Search

Glass-bonded sodalite is being developed as a ceramic waste form (CWF) to immobilize radioactive fission products, actinides, and salt residues from electrometallurgical treatment of spent nuclear reactor fuel. The CWF consists of about 75 mass% sodalite, 25 mass% glass, and small amounts of other phases. This paper presents some results and interpretation of physical measurements to characterize the CWF structure, and dissolution tests to measure the release of matrix components and radionuclides from the waste form. Tests have been carried out with specimens of the CWF that contain rare earths at concentrations similar to those expected in the waste form. Parallel tests have been carried out on specimens that have uranium or plutonium as well as the rare earths at concentrations similar to those expected in the waste forms; in these specimens UCl3 forms UO2 and PuCl3 forms PuO2. The normalized releases of rare earths in dissolution tests were found to be much lower than those of matrix elements (B, Si, Al, Na). When there is no uranium in the CWF, the release of cerium is two to ten times lower than the release of the other rare earths. The low release of cerium may be due to its tetravalent state in uranium-free CWF. However, when there is uranium in the CWF, the release of cerium is similar to that of the other rare earths. This trivalent behavior of cerium is attributed to charge transfer or covalent interactions among cerium, uranium, and oxygen in (U,Ce)O2.

The aqueous complexation reactions of trivalent lanthanide and actinide cations with the hexadentate ligand N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) have been characterized using potentiometric and spectroscopic techniques in 0.1 M NaClO4. At 25°C, the stability constant of Am(TPEN)3+ is two orders of magnitude larger than that of Sm(TPEN)3+, reflecting the stronger interactions of the trivalent actinide cations with softer ligands as compared to lanthanide cations.

The compounds [M(NO3)2(tpen)][NO3]·3H2O with M=La, Nd, Eu, Tb and Y have been synthesized. Single crystals have been grown of the compounds with M=La and Tb from water–ethanol solution. Their crystal structures have been determined. The compounds are isostructural; the metals are ten-fold coordinated, with six nitrogens from one tpen ligand and four oxygens from two nitrates. Growth of crystals with nitrogen coordinated to rare earths in water-containing solution is unusual and indicates that tpen is a good nitrogen donor even with hard cations.

The standard molar enthalpies of formation ΔfHm° of the fluorite oxide PrO2and the perovskite oxide SrPrO3have been determined by solution calorimetry. A combination of appropriate thermodynamic cycles leads to the values ΔfHm°(PrO2, 298.15 K) =−(959.8 ± 4.1) kJ·mol−1and ΔfHm°(SrPrO3, 298.15 K) = − (1588.4 ± 4.1) kJ·mol−1. Unusual stability is found for SrPrO3in terms of the enthalpy of reaction of the binary oxides to form the ternary oxide: ΔfHm°{SrO(cr) + PrO2(cr) = SrPrO3(cr)} = − 39 kJ·mol−1. This stability of SrPrO3is consistent with earlier thermochemical work on BaPrO3. The Pr members of the series of perovskite oxides AMO3(A = Sr, Ba; M = transition elements: Ce, Pr, Tb, actinides) are substantially more stable than expected from the trend established by the other members of the series, in which the enthalpy of reaction of the binary oxides to form the ternary oxide becomes less negative as the perovskite becomes more distorted. Electronic stabilization of the perovskite (PrO3)2−framework in these 4f1compounds is proposed as the origin of the additional thermodynamic stability of the perovskite oxides of Pr4+.

[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.

A systematic treatment of the thermochemical properties of binary and complex oxides of the lanthanides and actinides in terms of other wellcharacterized species and thermochemical cycles is given in this paper. Since the trivalent lanthanides provide a reference series with which the lanthanide and actinide sesquioxides can be compared, the trivalent ion energetics are considered first. Recent interest in monoxides, prompted by high pressure synthesis of lanthanide monoxides and interest in divalent actinide metals and oxides, has led us to include a treatment of the relative stabilities of monoxides and sesquioxides. The important tetravalent state is viewed from the perspective of the dioxides as well as the perovskites BaMO3. Since there are no higher valent lanthanides, systematic trends in pentavalent and hexavalent complex actinide oxides are not treated in this review.

Mesure des enthalpies de rupture des liaisons Th-coordinat sur les complexes Cp 3 ThR (R=groupe alkyl), par calorimetrie anaerobie

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.

We have measured the enthalpy of reaction of AmO 2 , using 241 Am and 243 Am, in 0.5 M H 2 SO 4 −0.1 M KI solution. From this datum and auxiliary thermochemical measurements we have calculated ΔH f °(AmO 2 , C, 298.15 K) = −932.2 ± 2.7 kJ mol −1 . We estimate ΔH f °(Am 4+ , aq, 298.15 K) = −406±6 kJ mol −1 and E °(Am 4+ /Am 3+ ) = 262 ± 0.09 V.

