LUMINESCENCE SPECTRA OF SINGLE-CRYSTALS OF CS2ZRCL6-UO2CL42- 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 (Eg) 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.