5f - 5f transitions of U4+ ions in high-field, octahedral fluoride coordination: The Cs2GeF6 : U4+ crystal

摘要

The U-F bond length, totally symmetric vibrational frequency, and 5f(2) energy levels of the Cs2GeF6:U4+ crystal are predicted through quantum-chemical calculations on the embedded (UF6)(2-) cluster. The U4+ ions substitute for much smaller Ge4+ retaining octahedral site symmetry, which is useful to interpret the electronic transitions. The structure of the 5f(2) manifold: its energy range, the crystal splitting of the 5f(2) levels, their parentage with free-ion levels, and the energy gaps appearing within the manifold, is presented and discussed, which allows to suggest which are the possible 5f(2) luminescent levels. The effects of Cl-to-F chemical substitution are discussed by comparison with isostructural Cs2ZrCl6:U4+. The energy range of the 5f(2) manifold increases by some 6000 cm(-1) and all levels shift to higher energies, but the shift is not uniform, so that noticeable changes of order are observed from Cs2ZrCl6:U4+ to Cs2GeF6:U4+. The comparison also reveals that the green-to-blue up-conversion luminescence, which has been experimentally detected and theoretically discussed on Cs2ZrCl6:U4+, is quenched in the fluoride host. The results of the Cs2GeF6:U4+ are used as a high-symmetry model to try to understand why efficient radiative cascade emissions in the visible do not occur for charged U4+ defects in low-symmetry YF3 crystals. The results presented here suggest that theoretical and experimental investigations of 4f/5f ions doped in octahedral, high-symmetry fluoride crystals may be conducted even when the mismatch of ionic radii between the lanthanide/actinide ions and the substituted cations of the host is considerably large. Investigations of these new materials should reveal interesting spectroscopic features without the difficulties associated with more commonly used low-symmetry fluoride hosts. (c) 2005 American Institute of Physics.