Intervalence charge transfer luminescence: The anomalous luminescence of cerium-doped Cs₂LiLuCl₆ elpasolite

The existence of intervalence charge transfer (IVCT) luminescence is reported. It is shown that the so called anomalous luminescence of Ce-doped elpasolite Cs₂LiLuCl₆, which is characterized mainly by a very large Stokes shift and a very large band width, corresponds to an IVCT emission that takes place in Ce³⁺–Ce⁴⁺ pairs, from the 5de_g orbital of Ce³⁺ to 4f orbitals of Ce⁴⁺. Its Stokes shift is the sum of the large reorganization energies of the Ce⁴⁺ and Ce³⁺ centers formed after the fixed-nuclei electron transfer and it is equal to the energy of the IVCT absorption commonly found in mixed-valence compounds, which is predicted to exist in this material and to be slightly larger than 10 000 cm⁻¹. The large band width is the consequence of the large offset between the minima of the Ce³⁺–Ce⁴⁺ and Ce⁴⁺–Ce³⁺ pairs along the electron transfer reaction coordinate. This offset is approximately