Crystal-field analysis, upconversion, and excited-state dynamics for(U4+,U3+):Ba2YCl7single crystals

Abstract

Single crystals of $({\mathrm{U}}^{4+},{\mathrm{U}}^{3+}):{\mathrm{Ba}}_{2}{\mathrm{YCl}}_{7}$ were grown by the Bridgman-Stockbarger technique. The ${\mathrm{U}}^{4+}$ and ${\mathrm{U}}^{3+}$ ions substitute for ${\mathrm{Y}}^{3+}$ and have ${C}_{1}$ site symmetry in ${\mathrm{Ba}}_{2}{\mathrm{YCl}}_{7}.$ Sixty energy levels of ${\mathrm{U}}^{4+}$ were assigned to $40000{\mathrm{cm}}^{\ensuremath{-}1}$ from absorption, excitation, and luminescence spectra, which encompasses all but the ${}^{1}{S}_{0}$ multiplet. A crystal-field analysis was performed by fitting 8 atomic and 19 crystal-field parameters to the experimental Stark levels. The values obtained from the fit had a standard deviation of $36.6{\mathrm{cm}}^{\ensuremath{-}1}$ and agreed with those calculated from the angular overlap model. Efficient luminescence was observed at 7 K from ${\mathrm{the}}^{4}{G}_{7/2}$ and ${}^{4}{F}_{9/2}$ levels of the ${\mathrm{U}}^{3+}$ ions and from the ${}^{1}{D}_{2}$ and ${}^{1}{I}_{6}$ of the ${\mathrm{U}}^{4+}$ ions with decay times of 0.5, 0.55, 9.2, and 4.3 $\ensuremath{\mu}\mathrm{s},$ respectively. For the ${\mathrm{U}}^{4+}$ ions, a very strong anti-Stokes emission was observed due to energy-transfer processes. The emission of the ${\mathrm{U}}^{3+}$ ions is strongly quenched by temperature in contrast to that of the ${\mathrm{U}}^{4+}$ ions for which emission was observed even at room temperature. Energy transfer between the ${\mathrm{U}}^{3+}$ and ${\mathrm{U}}^{4+}$ ions has been shown to occur in the $({\mathrm{U}}^{4+},{\mathrm{U}}^{3+})$ doped ${\mathrm{Ba}}_{2}{\mathrm{YCl}}_{7}.$