Line broadening studies for U 3+ and U 4+ ions in RbY 2Cl 7 single crystals

Abstract

Temperature-dependent line broadening measurements of emission and excitation transitions for two intrinsic sites U(1) and U(2) of U 3+ ions doped in a RbY 2Cl 7 single crystals as well as of U 4+ ions have been performed. Values of the electron phonon (EP) coupling parameter α ̄ were determined by a fit of experimentally observed line widths to an equation containing the temperature dependent broadening term due to the Raman two-phonon process. The α ̄ parameters for U 3+ ions in RbY 2Cl 7 are larger than those determined for this ion in LaCl 3 host crystals. This is due to shorter M–Cl distances in RbY 2Cl 7 which leads to a stronger interaction of uranium with the chlorine ions and to an increase of covalency. The relatively large α ̄ value determined for the 2H2 11/2 multiplet of U 3+ in RbY 2Cl 7 may result from the proximity of opposite parity 5 f 26 d 1 states. The α ̄ parameters obtained for the U 3+ ions are larger than those for U 4+. The latter ones are affected by a stronger crystal-field (CF), however the position of the first 5 f 26 d 1 or 5 f 16 d 1 states, which for U 3+ is observed at an energy of ∼15,000 cm −1 lower than for U 4+, is the dominating one among the factors influencing the EP coupling strength. The EP coupling parameters for all investigated transitions of the U 3+ ions are larger for U(2) than for U(1), which results mainly from the larger crystal field strength observed for the U(2) site. The differences in the EP coupling strength of the U 3+ ions in the U(1) and U(2) sites are in accordance with decay times observed for emission for both sites from the 4F 9/2 multiplet.