First-Principles Study on Structural Properties and 4f → 5d Transitions of Locally Charge-Compensated Ce3+ in CaF2

The structural properties and 4f → 5d transitions of Ce3+ in CaF2, with local charge compensation by an interstitial fluoride (Fi′) or an oxygen substitution for fluoride (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 results indicate that the incorporation of locally charge-compensated Ce3+ in CaF2 induces an anisotropic distortion of the structure around the dopant site. On the basis of the DFT-optimized structures, the Ce-centered embedded clusters are constructed, on which the wave function-based CASSCF/CASPT2/RASSI–SO calculations at the spin–orbit level are performed to obtain the Ce3+ 4f1 and 5d1 level energies. The calculated 4f–5d transition energies and relative intensities are in good agreement with available experimental results. From the present calculations, we conclude that the 5d1 level missing in the low-temperature absorption spectrum of the tetragonal Ce center with Fi′ compensation is the second-lowest one, and the absorption at this level is overshadowed by an adjacent cluster band usually assigned to Ce clusters and thus was not observed in experiments. We also assign the two closely spaced absorption lines around 3118.5 Å observed in experiments to the lowest two quasi-degenerated 4f → 5d transitions of the monoclinic center with Fi′ compensation rather than those of the trigonal center as proposed earlier. Finally, we analyze the structural and electronic reasons for the large reduction (∼2000 cm–1) of the lowest 4f → 5d transition energy from a Fi′ to a nearest-neighbor OF′ compensation, in terms of the changes in the centroid energy difference and crystal-field splitting.