Preparations of the complex chlorides Cs2NaAnCl6 (An = U, Np, Am, Cf), growth of a single crystal of Cs2NaUCl6, and lattice parameters of these face-centered cubic compounds are reported. The enthalpies of solution of these compounds in suitable media have been measured and their enthalpies of formation have been obtained. The enthalpy changes upon formation of these quaternary chlorides from the corresponding binary chlorides (ΔcplxH0m) are calculated and interpolated to calculate enthalpies of formation of Cs2NaCmCl6 and Cs2NaBkCl6. These results are combined with auxiliary structural, spectroscopic and thermodynamic data and with calculated lattice energies, to derive the third ionization potentials of the actinides U-Cf and the hydration enthalpies of their tripositive ions.

CsEuCl3 was prepared from stoichiometric amounts of CsCl and EuCl2. Powder X-ray diffraction data show that CsEuCl3 has a distorted perovskite structure. The diffraction data are most consistent with a tetragonal unit cell with a 0 = 5.588(4) A , c 0 = 5.619(8) A , Z = 1 , and space group P4 mm. Measurement of the heat of solution of CsEuCl3 in water leads to ΔHfo(CsEuCl3, c)=−1265.0±5.6 kJ/mol. The perovskite lattice does not appear to stabilize CsEuCl3 in comparison with the binary chlorides CsCl and EuCl2.

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): Ba2CaNpO6, a0 = (0.86235 ± 0.00036) nm; Ba2MgPuO6, a0 = (0.83318 ± 0.00058) nm; and Ba2CaPuO6, a0 = (0.86112 ± 0.00040) nm. Two previously reported compounds are isostructural with orthorhombic Ba2SrUO6: Ba2SrNpO6, a0 = (0.6221 ± 0.0040) nm, b0 = (0.6215 ± 0.0060) nm, c0 = (0.8863 ± 0.0028) nm; and Ba2SrPuO6, a0 = (0.6204 ± 0.0042) nm, b0 = (0.6188 ± 0.0062) nm, c0 = (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: Ba2CaUO6, −(3295.6 ± 4.8); Ba2SrUO6, −(3257.3 ± 4.4); Ba2CaNpO6, −(3159.2 ± 6.7); Ba2SrNpO6, −(3122.6 ± 6.7); Ba3PuO6, −(2997.7 ± 8.7); Sr3PuO6, −(3041.9 ± 8.0); Ba2MgPuO6, −(2994.5 ± 7.9); Ba2CaPuO6, −(3067.7 ± 8.0); and Ba2SrPuO6, −(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.

Thorium-ligand bond disruption enthalpies (D) have been determined in the series Cp'/sub 2/Th(thorium)R/sub 2/ (Cp' = eta/sup 5/-(pentamethylcyclo-pentadienyl-(CH/sub 3/)/sub 5/C/sub 5/) for a range of R groups including R = alkyl, aryl, metallacycle, hydride, and dialkylamide. Enthalpy data were obtained in an anaerobic batch-titration isoperibol reaction calorimeter from heats of solution in toluene followed by heats of alcoholysis with tert-butyl alcohol. Calculated Th-hydrocarbyl bond disruption enthalpies fall in the range 60 to 90 kcal/mol, with C/sub 6/H/sub 5/ > CHa3 approx. = CH/sub 2/Si(CH/sub 3/)/sub 3/ > CH/sub 2/CH/sub 3/ greater than or equal to CH/sub 2/C/sub 6/H/sub 5/ approx. = n-C/sub 4/H/sub 9/ approx. = CH/sub 2/C(CH/sub 3/)/sub 3/. Th-C bonds appear to be strengthened by ThOR coligands and possibly weakened by Th-Cl coligands. The strain energies in thoracyclobutanes Cp'/sub 2/Th(CH/sub 2/)/sub 2/C(CH/sub 3/)/sub 2/ and Cp'/sub 2/Th(CH/sub 2/)/sub 2/Si(CH/sub 3/)/sub 2/ are estimated to be ca. 16 and 8 kcal/mol, respectively. A Th-H bond is ca. 20 kcal/mol stronger than a Th-alkyl bond, while a Th-NR/sub 2/ bond is ca. 18 kcal/mol stronger. These data afford a considerably expanded understanding of organoactinide reaction patterns involving C-H activation, ..beta..-hydride elimination, and CO insertion as well as why they sometimesmore » differ from those of middle and late first-row transition elements. 3 figures, 2 tables.« less

Concentrations of up to 20 mol.% Dy have been substituted for tetravalent cerium in the perovskite BaCeO3. 161Dy Mossbauer spectroscopy demonstrates that the dysprosium in this host is present in the trivalent oxidation state. Effective magnetic moments obtained on the same samples confirm these findings. Line broadening as a function of dysprosium concentration, observed from both X-ray and Mossbauer measurements, is attributed to the presence of lattice defects (oxygen vacancies) which are necessary for charge compensation.

