Search

We report a systematic analysis of the crystal‐field spectra of four fluoride compounds containing tetravalent actinide ions. The first part of this work [J. Chem. Phys. 95, 7194 (1991)] provided interpretation of the absorption spectra of UF4, NpF4, and PuF4. To extend our analysis to heavier elements of the series, low‐temperature absorption spectra of AmF4 and CmF4, and site selective laser‐induced emission and excitation spectra of Cm4+:CeF4 and Bk4+:CeF4 were obtained. A model energy level calculation was found to be in good agreement with the experimental results. It is shown that the crystal‐field interaction in combination with spin–orbital coupling results in significant J mixing in the excited states, but ground state wave functions are still relatively pure in J character for the tetravalent actinide ions Am4+, Cm4+, and Bk4+. Trends in the parameters of the effective operator Hamiltonian are compared with those of a Hartree–Fock free‐ion model. Interpretation of the ground‐state splitting of t...

[N(CH3)4]2BkCl6 was synthesized from cold, concentrated HCl(aq) using 249Bk ( t 1 2 = 320 days ) separated from 249Cf on the milligram scale. For comparison [N(CH3)4]2ZrCl6 was also synthesized and single crystals were grown. Both compounds were studied by X-ray powder diffraction, and [N(CH3)4]2ZrCl6 was also studied by single-crystal X-ray diffraction. Powder patterns of [N(CH3)4]2BkCl6 have f.c.c. symmetry, a = 13.08(2) A , isostructural with K2PtCl6 and other [N(CH3)4]2MCl6 salts. [N(CH3)4]2ZrCl6 has f.c.c. symmetry, space group Fm3m (disregarding hydrogen positions) with a = 12.882(1) A and d(Zr-Cl) = 2.428(8) A . A transition to lower symmetry was found in [N(CH3)4]2ZrCl6 at − 110 °C by Guinier powder diffraction. Optical spectra of Bk4 + in [N(CH3)4]2BkCl6 in KBr and KCl pellets were recorded and interpreted.

The optical spectra of the lanthanides doped into single crystal LaF3 have been interpreted in terms of transitions within 4f N configurations. Energy matrices combining free‐ion terms with a crystal field for an approximate model which assumes C2v instead of the actual C2 site symmetry were diagonalized. Excellent correlations were obtained between experimental transition energies and the computed level structures. We also report the results of previously unpublished experimental spectroscopic investigations of Nd3+ and Sm3+:LaF3, as well as predicted energy levels for Pm3+:LaF3. The spectroscopic data for each ion were independently interpreted using an effective‐operator model, then the model parameters were intercompared. Systematic trends have been identified, and a comprehensive energy level diagram is presented.

Previous successful analyses of the observed crystal-field structure in Tm3+:LaF3 and Ho3+:LaF3 in an approximate C2v site symmetry have depended on a crystal-field model derived in an analysis of the spectrum of Er3+:LaF3. In the present investigation the predictions of this same model are compared with the observed crystal-field structure in Dy3+:LaF3. The energy parameters resulting from a least-squares fit of the corresponding spectral assignments are reported. To help clarify the physical situation, some comparisons are offered between C2v and D3h crystal-field models for these Ln3+: LaF3 systems.

A crystal field analysis of the energy level structure deduced in LaCl3: Am3+ and in the isostructural compound AmCl3 from spectroscopic measurements has been carried out using an effective operator Hamiltonian. Both free-ion and crystal field parameters were fitted to the data. While a shift in energy between transitions observed in the doped crystal and pure compound was noted, it was found that the same crystal field parameters were applicable to both samples. Shifts in centers of gravity of levels could be attributed to differences in free-ion structure. There is a clear similarity between spectra observed for the solid compounds studied and that observed for a frozen aqueous solution of Am3+. This indicates that the same symmetry-induced selection rules are operative and is consistent with a ninefold coordination by water molecules.

The first evidence for an isotope shift in the near infrared region of the spectrum of an actinide molecule, PuF6, is reported. Using samples of 239PuF6 and 242PuF6, isotope shifts were found in the absorption region near 8000 A. Shifts of 0.3 to 1.2 cm−1 were observed. The data are interpreted as indicating that some of the absorption bands in this region are vibronically induced electronic transitions which result from coupling of one or more quantum changes in the asymmetric ν3(F1u) and ν4(F1u) vibrations to electronic states of the nonbonding 5f electrons in the molecule.

