Actinide phase stability

A system particularly studied from a thermodynamic point of view is the monocarbide (AnCi x) system. A range of stability for carbon deficient compositions in the monocarbides is present in many metal-carbon phase diagrams. Table 6 shows the composition range at room temperature for actinide monocarbides. The non-stoichiometry range is very limited for uranium monocarbide for neptunium and plutonium monocarbides, the stoichiometric AmCj oo composition is not stable. 

Much stronger kinetic stabilization can be expected for processes leading to the inclusion of radionuclide ions into the colloid structure (Fig. 7, lower part). Spectroscopic indications for such processes have indeed been found again by TRLFS for the Cm(III) interaction with colloidal and particulate amorphous silica, calcite and CSH phases (Chung et al. 1998 Stumpf Fanghanel 2002 Tits et al. 2003). The incorporation of actinide ions into colloidal precursor clay phases has been recently investigated as a possible mechanism in natural... 

C.K.Jorgensen The Conditions for Total Symmetry Stabilizing Molecules, Atoms, Nuclei and Hadrons. - J. C. Green Gas Phase Photoelectron Spectra of d- and f-Block Organometallic Compounds. - R. Englman Vibrations in Interaction with Inpurities. - W.L.Smith, K, N. Raymond Actinide-Specific Sequestering Agents and Decontamination Applications.- Y Y G. Mourn, I.Moura, A. V.Xavier Novel Structures in Iron-Sulfur Proteins. 

The interaction of the actinides or of their hydrides with chalcogens or H2X, respectively, give a number of nonstoichiometric phases with a composition close to AnX, An2X3, A11X2, An2Xs, or A11X3. Their thermal stability decreases in the order S > Se > Te. The magnetic susceptibility and Mossbauer spectroscopy show that the latter compounds may be formulated as [An +]2(S )2(S-Sf-, [An +]2(S -)3(S-S)2-, and [An +](S2-)(S-S)2-. These compounds have coordination numbers of 8, 10, and... 

Th02—ternary oxides or oxide phases with tetravalent americium are stabilized. The solid-state reaction of Am02 with most group V elements yields compounds with trivalent americium which are isostructural with the analogous rare earth compounds. In the last types of reactions americium exhibits a typical actinide behavior. 

An = Th, U, Np, and Pu. In complexing with metal ions, the / -diketones form planar six-member chelate rings with elimination of the enol proton. The simpler / -diketones, such as acetylacetone (HAA), are fairly water soluble, but form complexes that may be soluble in organic solvents. This is especially true for the An ions which form strong complexes with HAA and can be effectively sequestered to the organic phase, making HAA a potentially useful extractant (See Table 27). The four stability constants in Table 27 for tetravalent actinides imply that four HAA ligands coordinate with each metal ion in the formation of the extracted neutral ML4 complexes. ... 

This involves the use of tertiary amine extraction of the An ions from acidic 11 M LiCl solutions. Spectroscopic studies have indicated that, in the cases of Am and Nd at least, the octahedral trianionic hexachloro complexes are extracted from 11 M LiCl. Stability constant data for the chloride complexing of Am , and Cfin media of ionic strength 1,0 have been reported. Tertiary amines also extract Pu and a study of extraction from nitrate media by trilaurylamine (TLA) in xylene has been reported. " This showed that the mass transfer rate was controlled by the reactions between Pu from the bulk phase and interfacially adsorbed TLA-HNOs. The separation of individual transplutonium elements from the Tramex actinide product may be achieved using ion exchange or precipitation techniques." ... 

Thorium phosphate-diphosphate Th4(P04)4P207 (TPD, Pcam) is an actinide host phase due to its very high chemical durability and radiation stability [165-167]. TPD is synthesized by drying of thorium nitrate and phosphorus acid or ammonium phosphate solution, cold pressing at 300-800 MPa, and sintering of pellets at 1100-1250 for 10-30 hours. Th" in the TPD structure may be replaced by other tetravalent actinides but its isomorphic capacity is reduced with decreasing cationic radii in the following sequence > Np" > Pu". ... 

The structure and behavior of lanthanides and actinides in ILs has recently been summarized in detail by Binnemans and the reader is referred to this excellent review for more details [71]. Among others, it has been pointed out that solvation of the metal ions by the IL components is a major issue in f-element coordination chemistry [71, 79, 218, 219], Understanding the solvation is also a key to understanding f-element extraction processes. Among others, theoretical studies suggest that Ln3+ (Ln = La, Eu, Yb) are surrounded by six PF6 anions in Bmim-PF6 and by 11-13 imidazolium ions in the second ionic sphere. The same authors also suggest that free lanthanide cations, that is, cations without a solvation shell, are poorly soluble in Bmim-PF6 [149, 152, 220-223], They also report that, while [LnCls]3 complexes are unstable in the gas phase, they can be stabilized by solvation in ILs. 

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