Clayton W. Williams

Analysis of the crystal‐field spectra of the actinide tetrafluorides. I. UF4, NpF4, and PuF4

An interpretation of the low‐temperature absorption spectra of AnF4 (An=U, Np, Pu) is presented. Using an effective operator Hamiltonian with orthogonalized free‐ion operators and initializing crystal‐field parameter values based on a superposition model calculation for An4+ sites with C2 symmetry, good agreement between the model calculations and experimentally observed absorption band structure could be obtained. Correlations with published magnetic and heat capacity measurements are discussed.

The role of triaxiality in the ground states of even-even neutron-rich Ru isotopes

Abstract Partial decay schemes for the very neutron-rich nuclei 108,110,112 Ru and 114 Ru, about which nothing was previously known, have been determined from the study of prompt, triple-γ coincidences in 248 Cm fission fragments. Levels with probable spin up to 10ħ have been observed and γ-ray branching ratios obtained. Ratios of reduced electric quadrupole transition probabilities obtained from the branching ratios are in good agreement with predictions of a simple model of a rigid triaxial rotor.

Berkelium redox speciation

Summary The inner-sphere hydration environments of the Bk3+·nH2O and Bk4+·n′H2O aquo ions in 1 M HClO4 were determined and are viewed in the context of other 5f (actinide) and 4f (lanthanide) aquo ions. The formal potential of the Bk4+/Bk3+ redox couple and the reorganization of the hydration sphere, in terms of the average Bk–OH2 interatomic distances and number of coordinated water molecules (n and n′), were determined through in situ X–ray absorption spectroelectrochemistry. The participation of H2O in the electrochemical conversion and the rearrangement of the hydration environment upon reduction–oxidation are summarized in the equation: [Bk(OH2)8]4+ + H2O + e− ⇆ [Bk(OH2)9]3+. The change of hydration about 249Bk between the trivalent and tetravalent states is believed to reflect predominantly steric factors related to the difference of ionic radii, rather than electronic or bonding influences.

The synthesis and characterization of CmBa2Cu3O7

Abstract In order to help evaluate possible mechanisms of superconductivity, we have prepared a new member of the RBa 2 Cu 3 O 7 series, CmBa 2 Cu 3 O 7 . The X-ray powder diffraction pattern was obtained for a single phase compound, isostructural with the orthorhombic phase of YBa 2 Cu 3 O 7 . Magnetic susceptibility measurements show that CmBa 2 Cu 3 O 7 is not superconducting. There is evidence of long-range magnetic ordering at 22 K. These results are compared with the behavior of PrBa 2 Cu 3 O 7 , the only other nonsuperconducting compound with the YBa 2 Cu 3 O 7 structure.

Thermodynamics of actinide perovskite-type oxides. II: Enthalpy of formation of Ca3UO6, Sr3UO6, Ba3UO6, Sr3NpO6, and Ba3NpO6

Abstract The molar enthalpies of formation ΔfHmo of several ternary oxides (ordered perovskites) of hexavalent uranium and neptunium with alkaline earths have been determined at 298.15 K. The results for ΔfHmo/(kJ·mol−1) are as follows: Ca3UO6, −(3301.9±4.9); Sr3UO6, −(3262.6±4.4); Ba3UO6, −(3210.7±5.8); Sr3NpO6, −(3125.3±5.8); and Ba3NpO6, −(3086.0±7.7). The stabilities of these complex oxides with respect to the parent binary oxides are discussed. These results are compared with those found for related complex oxides.

Powder neutron diffraction and magnetic susceptibility of 248CmO2

Abstract We report the first neutron diffraction experiments on a sample of approximately 55 mg of CmO2, using the rare isotope 248Cm. Preparative techniques have assured that the sample is close to stoichiometry and X-ray patterns show only the expected f.c.c. lines. The neutron diffraction experiments confirm that the sample is nearly stoichiometric and rule out the possibility that the oxygen atoms form a superlattice to accommodate a mixture of Cm3+ and Cm4+ ions. Magnetic susceptibility measurements give a μeff value of (3.36 ± 0.06)μB for the effective paramagnetic moment, consistent with many other determinations of this property in Cm(IV) compounds, which have been interpreted in the past as indicating substantial Cm3+ impurity. In view of the contradiction between bulk susceptibility on the one hand and thermogravimetric and diffraction results on the other hand, we suggest that a re-evaluation of the electronic ground states in the actinide dioxides may be needed. Some other contradictory experimental results are given.