The results of model crystal field calculations in C 2 v symmetry for Pr 3+ -LaF 3 and Tm 3+ -LaF 3 are compared with experimental data for the nearly complete f 2 (f 12 ) configurations. It is shown that the method used results in computed levels that generally correlate closely with level energies deduced from experiment. However, a number of important new insights are gained and some previous assignments are questioned.

The electronic transition intensities of the LiYF4Eu3+ matrix are explained by mainly two mechanisms: the induced electric dipole and the magnetic dipole mechanism. The induced electric dipole transition intensities are simulated by means of a set of phenomenological Bλkq parameters.

The optical absorption spectra of polycrystalline samples of both α- and β-UF5 have been measured in the range 2700 - 500 nm at 4, 77 and about 293 K using a recording spectrophotometer. It was assumed that the crystal field in both cases was dominated by weak covalent interactions; consevuently crystal-field analyses were developed based on the angular overlap model. Crystal-field parameters consistent with the evperimental data were derived.

An unsolved problem in actinide spectroscopy is the lack of a consistent systematic interpretation of tetravalent actinide ion spectra. We are addressing this problem by modelling the spectra of AnF4 and An4+:CeF4. This family exhibits a single crystallographic structure, and the symmetry of the actual C2 and Cs sites can be approximated as D4d, where the crystalfield potential has only three terms. The spectrum of Bk4+:CeF4 suggests a value of about 2000 cm−1 for B02. We have adopted this constraint and will report our progress toward a systematic free-ion and crystal-field parametrization including spectra of UF4 and NpF4.

In recent years sufficient new spectra of actinides in their numerous valence states have been measured to encourage a broader scale analysis effort than has been attempted in the past. Theoretical modelling in terms of effective operators has also undergone development. Well-established electronic structure parameters for the trivalent actinides are being used as a basis for estimating parameters in other valence states and relationships to atomic spectra are being extended. Recent contributions to our understanding of the spectra of 4+ actinides have been particularly revealing and supportive of a developing general effort to progress beyond a preoccupation with modelling structure to consideration of the much broader area of structure-bonding relationships. We summarize here both the developments in modelling electronic structure and the interpretation of apparent trends in bonding.

Absorption spectra of polycrystalline samples of LiUF6, α‐NaUF6, and CsUF6 have been recorded at 4, 77, and 298 K. This group of compounds has a common sixfold U–F coordination that approaches an octahedral site symmetry for LiUF6 but exhibits increasing trigonal distortion (D3d symmetry) along the indicated series. Spectra have been systematically interpreted as consisting of sequences of vibronic transitions coupled to well‐defined electronic excited states. Crystal‐field calculations have been performed.

Neptunium (IV) hydroxysulfate, Np(OH) 2 SO 4 , was synthesized using hydrothermal techniques. The X-ray powder diffraction pattern indicates that the compound is isomorphous with the Th(IV) and U(IV) analogs. Cell constants for the three compounds clearly show the effects of the actinide contraction. Visible and near-ir spectra are consistent with the presence of Np(IV) and are compared to spectra of Np(IV) in acidic solution. The ir spectrum contains bands which are assigned to the hydroxy and sulfate groups.

Diagonalization of the complete crystal field matrices in both S 4 and the approximate D 2 d symmetry produces the crystal field energy levels and wavefunctions of the Nd 3+ -ion doped in a LiYF 4 single crystal. Quite a good simulation can be expected, because of the simultaneous fitting of the freeion and crystal field parameters to a set of about 120 experimental energy levels. The composition of the wavefunction is further checked by a calculation of the intensities for both magnetic and induced electric dipole absorption transitions. The last mechanism, described by the Judd-Ofelt theory, introduces a set of phenomenological B λkq parameters.

The absorption spectrum of aqueous curium (III) has been studied in 0·04 N HClO4, using the isotope 244Cm. Three prominent absorption peaks have been discovered at 3754 A, 3812 A and 3965 A, in addition to absorption in the far ultra-violet. These same peaks are found in HCl, HNO3 and H2SO4 solutions. The general characteristics of the spectra in these three acids and some general observations on the radiation decomposition of aqueous curium solutions are reported.

