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 Ce(3+)-Ce(4+) pairs, from the 5de(g) orbital of Ce(3+) to 4f orbitals of Ce(4+). Its Stokes shift is the sum of the large reorganization energies of the Ce(4+) and Ce(3+) 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 Ce(3+)-Ce(4+) and Ce(4+)-Ce(3+) pairs along the electron transfer reaction coordinate. This offset is approximately 2√3 times the difference of Ce-Cl equilibrium distances in the Ce(3+) and Ce(4+) centers. It is shown that the energies of the peaks and the widths of IVCT absorption and emission bands can be calculated ab initio with reasonable accuracy from diabatic energy surfaces of the ground and excited states and that these can be obtained, in turn, from independent calculations on the donor and acceptor active centers. We obtained the energies of the Ce(3+) and Ce(4+) active centers of Ce-doped Cs2LiLuCl6 by means of state-of-the-art wave-function-theory spin-orbit coupling relativistic calculations on the donor cluster (CeCl6Li6Cs8)(11+) and the acceptor cluster (CeCl6Li6Cs8)(12+) embedded in a quantum mechanical embedding potential of the host. The calculations provide interpretations of unexplained experimental observations as due to higher energy IVCT absorptions, and allow to reinterpret others. The existence of another IVCT emission of lower energy, at around 14,000-16,000 cm(-1) less than the 5dt(2g) emission, is also predicted.