[en] Temperature-dependent line broadening measurements of emission and excitation transitions for two intrinsic sites U(1) and U(2) of U³⁺ ions doped in a RbY₂Cl₇ single crystals as well as of U⁴⁺ ions have been performed. Values of the electron phonon (EP) coupling parameter α-bar 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-bar parameters for U³⁺ ions in RbY₂Cl₇ are larger than those determined for this ion in LaCl₃ host crystals. This is due to shorter M-Cl distances in RbY₂Cl₇ which leads to a stronger interaction of uranium with the chlorine ions and to an increase of covalency. The relatively large α-bar value determined for the ²H2_11/2 multiplet of U³⁺ in RbY₂Cl₇ may result from the proximity of opposite parity 5f²6d¹ states. The α-bar parameters obtained for the U³⁺ ions are larger than those for U⁴⁺. The latter ones are affected by a stronger crystal-field (CF), however the position of the first 5f²6d¹ or 5f¹6d¹ states, which for U³⁺ is observed at an energy of ∼15,000 cm^-1 lower than for U⁴⁺, is the dominating one among the factors influencing the EP coupling strength. The EP coupling parameters for all investigated transitions of the U³⁺ 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 U³⁺ 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

No abstract available

[en] Uranium(III) chloro complexes of the formulas SrUCl₅ and Ba₂UCl₇ were synthesized and characterized by electronic absorption spectra, X-ray powder diffraction and magnetic susceptibility investigations. Ba₂UCl₇ crystallizes in the monoclinic system, P2₁/c, with a=7.20 A, b=15.61 A, c=10.66 A, β=91.1 and Z=4. Magnetic susceptibilities of polycrystalline samples of the compounds were measured on a SQUID magnetometer in the 1.7-300 K range. The inverse magnetic susceptibility versus temperature plots of SrUCl₅ and Ba₂UCl₇ follow the Curie-Weiss law above 90 and 105 K, respectively. The following paramagnetic constants from the Curie-Weiss law C=χ_M(T-Θ) have been obtained: C=1.653 emu.K.mol^-1, Θ=-127 K, μ_eff=2.84√(c)=3.65 B.M. (for SrUCl₅) and C=1.310 emu.K.mol^-1, Θ=-95 K, μ_eff=3.25 B.M. (for Ba₂UCl₇). Solid state electronic spectra of the compounds have been recorded in the 4000-30000 cm^-1 range and are discussed. (orig.)

[en] 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 U³⁺:LaCl₃, Nd³⁺:LaCl₃ and U³⁺:LaBr₃ single crystal spectra. The values of the electron-phonon coupling parameter α-bar were determined for U³⁺:LaCl₃ and Nd³⁺:LaCl₃ 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 LaCl₃ the values of the α-bar parameters are considerably lower than in K₂LaCl₅, and the value of α-bar for U³⁺ in the LaCl₃ host is markedly larger as compared with that of Nd³⁺. Factors influencing these differences are discussed. With a temperature increase a blue shift of the absorption lines of the U³⁺ ions in LaCl₃ and LaBr₃ is observed. A comparison has been performed among the electron-phonon coupling parameters obtained from an analysis of the line widths of the U³⁺:LaCl₃ 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 U³⁺ doped LaCl₃ and LaBr₃ crystals. The electron-phonon coupling is stronger for U³⁺ in the tribromide as compared with the trichloride host which is mainly due to a larger covalency of the first one