The absorption spectrum of Cm(III) in HC1O/sub 4/, HlCl, HNO/sub 3/, and H/sub 2/SO/sub 4/ in the region 220 to 1400 m mu was studied. A number of new Cm(III) bands were found using larger amounts of curium than were available previously. The relative complexing of Cm(III) in HCl, HNO/sub 3/ and H/sub 2/SO/ sub 4/ is indicated and these results are compared with those for Pu(III) and Am(III). Radiation decomoposition of the solvent limited the study in concentrated acid solutions. (auth)

The α-half-life of 244 Cm, determined from specific activity measurements, and the α-half-lives of 245 Cm and 246 Cm, determined by mass spectrometric analysis of curium and its plutonium daughters, have been determined to be 17·59 ± 0·06 years, 9320 ± 280 years and 5480 ± 170 years, respectively.

Author(s): Gonway, J.G.; Fried, S.; Latimer, R.M.; Mclaughlin, R.; Gutmacher, R.G.; Carnall, W.T.; Fields, P.

In this systematic examination of some of the spectroscopic properties of the f-elements we deal with both the trivalent lanthanides and actinides. We summarize the present status of our energy level calculations in single crystal matrices and in aqueous solution, and compare the predicted crystal-field structure in certain low-symmetry sites with that observed. Some interesting new structural insights are thereby gained. The state eigenvectors from these calculations are then used in part in reassessing and interpreting the intensities of transitions in aqueous solution via the Judd-Ofelt theory. The parameters of this theory derived from fitting experimental data are compared with those computed from model considerations. Finally, we discuss some recent contributions to the interpretation of excited state relaxation processes in aqueous solution. 79 references, 23 figures, 17 tables.

Absorption spectra of polycrystalline samples of LiUF6, α‐NaUF6, and CsUF6 have been recorded at 4, 77, and 298 K. This group of compounds has a common sixfold U–F coordination that approaches an octahedral site symmetry for LiUF6 but exhibits increasing trigonal distortion (D3d symmetry) along the indicated series. Spectra have been systematically interpreted as consisting of sequences of vibronic transitions coupled to well‐defined electronic excited states. Crystal‐field calculations have been performed.

In this systematic examination of some of the spectroscopic properties of the f-elements we deal with both the trivalent lanthanides and actinides. We summarize the present status of our energy level calculations in single crystal matrices and in aqueous solution, and compare the predicted crystal-field structure in certain low-symmetry sites with that observed. Some interesting new structural insights are thereby gained. The state eigenvectors from these calculations are then used in part in reassessing and interpreting the intensities of transitions in aqueous solution via the Judd-Ofelt theory. The parameters of this theory derived from fitting experimental data are compared with those computed from model considerations. Finally, we discuss some recent contributions to the interpretation of excited state relaxation processes in aqueous solution.

CsPuF/sub 6/ was prepared and verified to be isostructural with corresponding compounds of uranium and neptunium. Its decomposition was studied in an inert gas atmosphere and in vacuum. Its spectrum has been measured in the region 400-2000 nm. The energy level structure of Pu/sup 5 +/ in the trigonally distorted octahedral CsPuF/sub 6/ site was computed from a predictive model and compared with the observed spectrum. 20 references, 4 figures, 3 tables.

The atomic spectroscopic data available for plutonium are among the richest of any in the periodic system. They include high-resolution grating and Fourier-transform spectra as well as extensive Zeeman and isotope-shift studies. The present status of the term analysis is summarized, and the configurations that have been identified are cited. A least-squares adjustment of a parametric Hamiltonian for configurations of both Pu I and Pu II has shown that almost all of the expected low levels are now known. The use of a model Hamiltonian applicable to both lanthanide and actinide atomic species has been applied to the low configurations of Pu I and Pu II making use of trends predicted by ab initio calculations. This same model has been used to describe the energy levels of Pu/sup 3 +/ in LaCl/sub 3/, and an extension has permitted preliminary calculations of the spectra of other valence states. 46 references, 9 figures, 7 tables.

The crystal field level structure in Ho3+-LaF3 has been analyzed assuming an approximate C2v-site symmetry. Both free-ion and crystal field parameters of an effective operator Hamiltonian were fit to the data, and close correlation between experimental and computed level energies was obtained. Starting point for the interpretation was a model which included the crystal field parameters for Er3+-LaF3. The results of a partial re-analysis of the energy level structure in Er3+-LaF3 are also reported.