Comparison between electron–phonon coupling strengths of U 3+ and Nd 3+ ions doped in LaCl 3 and U 3+ in LaBr 3 single crystals

Abstract

Temperature-dependent line width and line shift measurements between 7 and 280 K have been performed for a number of absorption transitions in the 4000–21,000 cm −1 energy range of the U 3+:LaCl 3, Nd 3+:LaCl 3 and U 3+:LaBr 3 single crystal spectra. The values of the electron–phonon coupling parameter α ̄ were determined for U 3+:LaCl 3 and Nd 3+:LaCl 3 by a fit of experimentally observed line widths to an equation containing the temperature dependent broadening due to the Raman two-phonon process. For both ions diluted in LaCl 3 the values of the α ̄ parameters are considerably lower than in K 2LaCl 5, and the value of α ̄ for U 3+ in the LaCl 3 host is markedly larger as compared with that of Nd 3+. Factors influencing these differences are discussed. With a temperature increase a blue shift of the absorption lines of the U 3+ ions in LaCl 3 and LaBr 3 is observed. A comparison has been performed among the electron–phonon coupling parameters obtained from an analysis of the line widths of the U 3+:LaCl 3 single crystal and those determined from temperature induced line shifts as well as between the magnitudes of the absolute increase in line width and line shifts in the 7–290 K temperature range for U 3+ doped LaCl 3 and LaBr 3 crystals. The electron–phonon coupling is stronger for U 3+ in the tribromide as compared with the trichloride host which is mainly due to a larger covalency of the first one.