  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 in Ce3+-Ce4+
pairs, from the 5deg 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 10000 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 2*sqrt(3) times the difference of
Ce-Cl equilibrium distances in the Ce3+ and Ce4+ 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 Ce3+ and Ce4+ 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 reinterpreting
others. The existence of another IVCT emission of lower energy, at around
14000-16000 cm-1 less than the 5dt2g emission, is also predicted.
