Luminescence Spectra of Single Crystals of Cs₂ZrCl₆:UO₂hCl₄²⁻ at Low Temperatures. Vibronic Structure of UO₂²⁺ Doped in a Cubic Host

Low-temperature (20 K-120 K) high-resolution luminescence spectra of UO₂Cl₄²⁻ doped in cubic crystals of Cs₂ZrCl₆ are reported. The luminescence is associated with the A_1g ← E_g electronic origin transition and vibronic transitions associated with this origin. The vibronic structure is consistent with the UO₂Cl₄²⁻ ion having D_4h symmetry. The distribution of intensity among members of vibronic progressions associated with the UO₂²⁺ stretching frequency was used to determine Huang-Rhys parameters for both the ground (A_1g) and excited (E_g) states. The parameters are consistent with a difference of U-O bond length of ca. 7 pm between the ground and excited states. The energy of the electronic origin transition is less than that of the neat CS₂UO₂Cl₄ crystal [Denning, R. G.; Snellgrove, T. R.; Woodwark, D. R. Mol. Phys. 1976, 32, 419–442] by some 400 cm⁻¹. This is explained by structural and Coulombic effects on the UO₂Cl₄²⁻ ion as it is accommodated in the Cs₂ZrCl₆ lattice.