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Uranium (3+) doped single crystals of Cs2NaYCl6 and Cs2LiYCl6 with a 2.0% and 0.1% U3+ concentration have been obtained by the Bridgman-Stockbarger method. Luminescence spectra of the crystals were recorded at 160, 70, and 15 K. The emission bands observed in the visible and near infrared regions have been assigned to transitions from the lowest components of the I11/24, F3/24, and G7/24 multiplets to the crystal-field components of the I9/24 ground level. Absorption spectra were recorded from 4 000 to 25 000 cm−1 at 4.2 K. The zero phonon transitions were identified from an analysis of the vibronic side bands as well as in least-squares fits by applying a semiempirical Hamiltonian representing the combined atomic and crystal-field interactions for an ion of the 5f3 configuration and Oh symmetry. Eight energy level parameters were varied simultaneously in the least-squares adjustments yielding a mean error of 57 and 61 cm−1 for U3+:Cs2LiYCl6 and U3+:Cs2NaYCl6, respectively. The analysis of the spectra enabled the assignment of 25 crystal field levels for U3+:Cs2NaYCl6 and 27 for U3+:Cs2LiYCl6. The received total splitting of the ground level is equal to 625 and 595 cm−1, respectively.

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Laser selective excitation and emission spectra as well as σ- and π-polarized absorption spectra have been recorded for U3+ ions diluted in a CsCdBr3 host crystal. Three distinct U3+ sites were observed. For the principal site, which was attributed to a symmetric dimer center, 45 crystal-field energy levels in the 0−13250 cm-1 energy range were assigned. These experimentally determined levels were fitted to thirteen parameters of a semiempirical Hamiltonian representing the combined atomic, one-electron crystal field as well as two-particle correlation crystal-field (CCF) operators, with an rms deviation of 41 cm-1. Above 14000 cm-1 strong electric-dipole allowed 5f3−5f26d transitions were observed. The lifetime of the 4G7/2 fluorescing level has been measured for each of the three U3+ sites at liquid-helium temperatures and are 2.5, 4.3, and 7.6 μs. The emission is strongly influenced by temperature due to strong phonon coupling of the 5f3 states with the nearby 5f6d1 states. Visible upconversion fluorescence observed when pumping the 4I9/2 − (4F7/2 + 4I15/2) absorption transitions of the principal site A is attributed to excited-state absorption (ESA).