Structure, optical absorption, and luminescence energy calculations of Ce3+ defects in LiBaF3

We address two remarkable features in the optical behavior of Ce3+ defects in LiBaF3: the fourfold splitting of the Ce3+ 5d manifold in a cubic system , and the unusually large Stokes shift, of around 1 eV (approximate to 9000 cm(-1)), between the energy of the lowest Ce3+ 4f --> 5d absorption line a nd its 5d --> 4f luminescence energy. To this end we investigated the elect ronic properties and the structure of several possible luminescence center configurations in LiBaF3:Ce3+, each consisting of a Ce3+ substitution at a Ba or Li site, plus an appropriate charge-compensating defect. Using a plan e-wave pseudopotential density-functional-based method to optimize the geom etry of a supercell consisting of 3 x 3 x 3 LiBaF3 unit cells, containing a single luminescence center, the equilibrium structures of these defect com plexes were determined. We performed nb initio cluster calculations at the Hartree-Fock level to determine the optical-absorption energies of the Ce3 4f --> 5d transitions in these different geometries. Comparison of these e nergies with the results of optical-absorption measurements on LiBaF3:Ce3shows that the most likely luminescence center configuration consists of Ce 3+ at a Ba site, charge compensated by the substitution of one of its neare st-neighboring Ba ions by a Li+ ion. For this configuration we have repeate d the cluster and supercell calculations with Ce3+ in the [Xe]5d(1) excited -state electronic configuration to determine the Ce3+ 5d-->4f luminescence energy and to study effects that can explain the large Stokes shift in this material. These calculations predict an extensive lattice relaxation, indu ced by the excitation of the Ce3+ ion, and yield a Stokes shift of 0.61 eV (compared to 1 eV found from experiment). The origin of this large Stokes s hift is identified as a strong coupling of the crystal-field splitting of t he Ce3+ 5d manifold to the displacement of four of its F nearest neighbors.