4f-5d transitions of Pr³⁺ in elpasolite lattices

The 4f5d→4f² emission spectra of Cs₂MPrCl₆ (M = Na,Li) and CS₂NaYCl₆:Pr³⁺ have been recorded at temperatures down to 10 K. The spectra of Pr³⁺ in the cubic host Cs₂NaYCl₆ are the most clearly resolved, and 15 transitions to terminal crystal field levels of symmetry representations Γ_5g and Γ_4g have been observed and assigned, thereby inferring that the symmetry representation of the lowest 4f5d crystal field level is Γ_3u. Each transition is characterized by strong progressions in two totally symmetric vibrational modes. The relative displacement of the potential energy curves for the 4f² and 4f5d crystal field levels, along the α_1g internal mode coordinate, is small, being only about 5 pm. The 10-K ultraviolet absorption spectra of CS₂NaYCl₆:Pr³⁺ are assigned to transitions from the [³H₄] Γ_1g electronic ground state to terminal Γ_4u crystal field levels of 4f5d. Nontotally symmetric gerade vibrational modes only provide minor intensity contributions. The large energy gap between the d-f emission and f-d absorption spectra of Pr³⁺ in the cubic elpasolite host is rationalized. The 8-K excitation spectra of Cs₂NaPrCl₆ and Cs₂NaYCl₆:Pr³⁺, excited by synchrotron radiation, show that the transitions to 4f5d fall into two groups. The energy levels and wave vectors of the (independent) 4f² and 4f5d configurations of Pr³⁺ have been calculated using a model which includes spin-orbit coupling and crystal field and Coulomb interactions, as well as the configuration interaction of 4f² with 4f6p. Using the eigenvector of the predominantly high-spin, lowest excited crystal field level of 4f5d, the emission intensities are reasonably well simulated. However, the refinement of the 4f²→4f5d absorption intensities requires a more detailed knowledge of the crystal field energy level scheme of 4f5d. The configuration interaction of 4f5d with 4f6s and 4f5g is discussed.