Fluorescence studies of neptunium and plutonium hexafluoride vapors

The first observation of fluorescence from gas phase, electronically excited, transuranic hexafluorides is reported. Fluorescence peaking at 1360 nm was observed from 237NpF6 gas excited at 1064 nm. The measured fluroescence lifetime was 3.53±0.01 ms, independent of NpF6 pressure. Fluourescence peaking at 2300 nm has been observed from 242PuF6 gas excited at 1064 nm. The measured fluorescence lifetime was 204±12 μs, independent of PuF6 pressure. In both cases the emitting state is assigned as a vibronic component of the first excited electronic state of the hexafluoride based on previously reported absorption spectra and energy level calculations. Fluorescence in the 1900 and 4800 nm region was observed from PuF6 gas excited at 532 nm. The lifetime of this fluorescence was 86±4. The emitting state giving rise to this shorter‐lived fluorescence was not identified. Estimated fluorescence quantum yields are reported.

Isotope effects in the molecular spectrum of plutonium hexafluoride

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.

F-electron localization/delocalization phenomena in PrBa2Cu3O7 and CmBa2Cu3O7

Inelastic neutron scattering has been employed in the study of the electronic properties of Pr3+ in PrBa2Cu3O7 (a semiconductor). The absence of any sharp, well-defined crystal field excitations at energies less than 150 meV is contrasted with data obtained for NdBa2Cu3O7 (a 92 K superconductor) and PrScO3 (an insulating perovskite). The absence of sharp crystal field transitions for PrBa2Cu3O7 is attributed to a broadening of these levels via an f-electron-CuO (planar)- electron interaction. It is suggested that this interaction is responsible for the absence of superconductivity in PrBa2Cu3O7. The absence of a superconducting transition in CmBa2Cu3O7 is used as supporting evidence for this hypothesis.

Thermodynamics of actinide perovskite-type oxides III. Molar enthalpies of formation of Ba2MAnO6 (M = Mg, Ca, or Sr; An = U, Np, or Pu) and of M3PuO6 (M = Ba or Sr)☆

Abstract Nine complex oxides (ordered perovskites) of alkaline earths and hexavalent actinides have been characterized by X-ray powder diffraction and solution calorimetry. Three of these are new compounds, all indexed as face-centred cubic (space group Fm3m): Ba 2 CaNpO 6 , a 0 = (0.86235 ± 0.00036) nm; Ba 2 MgPuO 6 , a 0 = (0.83318 ± 0.00058) nm; and Ba 2 CaPuO 6 , a 0 = (0.86112 ± 0.00040) nm. Two previously reported compounds are isostructural with orthorhombic Ba 2 SrUO 6 : Ba 2 SrNpO 6 , a 0 = (0.6221 ± 0.0040) nm, b 0 = (0.6215 ± 0.0060) nm, c 0 = (0.8863 ± 0.0028) nm; and Ba 2 SrPuO 6 , a 0 = (0.6204 ± 0.0042) nm, b 0 = (0.6188 ± 0.0062) nm, c 0 = (0.8822 ± 0.0032) nm. The standard molar enthalpies of formation Δ f H m o (kJ·mol −1 ) at 298.15 K of these nine compounds are as follows: Ba 2 CaUO 6 , −(3295.6 ± 4.8); Ba 2 SrUO 6 , −(3257.3 ± 4.4); Ba 2 CaNpO 6 , −(3159.2 ± 6.7); Ba 2 SrNpO 6 , −(3122.6 ± 6.7); Ba 3 PuO 6 , −(2997.7 ± 8.7); Sr 3 PuO 6 , −(3041.9 ± 8.0); Ba 2 MgPuO 6 , −(2994.5 ± 7.9); Ba 2 CaPuO 6 , −(3067.7 ± 8.0); and Ba 2 SrPuO 6 , −(3023.7 ± 8.2). The stabilities of these complex oxides are discussed with respect to the parent binary oxides, to the aqueous ions, and to other complex oxides of hexavalent actinides.