Optical absorption and luminescence spectroscopy of U3+ in K2LaX5 (X=Cl,Br,I).

Abstract

The title compounds were synthesized and high-resolution absorption and luminescence spectra measured in the near-infrared, VIS, and near UV regions. The visible absorption spectra are dominated by very intense 5f\ensuremath{\rightarrow}6d bands overlapping with f-f transitions. The onset of the first f-d absorption is shifted from 46 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in ${\mathrm{K}}_{2}$${\mathrm{LaCl}}_{5}$:${\mathrm{Nd}}^{3+}$ to 15 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in ${\mathrm{K}}_{2}$${\mathrm{LaCl}}_{5}$:${\mathrm{U}}^{3+}$. Crystal-field splittings in corresponding $^{2\mathit{S}+1}$${\mathit{L}}_{\mathit{J}}$ multiplets are greater by typically a factor of 2 in the ${\mathrm{U}}^{3+}$ doped crystal, thus reflecting the larger extension and stronger interaction of the 5f electrons with the ligands. f-f transitions are typically two orders of magnitude more intense in ${\mathrm{K}}_{2}$${\mathrm{LaCl}}_{5}$:${\mathrm{U}}^{3+}$ than in ${\mathrm{K}}_{2}$${\mathrm{LaCl}}_{5}$:${\mathrm{Nd}}^{3+}$. Along the halide series ${\mathrm{K}}_{2}$La${\mathit{X}}_{5}$:${\mathrm{U}}^{3+}$ (X=Cl,Br,I) the differences in the position of corresponding f-d and f-f transitions, crystal-field splittings, vibronic intensities, and excited-state lifetimes can be explained with the increasing covalency, the decreasing phonon energies, the increasing electron-phonon coupling, and the increasing U-X distances. The f-d excited states provide a nonradiative bypass of some f-f excited states in the case of all these halide lattices. The excited-state dynamics are determined by a delicate interplay of radiative and nonradiative relaxation processes, they are strongly dependent on the nature of X. Multiphonon relaxation processes are least competitive in the iodide due to the very low value of 106 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for the highest-energy phonons. A cross-relaxation mechanism determines the dynamics of the iodide at room temperature. \textcopyright{} 1996 The American Physical Society.