First-principles study on site preference and 4f → 5d transitions of Ce(3+) in Sr3AlO4F

The local structures and 4f → 5d transition energies of Ce(3+) located on the two crystallographic strontium sites of Sr3AlO4F, with charge compensation by means of nearby sodium substitutions for strontium (NaSr') or oxygen substitutions for coordinating fluorine (OF'), have been studied using the density functional theory (DFT) within the supercell model and the wave function-based embedded cluster calculations, respectively. The DFT total energy calculations show that Ce(3+) prefers strongly to occupy the eight-coordinated (Sr2) site over the ten-coordinate (Sr1) site. On the basis of the results from embedded cluster calculations at the CASPT2 level with the spin-orbit effect, the experimentally observed excitation bands are identified in association with the charge-compensated cerium centers. Especially, the two bands observed at ∼404 and ∼440 nm have been both assigned to the Ce(3+) located at the Sr2 sites but with compensation by one and two nearest-neighbor OF' substitutions, respectively, rather than to the Ce(3+) on the Sr1 and the Sr2 sites, respectively, as proposed earlier. Furthermore, the structural and electronic reasons for the red shift of the lowest 4f → 5d transition caused by coordinating OF' substitutions are analyzed in terms of the variations in centroid energy and crystal-field splitting of the 5d(1) configuration with the local environment. Finally, the thermal quenching of 5d luminescence at relatively high Ce(3+) concentrations is discussed on the basis of the electronic properties calculated with the hybrid DFT method.