[en] Uranium(3+) doped Cs₂NaYBr₆ single crystals with a 2.1 and 0.09 mol% U³⁺ concentration have been obtained by the Bridgman-Stockbarger method. The U³⁺ ions replaced the Y³⁺ ions at an octahedral site of O_h symmetry. Luminescence spectra of the crystal were recorded at 4.2 K. The emission bands observed in the visible and near infrared regions have been assigned to transitions from the lowest components of the ⁴G_7/2 and ⁴F_3/2 multiplets to the crystal-field components of the ⁴I_9/2 ground state. Absorption spectra were recorded in the 3900-35,000 cm^-1 range at 4.2 K. The analysis of these spectra enabled the assignement of 28 energy levels, which were fitted to seven parameters of an empirical Hamiltonian representing the combined atomic and crystal-field interactions, with a rms deviation of 43 cm^-1. The energies of the most prominent vibronic features have been measured and assigned to the specific vibrational modes of the UCl₆^3- moiety or to lattice modes. Above 14,000 cm^-1 vibronic transitions associated with the electric dipole allowed 5f³-5f²6d¹ transitions were observed. The unresolved zero-phonon lines are accompanied by a number of vibrational progressions combined with the even S₁(a_1g) mode

[en] Uranium(III) chloro complexes of formulae CsUCl₄ and Cs₂LiUCl₆ were synthesized and characterized by electronic absorption, X-ray powder diffraction and magnetic susceptibility investigation. Cs₂LiUCl₆ crystallizes in the regular system Fm3m, with a=10.671 A, V=1218.03 A³ and Z=4. The powder diffraction data for CsUCl₄ could not be indexed unambiguously. The following paramagnetic constants were obtained: C=1.215 emu K mol^-1, Θ=-32.7 K, μ_eff=3.13 B.M. (for CsUCl₄) and C=1.571 emu K mol^-1, Θ=-103 K, μ_eff=3.56 B.M. (for Cs₂LiUCl₆). Solid state electronic spectra of the compounds were recorded in the range 4000-30000 cm^-1 and are discussed. An analysis of the 5f³→5f²6d¹ transitions in the absorption spectrum of Cs₂LiUCl₆ is presented. (orig.)

[en] The absorption spectrum of a thin film of uranium(III) formate was measured at 4 and 300 K in the 4000-23000 cm^-1 range. The energy levels of U³⁺ were assigned and fitted to a semiempirical Hamiltonian representing the combined atomic and crystal-field interactions at the C_3v symmetry site. Ten energy level parameters were varied simultaneously in least-squares adjustments yielding a mean error of 23 cm^-1. The analysis enabled the determination of the crystal-field parameters and the assignment of 49 crystal-field levels. The calculated total splitting of the ground level is equal to 520 cm^-1. An analysis of the 5f³→5f³ band intensities based on the Judd-Ofelt theory is presented. (orig.)

[en] Single crystals of Nd³⁺:RbY₂Cl₇ were grown by the Bridgman-Stockbarger method. The host crystal contains two slightly inequivalent Y³⁺ ions, each with an approximate C_2v site symmetry. Anti-Stokes emission from the ⁴G_7/2 and ⁴D_3/2 levels was observed after laser excitation of the ⁴F_3/2 and ⁴F_9/2 multiplets. Laser excitation at 413 cm^-1 or 453 cm^-1 above the ⁴F_3/2 multiplet resulted in emission from the ²P_1/2 level. Laser site-selective upconverted emission spectra have been measured, as well as their emission transients and power dependence. Possible excited state absorption and energy-transfer upconversion (ETU) mechanisms are proposed and discussed. Due to the smaller crystal field and a somewhat different energy level structure for the Nd³⁺ ions in RbY₂Cl₇ as compared with those observed for Nd³⁺ in fluoride or oxide hosts, the ⁴G_7/2 and ⁴D_3/2 multiplets are populated under ⁴F_3/2 excitation in a three and four step ETU process, respectively, instead of in a two and three step process as observed for the lighter hosts

No abstract available

[en] High-resolution absorption spectra of thin films of UCl₃ and UBr₃ have been measured at 4.2 K in the 4000-30 000 cm^-1 range. The determined crystal-field lines were used in a computational analysis employing free-ion operators, one-electron crystal-field operators as well as two-particle correlation crystal-field operators. The performed analysis enabled the determination of the Hamiltonian parameters and an unambiguous assignment of 57 and 46 crystal-field levels with a mean error of 31 cm^-1 for UCl₃ and UBr_3, respectively