Analysis of vacuum ultraviolet electronic spectra of Ce³⁺ and Pr³⁺ ions in Ca₉Lu(PO₄)₇: Crystal-field calculations and simulation of optical spectra

The 4f-5d excitation and emission spectra of Ce³⁺ and Pr ³⁺ ions in Ca₉Lu(PO₄)₇ as recently reported (2012 J. Phys.: Condens. Matter 24 385502) were further analyzed and simulated by employing the effective Hamiltonian model for the 4f^N and 4f^N-15d electronic configurations of impurity lanthanide ions and the exchange charge model of crystal-field theory. The multi-site effect on the 4f-5d transition spectra was explicitly discussed from the points of view of the local structure and site occupation ratios of lanthanide ions in Ca ₉Lu(PO₄)₇. An excellent agreement between the predicted and measured spectra confirms the validity of the performed calculations. Based on these energy level and intensity calculation results, the radiative lifetimes of the 5d-4f emissions of Ce³⁺ and Pr ³⁺ ions have been modeled to show nearly independent temperature trends. Comparison with the measured lifetimes suggests the nonradiative relaxation process in this host is probably related to the intrinsic defect states. In addition to the studies of the 4f-5d transitions, a general theoretical scheme to calculate the lowest 4f-6s transition energy of the Ce³⁺ ion was proposed for the first time on the basis of the ligand polarization model. The predicted 6s energy position of the Ce³⁺ ion in Ca₉Lu(PO₄)₇ is solid evidence corroborating our previous spectroscopic assignment.