands were found. The trends are interpreted by models involving the Madelung potential and Pauling repulsion at the lanthanide site together with the Coulomb and isotropic exchange interactions within the lanthanide ion. The results provide information on the approximate location of the lowest 4fⁿ⁻¹5d level relative to the bottom of the conduction band. The systematic variation with type of lanthanide and host lattice is discussed. Combining the results with information on the systematic variation in the fd transition energies, all energy levels for each divalent lanthanide can be approximately positioned relative to the conduction and valence band.