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.

An electrostatic method to determine variable atom parameters in ionic crystals with experimentally determined unit cell parameters and space group is proposed. The atom parameters are usually chosen to give the maximum Madelung constant. However, when these atom parameters generate interatomic distances at least one of which is less than a critical distance, which comes from repulsion between atoms, the atom parameters corresponding to that distance are assigned. Applicability was examined for three cases: TiO2 (rutile), UCl3, and β-Rb2GeF6. Agreement between the atom parameters of this method and of literature was good. Some discussion is presented on the basis of this method. In ionic crystals, the atoms with variable parameters are set first using the geometrical arrangement which is the most stable in an electrostatic sense, and then “real” distances are fixed under the interaction of repulsive forces.

Measurements of the enthalpy of solution of SrCl2 in water and in 1 mol·dm−3 HCl solution lead to ΔHfo(SrCl2, c, 298.15 K) = −(832.7±0.9) kJ·mol−1, and to ΔHfo(Sr2+, aq, 298.15 K) = −(549.7±0.9) kJ·mol−1. Literature data have been assessed to calculate corresponding enthalpies of formation for the other alkaline-earth ions as well as the enthalpies of formation of the alkaline-earth dichlorides in 1 mol·dm−3 HCl.

Recent accomplishments are reviewed and current challenges in the syntheses and elucidation of the bonding and electronic structure of metals, inorganic compounds and organometallic compounds containing the rare earth and actinide elements are identified.

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.

Complex oxides and halides of the lanthanides and actinides have been studied extensively and reviewed thoroughly. Trivalent lanthanides and tetravalent actinides are readily prepared as complex halides; these two classes of complex compounds have been thoroughly investigated. Among complex oxides the most important groups have been those of the trivalent lanthanides and the hexavalent actinides. A number of recent research programs have been carried out on classes of complex compounds of unusual oxidation states. For example, exotic oxidation states such as Ln(II) and An(V) have been the focus of study of CsLnX 3 and MAnO 3 compounds (M = LiCs). In this category should also be placed the complex tetravalent oxides BaLnO 3 and BaAnO 3 , since these compounds are relatively unstable (for different reasons). This paper identifies some of the highlights of recent reviews cited above, and compares and contrasts the structural and thermodynamic properties of these important classes of compounds. ‘Mixed oxides’, such as LnLn′O 3 and An x An′ 1−x O 2 , are not treated here. Important predictions and areas for new research opportunities are identified.

The compounds K2PuCl5, Rb2PuCl5 K2PuCl6, and Rb2PuCl6 have been prepared. The latter two compounds were verified to contain only Pu(IV) by chemical analysis and spectrophotometry in solid and solution. The crystal structure of these compounds were determined by x-ray powder (Debye-Scherrer and Guinier) diffraction. The results are as follows: • K2PuCl5: Orthorhombic, Pnma, K2PrCl5 type [1]. a = 12.67 ± 0.003, b = 8.728 ± 0.00202, c = 7.970 ± 0.002 A. • Rb2PuCl5: Orthrohombic, Pnma, K2PrCl5 type [1]. a = 13.078 ± 0.005, b = 8.908 ± 0.003, c = 8.177 ± 0.004 A. • K2PuCl6: Monoclinic, C2/m, C2/m, distorted K2PtCl6 type. a = 10.45 ± 0.03, b = 10.12 ± 0.03, c = 10.47 ± 0.05 A; β = 92.7 ± 0.2 ° (tentative assignment). • Rb2PuCl6: Hexagonal, P63mc, Rb2GeF6 type [2]. a = 7.374 ± 0.005, c = 11.91 ± 0.001 A.

Nonstoichiometric calcium monouranate, CaUO3.50, was prepared by reducing CaUO4 in a hydrogen stream at 900°C. CaUO3.64 and CaUO3.81 were prepared by heating mixture of CaUO3.50 and CaUO4 in evacuated quartz tubes at 900°C for 50 h. Nonstoichiometric strontium monouranates with compositions SrUO3.62, SrUO3.70, and SrUO3.87 were prepared by heating mixture of SrUO3.52 and α-SrUO4 in evacuated tubes at 300°C for 100h. Chemical analyses were carried out for the products. These are all rhombohedral (R 3 m). The lattice c-parameter decreases at a much larger rate than the a-parameter with increasing oxygen amount, but there exist two-phase regions at CaUO3.81 and SrUO3.87. The effect of heating conditions for these phases was examined. The enthalpies of formation of these compositions were measured. The variation of these data as a function of nonstoichiometry is discussed.